Download Lymphatic System These notes are intended as a comprehensive

Lymphatic System
These notes are intended as a comprehensive yet basic summary of defense mechanisms of the
body, aimed at the level of this unit. The lymphatic system is studied again in Human Organs and
Systems
910.214,
the
second
semester
histology
unit.
To keep the file size and download time manageable, this page does not have pictures. Refer to
PowerPoint illustrations.
The term lymphatic system has different connotations depending on the context. In gross
anatomy we speak of lymph and lymphatic drainage including regional lymph nodes.
Histologically it involves a study of lymphatic structures and organs like Tonsil, Peyer’s patches,
appendix, lymph nodes, spleen and the thymus. In a broader context we also study the
mechanisms involved in defence and immunity. We take the larger context in the present
discussion and consider the defence mechanisms of the body in general, with a focus on the
histology of the lymphatic structures and the regional lymphatic drainage. The histology lectures
deal specifically with an overview of the lymphatic system and immune mechanisms. The
regional lymphatics are outlined in the gross anatomy lab.
The body has to deal with a variety of external harmful influences which include attacks by
micro-organisms – viruses, bacteria, uni- and multicellular parasites; physical and chemical
agents – radiation, environmental pollutants, substances of plant origin and so on. The body also
has to deal with unwanted internal entities like debris of its own dead cells – whether due to
natural cell death or damage from injuries. The body also tries to combat its own cells which
have become dangerous, like cancer cells.
Defence functions in the body exhibit a large spectrum of mechanisms.
First and foremost in the array of these mechanisms are the natural barriers. All surfaces of the
body exposed to external environment are covered or lined by epithelia. Externally the epidermis
of the skin is a stratified squamous keratinised epithelium. The inner surface of the digestive
system, lined by epithelium, is also exposed to external environment through the food that we
ingest. The constant influx of air exposes the respiratory system to a lot of dust and the attendant
micro-organisms that go with it. The urinary and reproductive systems have external apertures
and are similarly exposed. The epithelia of these systems are excellent biological barriers as they
selectively control what passes through them. Despite the protection they provide, these barriers
can break down, either by injury or by attacks by organisms. (Epithelia also line closed internal
cavities of the body like heart, blood vessels or coelomic cavity, but these are not under
consideration here).
Blood carries a great variety of defence weapons of the body. The white blood cells are largely
involved in this function. Some of them actually attack external offenders while some assist in
this function by facilitating defence reactions. A brief survey of white blood cells is in order.
These are of two main types : granulocytes and agranulocytes.
Granulocytes.
Granulocytes have “granules” or particles in their cytoplasm. These granules are mainly of
digestive enzymes which help in the digestion of ingested foreign matter. Based on the staining
properties of these granules, these cells are further divided into subtypes called neutrophils
(“neutral staining granules”), eosinophils and basophils. Neutrophils are mainly active in
ingesting bacteria and destroying them. Neutrophils are also called polymorphonuclear
leucocytes (polymorphs, in short). Quite a mouthful isn’t it? The name simply means “white
cells (leucocytes) with nuclei having many shapes”. A mature neutrophil has a nucleus which has
four or five segments (pieces) joined by slender connections. A young neutrophil has fewer. You
may stick to the easier-on-the-tongue term neutrophil. Neutrophils are the most numerous of
white cells, making up 55 to 60 per cent of all leucocytes.
Eosinophils are also phagocytic (“cells which eat”), but they eat up mainly products of antigenantibody reactions. The granules of eosinophils stain reddish by taking up acidic stains like
eosin.
This introduces the terms antigen and antibody. An antigen is any substance that provokes an
immune response. Not a very helpful definition, is it? Well, in simpler words, it is a substance
‘foreign’ to the body – the body reacts by producing a protein called antibody which renders the
antigen harmless by a variety of mechanisms.
The third variety of granulocytes is the basophil. Basophil granules are blue or purple in colour.
Unlike the other two types of granulocytes, basophil granules contain chemicals which dilate
blood vessels and prevent clotting (coagulation) of blood. Considering that blood carries so many
defence weapons, a part of the body under attack must have a good blood supply – hence the
need for dilatation of blood vessels. Blood flow is slower (sluggish) in dilated vessels, and
sluggish flow predisposes to clotting of blood which must be prevented.
Agranulocytes.
The term "agranulocytes" means cells without granules in the cytoplasm. There are two main
types – lymphocytes and monocytes.
Lymphocytes are in general small cells (though there are ‘medium’ and ‘large’ lymphocytes
too). Their nucleus is round and relatively rather large and seems to fill the cell with little
cytoplasm around. In case of an attack, some of the lymphocytes can grow large as they go into
action. 35 to 40 per cent of circulating leucocytes are lymphocytes. There are two major types of
lymphocytes, called T and B lymphocytes. They cannot be distinguished by routine staining
methods. More about lymphocytes later – after all, in the ‘lymphatic system’ we are going to talk
a lot about them!
Monocytes are very few when it comes to counting circulating cells in the blood. They are
characterized by a kidney-shaped nucleus. However, these cells come out of blood capillaries
and are transformed into macrophages – the large (‘macro’) cells in connective tissues which eat
(‘phage’) almost everything. They do much more than just eat up things, as we shall see shortly.
The term mononuclear phagocyte is often used for the macrophage. It can be confusing, therefore a little
explanation. This term probably relates with the origin of macrophages from monocytes. Even then, why
should the monocyte be so called? Again, probably this refers to the shape of the monocyte nucleus
which is kidney-shaped, yet contrasts with multiple segments of the nucleus of the most numerous type,
the neutrophil. Mind you, the neutrophil has a single nucleus, but in the mature form the nucleus appears
to be segmented, the segments joined by slender rods of chromatin. The whole explanation simply tells
the tortuous way terminology has evolved. If you find this confusing, simply ignore the term ‘mononuclear
phagocyte’, and call the macrophage a macrophage!
As a defence mechanism, most of the leucocytes can push their way out of blood capillaries so
that the leucocytes enter the tissues under attack.
From the numbers mentioned above, you can see that eosinophils, basophils and monocytes
make up but a small fraction of circulating leucocytes.
Phagocytic cells are by no means the only defence weapons in blood. The liquid part of
blood, plasma, is an extraordinary, amazing mixture of proteins, electrolytes, chemical
messengers and what not. Among the plasma proteins are a group called globulins. A fraction of
the globulins are the antibodies. This globulin fraction is called gamma globulin or
immunoglobulin. As mentioned before, the antibodies combat antigens – a specific antibody
against every antigen conceivable. Naturally, in the event of an external attack, when blood flow
to the area under attack increases, fluid from blood oozes out into the tissues, carrying
antibodies. Antibodies are produced by cells called plasma cells. (It is an unfortunate
coincidence, but the name plasma cell has nothing to do with plasma!). Plasma cells are in fact
modified B lymphocytes.
The epithelial barrier and the general nature of neutrophil defence (they can attack almost any
bacteria) may be called non-specific defence mechanisms. Lymphocytes, on the other hand, can
be trained to attack specific invaders either directly or through antibodies. They therefore are the
main instruments of what is known as specific immunity.
Needless to say then, the entire gamut of defence reactions is a very complex one. About
thirty or forty years ago this whole discussion could perhaps have occupied a few pages of a
textbook. Our knowledge of the immune system has grown by leaps and bounds (pardon the
cliché!) From a simple anatomical and histological description we have progressed towards
understanding interactions between cells, how cells ‘talk’ to each other by molecular messengers,
the detailed molecular structure of antibodies and a host of other compounds. So rapid is this
progress that, as someone would say, all that is written here is obsolete by the time you finish
reading it. The vast quantity of research has also generated a large terminology which may be
daunting for the beginner. But fear not, we are aiming at establishing a basic framework of this
entire world of immunity so that you can then progress to understanding its beautiful landscapes
in the more advanced units if you choose to. Some of the terms are not even mentioned in this
outline, those that are mentioned are explained in as simple language as possible.
Oversimplification brings with it the risk of inaccuracy; care has been taken to avoid it!
Inflammation
Before we proceed to the outline of the immune system, a brief description of a basic defence
reaction is in order. We take the example of a common bacterial infection of the skin to
understand its fundamental principles.
Imagine that the barrier of the epidermis is breached by a species of bacteria. The invaders find
themselves in the nutrient-rich territory of the underlying loose connective tissue. The very
presence of the foreign organisms is troublesome to the body. Some bacteria may even produce
chemicals (toxins) that are harmful to the body. Loose connective tissue ordinarily does have
some macrophages resident in it. Remember, macrophages are derived from monocytes in the
blood. But the affected part needs more than that. Since the defence weapons are located in blood,
the area under attack needs more blood supply. Loose connective tissue also has a special
type of cells called mast cells. The granules in mast cells are made of a chemical called
histamine. In a burst of activity, the mast cells release histamine granules in the tissue, and, in
effect, lose their granules. In other words, the mast cells ‘degranulate’. Histamine causes
dilatation of the microscopic blood vessels in the loose connective tissue. Blood flow therefore
increases, and leucocytes and antibodies emerge from the blood vessels to fight the
bacteria.
The increased blood flow and the efflux of blood components from the vessels changes the
appearance of the tissue. The increase in blood flow makes it appear red and also causes it to be
warmer than the surroundings. The extra fluid coming out of the vessels causes swelling of the
area under attack. Bacterial presence, their toxins and the appearance of other chemicals cause
painful. The overall result is a red looking spot, swollen, warm and painful. These four
features together are described as inflammation.
A bacterial attack implies a lot of neutrophils. As the neutrophils ‘phagocytose’ the bacteria,
their granules disappear. Neutrophils cannot synthesise new granules – they perform their duty
and die. Dead bacteria and leucocytes must not be allowed to litter the tissue with garbage, and
the macrophages come in force to clear the battleground. In addition to the existing
macrophages, new macrophages can be formed by monocytes which come in from the blood
vessels. After damage control comes the repair stage. Fibroblasts lay down new matrix and the
tissue is restored.
At times the process goes on too long, cell and bacterial debris accumulates faster than
macrophages clear it. The result is a thick semifluid mass, that is, pus. As pus accumulates its
pressure builds up and finally the pocket of pus ruptures. Once again, repair follows.
Tissue fluid and lymph.
Another important aspect of the lymphatic system is the formation and flow of lymph itself.
Let us consider the circulation of blood through the smallest vessels, the capillaries. A network
of capillaries supplies blood to the tissues. While oxygen is supplied directly from the RBCs in
the capillaries, other components of blood leave the capillaries and form a part of tissue fluid
which bathes the cells. Most of this fluid returns to the blood vessels (post-capillary venules).
Some fluid, however, remains in the tissues and is collected as lymph by “blind” vessels called
lymphatics. Lymph has a composition similar to plasma, except that it has very little protein. It
is logical to expect that lymph should be returned to blood circulation at some stage, otherwise
there will be a reduction in the circulating volume of blood and accumulation of fluid in the
tissues. Since lymph is collected from tissue fluid, it is likely to contain debris and even micro-
organisms in the event of an infection. It is therefore mandatory that all lymph be screened
before being returned to blood. This is done by small structures called lymph nodes. We shall
discuss the histological basis of this screening by lymph nodes a little later. Lymph from the
entire body is emptied into veins at the junction between the neck and the thorax by two major
ducts. This is an elementary introduction, but it gives us enough foundation to go ahead with the
study of immune mechanisms.
Specific Immunity
The key players in specific immunity are the lymphocytes. They are helped by other cells in a
variety of ways. To combat foreign antigens, lymphocytes must be trained to recognize the
body’s own constituents, particularly proteins, as “self”. This happens during intrauterine life.
Later, when a foreign antigen is spotted, it is usually “presented” to lymphocytes by cells
specialised for this function. These are the antigen presenting cells (APCs). Lymphocytes then
learn to produce antibodies against the antigen. Finally, the lymphocytes must “remember” how
to do this in the event the same antigen is met with in the future. We can summarise this process
in a stepwise manner :
 Recognition of ‘self’ and ‘non-self’
 Attack
 Memory
You may think of the three R’s of immunity : Recognise, React and Remember!
It is convenient to understand the entire process first, we shall deal with “lymphocyte training” a
little later.
The Antigen Presenting Cells
The single, most easily understood antigen presenting cell type is the macrophage. Special
macrophages in specific locations, for example, in the liver and the lungs, have specific names.
APCs are also found in the epidermis and other locations. Once again we can take the example of
a bacterial attack. Macrophages engulf and ingest bacteria. Enzymes in the cytoplasm fragment
them. This is called antigen processing. The cells then “display” antigen molecules by attaching
them to proteins on the cell surface. The proteins to which antigens are attached belong to a
special category. By themselves they are the key to recognition of self from non-self. These
special proteins are called “major histocompatibility complex” or MHC in short. Now that is
another mouthful. Let us explain it in brief.
This term, MHC, came into being in the context of rejection of tissue transplant. The word
‘histocompatibility’ simply means tissue matching. MHC proteins form a group, and each person
has a unique combination of individual proteins. Cells of a transplanted tissue may have some
proteins in common with the recipient (“host”), but those that are not can be recognized as nonself by the host’s immune cells and thus the transplanted cells are attacked and destroyed,
resulting in rejection of the transplant. With the term MHC we also come across another
peculiarity of the language of immunologists. You know that all proteins are coded by DNA
sequences. You may say that the DNA sequence for a protein is the gene for that protein. When
immunologists say MHC, they may mean the genes or the actual proteins. If a DNA sequence
has been actually transcripted and translated and the protein synthesized by a cell, it said that the
cell has ‘expressed’ that gene. In our discussion we are more concerned with the proteins than
with the genes, and when we use the term MHC we mean the proteins! Incidentally, for those
who are interested, MHC genes are on chromosome number 6. You may also note another point
– I said that a combination of MHCs is unique to an individual. Well, almost! Identical twins
have exactly the same genetic make-up, so their MHCs are also identical. The best donor for a
tissue transplant is an identical twin… but then not many are lucky enough to have one!
Now a bit further… MHC proteins can be divided into two broad groups – Class I (that is a
capital Roman numeral one!) and Class II. MHC Class I molecules are present on all nucleated
cells of the body – that is, all cells except RBCs. MHC Class II molecules are present only on
Antigen Presenting Cells. You need not worry about these two ‘classes’, awareness of MHC on
APCs is enough!
So, coming back to our point : APCs present antigens by attaching them to MHCs. Whom do
they present to? The answer is, lymphocytes.
Cell-mediated and Humoral Immunity – The Basic Difference.
Lymphocytes are of two major types, called T lymphocytes and B lymphocytes. We shall explain
these names soon. Further, among T lymphocytes are several subgroups.
T lymphocytes of one of these types are called cytotoxic T lymphocytes. They actually attack
foreign cells or even the bodies own cells which are abnormal (like cancer cells). This they do by
several molecules called by rather expressive names – perforins which ‘punch’ holes in the
enemy cell membrane, lymphotoxins which are “poisonous” to enemy cells because they destroy
the latter’s DNA and so on.
The concept here is that cytotoxic T cells (TC for short) are like soldiers who personally kill
the enemy. Therefore, the mechanism of immunity through T cells is called cell-mediated
immunity (also known as cellular immunity).
The other types of T cells help in the immune processes in their own way :
Suppressor T cells (TS) prevent the immune reaction from running out of control.
Memory T cells (TM) … well, obviously, are the cells that remember the antigen!
Helper T cells (TH) are a special class. Though they are T cells, they are involved in all kinds of
defence reactions. In non-specific defence, for example, they have functions like activating
macrophages. They also help in cellular and humoral immunity by selecting appropriate cells for
attacking foreign cells.
B lymphocytes function in a different manner. In the tissues of the body, they transform into
plasma cells which produce antibodies. Antibodies are proteins, and are synthesised in granular
(rough) endoplasmic reticulum (gER or rER). Golgi apparatus is also required for sorting and
packing the products for delivery outside the cell.
Plasma cells are therefore larger than lymphocytes, have bluish (basophilic) cytoplasm due to
rER and a pale area that marks the Golgi complex. The nucleus of a plasma cell is characteristic
– largely pale, with chromatin granules near the boundary giving it a ‘clock-face’ or ‘cart-wheel’
appearance.
Antibodies are like weapons of chemical warfare. They circulate in the blood and destroy
antigens.
Some more fundamental concepts.
The ‘T’ of T lymphocytes comes for the thymus. These cells are “trained” in the thymus. We
understand the ‘B’ of B lymphocytes in a roundabout manner. The first structure known to be the
source of B lymphocytes is a lymphoid organ found near the cloaca of birds. (Cloaca is the
common opening of urogenital and digestive systems in non-mammalian vertebrates and in a few
mammals called monotremes). This organ was described by Fabricius in the 16th century, though
its importance was recognized much later. The lymphoid organ is called bursa of Fabricius,
though we now use the term bursa for an entirely different type of structure! Anyway, the ‘B’ of
B lymphocytes refers to this ‘bursa’. Mammals do not have such an organ, and bone marrow and
some gut-associated lymphoid tissues are considered its equivalent. We can therefore say that the
letter ‘B’ in this context means “Bursa-equivalent organ”.
More terms and explanations.
A lymphocyte that is capable of reacting to a challenge (antigen!) is immunocompetent. When a
lymphocyte capable of attacking a particular antigen divides, it produces generations of cells
with exactly the same capability. This is called a “clone”. The concept of clone features in many
different contexts. You will learn more about this in 214.
CD
CD stands for “Cluster Determinant”. The term refers to proteins on the surfaces of
lymphocytes. A large number of such proteins have been identified and are given internationally
accepted numbers (instead of names). Again, it can refer to the gene for the protein or the protein
itself (“expression” of the gene). The point is, different types of lymphocytes express specific
CD proteins. Some CD proteins are specific to a stage in the life of a lymphocyte. Lymphocyte
types or stages are often indicated by the CD number. Thus, helper T cells exhibit a protein
number CD4, cytotoxic T cells have CD8 and so on. Since the CD proteins are like identification
tags they are also called markers.
“Identification” of CD proteins is something like reverse engineering. If such proteins are injected in
other animals, these animals will treat them as antigens and produce antibodies against them! If human
lymphocytes are exposed to these antibodies their CD proteins will react with the antibodies,
“identifying” them. In the almost surreal language of immunology, the CD proteins are often referred to
as antigens. Cells can be described as CD4-positive or CD4+, CD8+ and so on.
This is not core material for 212.
Macrophage, macrophage system, RE system.
The term reticulo-endothelial cell system is largely given up now. It dates back to the days when
endothelial cells were considered to very similar to macrophages. No doubt, some endothelial
cells do have phagocytic capacity, but they do not entirely fit into the role of macrophages. The
“reticular” part of the term originates from the fact that we are dealing with fine reticular
connective tissue as the framework of most lymphoid organs.
Macrophages themselves are found in many situations. In the connective tissues they are
abundant and are ‘local residents’. In the liver macrophages are seen in large capillaries
(sinusoids) and are called Kupffer cells. In the lungs macrophages have the additional job of
clearing dust from the air which otherwise would clog the gas exchange mechanism. All these
used to be included in the so-called macrophage system on the presumption that they are
different cells. We now know that they are all macrophages. In some locations, where they have
to clear very large pieces of debris, macrophages fuse together to form large multinucleated cells
called giant cells. An example of a giant cell with normal duties is the osteoclasts in bone. For
this discussion, we focus on the macrophage in its principal role as an antigen presenting cells.
Keep in mind that there are special types of APCs which are also given special names like
dendritic cells, follicular dendritic cells (in the lymphoid follicles or nodules – explained later)
and others.
Lymphoid Organs (also called lymphatic organs).
As we have seen, specific immunity revolves around lymphocytes. Aggregations of lymphocytes
and associated cells form visible structures called lymphoid organs. Each type of lymphoid organ
has characteristic structural and functional features. Of these, the thymus stands out as a unique
structure. The spleen has special features of its own. And of course, there is only one thymus and
one spleen! Lymph nodes are scattered all over the body, and though tonsils are specifically
around the pharynx, similar aggregations are to be seen associated with a lot of epithelia.
The thymus and bone marrow (or Bursa-equivalent organs) are the sites where lymphocytes are
trained, and are called as primary lymphatic organs. Other lymphatic organs, which are the site
of functioning of the immunocompetent lymphocytes are called secondary lymphatic organs.
The idea here is not to confuse you, but to explain terms which are commonly used!
The structure of lymph nodes is perhaps easier to understand, and this will be described first.
Lymph nodes.
We have mentioned earlier that lymph, formed in almost all tissues of the body needs to be
‘screened’ before it is returned to the bloodstream. This is done by lymph nodes. Both cellular
and humoral immune mechanisms play a role in this, and lymph nodes (like other
secondary lymphatic organs) therefore have both T and B lymphocytes. The macrophage is
the principal antigen-presenting cell, and these are also present in significant numbers in lymph
nodes. (Other APCs are also present, notably a cell type called follicular dendritic cell – see
below for an explanation of the term ‘follicle’). All these make the lymph node a highly
“cellular” structure. The cells are supported by an extensive connective tissue network. In
addition, lymph nodes have a special vascular pattern.
Overall features.
The first feature of a lymph node that strikes us – whether we look at the cut surface of a lymph
node or examine a stained slide with the unaided eye (or the lowest magnification of a
microscope) is that it has two zones – an outer dense or darker zone called the cortex and an
inner lighter area called the medulla.
Under a microscope we observe another feature : in the cortex there are a number of somewhat
circular masses called follicles or nodules. Each follicle often shows an outer dark zone and an
inner paler zone called germinal centre. The deeper part of the cortex, close to the medulla, is
called the paracortex. The lighter medulla shows interlacing columns of cells (“medullary
cords”) alternating with fairly large lymphatic channels, the medullary sinusoids. Let us go stepby-step.
Digressing a little, let us note and remember that lymphoid follicles or nodules are a feature of
most secondary lymphatic organs.
The main supporting framework. A lymph node is an encapsulated structure – it is covered by
a capsule (envelope) of a thin layer of dense connective tissue. We see that from the capsule a
number of incomplete partitions running towards the centre. These partitions are called
trabeculae (singular – trabecula). The capsule and trabeculae, being made of dense connective
tissue, are easily seen as pink structures.
Fine supporting structure : The rest of the lymph node is filled with a delicate network of
reticular fibres. Reticular fibres are not stained by H&E and therefore not visible in such
preparations. Reticular fibres are also produced by fibroblasts. (Such fibroblasts were earlier
called reticular cells, but this term can be very confusing, as it is applied to a diverse variety of
cells. We shall avoid it!)
Lymphatic vessels. A lymph node receives lymph from a number of lymphatic vessels called
afferent (“coming towards”) lymphatics almost all over its surface. These pour lymph into a
cavity just under the capsule. This large but thin, continuous cavity is called subcapsular sinus.
From this sinus, lymphatic vessels descend into the cortex. In the medulla they form a network of
channels called the medullary sinusoids. They finally join up to form one or more efferent
(“going away”) lymphatics which exit the lymph node from a restricted region called the hilum.
Blood vessels. The arteries and veins of a lymph node pass through the hilum. The arteries
ascend through the medulla, form capillaries which loop towards the edge and turn back to unite
and form little veins (venules) which finally form the veins. The venules in the paracortex have
tall endothelial cells (cuboidal or columnar as against the flat endothelial cells of most blood
vessels). These tall post-capillary venules (TPCVs) are the site where lymphocytes enter
lymphatic tissues from blood.
Cells. Within the framework of connective tissue and among the vessels, all the space in a lymph
node is filled with cells. The density of cells and their sizes varies and gives rise to the
appearance of cortex and medulla. The principal cell type is the lymphocyte. Both B and T
lymphocytes are present.
T lymphocytes are seen predominantly in the paracortex. The paracortex is also called as
“thymus-dependent” zone.
Most other areas are populated with B lymphocytes. Mature B cells (that is, plasma cells!) are
predominant in medullary cords. Immature B lymphocytes are mainly seen in the mantle zones.
Cells in the germinal centres of the follicles are larger and dividing lymphocytes.
Macrophages, as antigen-presenting cells, are naturally present in lymph nodes – especially in
the sinusoids and the germinal centres.
Besides these, white blood cells are often found outside blood vessels in lymph nodes.
Mind you, these are cell types involved in lymph node function. This list does not include cells
of the supporting connective tissues (fibroblasts) and endothelial cells of the vessels.
You may come across the term “reticulo-endothelial cells”. This is one of the unfortunate relics
of old terminology – it was believed that endothelial cells are phagocytic and were called by this
name. Current knowledge tells us that they are not. The confusion probably arose in the early
years because macrophages are seen close to endothelial cells in the lymphatic channels.
The term “reticular cells” has been mentioned earlier. These terms are best avoided.
Lymph node function.
We have said earlier that lymph may carry infecting micro-organisms and other debris and
therefore must be screened before being returned to the blood stream. As lymph percolates
through the system of lymphatic channels within the node, unwanted materials are engulfed by
macrophages and destroyed or exposed to the powerful defence mechanisms in the form of
lymphocytes. A lymph node that is ordinarily resting can increase its size and alter its structure
to meet such a challenge. Germinal centres appear in the follicles when cells divide.
The lymphocyte population in a lymph node is not static. Lymphocytes can enter via blood
vessels, especially the TPCVs, populate various zones of the node, react to antigens and can go
back into circulation.
The events in, and functions of a lymph nodes can be summarized thus :







It provides a network of channels through which lymph slowly percolates.
This allows foreign material in the lymph to be exposed to macrophages.
Macrophages process antigens in foreign material and present them to lymphocytes.
Entry of lymphocytes into the lymph node via blood vessels.
Production of lymphocytes of both types by division.
Allows interaction between APCs and lymphocytes.
Onset of the immune response.
Do not remember this list; just think of the cell types in the lymph node and the role of blood
vessels. Also remember that the blood vessels of a lymph node are the source of oxygen and
nutrients!
Despite these defence mechanisms, a lymph node itself may be the site of infection when it drains the
micro-organisms from a drainage area. Such infected lymph nodes are inflamed and painful.
Microscopically such a lymph node also shows large numbers of neutrophils if the infection is bacterial.
More often than not it subsides, but pus may form if there is a massive reaction. Relatively minor injuries
on the limbs can cause such painful enlargement of lymph nodes in the groin (for the lower limb) and the
axilla (for the upper limb, thoracic wall and part of abdominal wall). Lymph nodes may also trap
cancerous (malignant cells) from tumours in their drainage territory. Malignant cells have a tendency to
break loose from a tumour and travel be blood or lymphatic vessels. The trapped malignant cells can
multiply within the lymph nodes causing them to enlarge. Malignant growths can occur in the lymphatic
tissues themselves. If affected lymph nodes are removed surgically, the resulting scar tissue can block
lymphatic drainage especially if new lymphatics fail to form as a result of extensive surgical injury. This
can lead to swelling of drainage area. This is sometimes seen as a complication of surgery for cancer of
the breast. Lymphatic vessels can also be inflamed and blocked. If superficial lymphatics are inflamed
they are seen as red lines on the skin. One of the familiar causes of blockage of lymphatic vessels is the
ova (eggs) of a tiny parasite called Wuchereria bancrofti. Such a block leads to massive swelling
commonly in the lower limbs and scrotum – the condition is called elephantiasis.
Lymphoid Tissue Associated with Epithelia
All over the body there are numerous sites where epithelia are exposed to external environment.
The skin is the obvious first example, with its associated lymphoid tissue. Lymphoid tissue
associated with internal epithelia is often abbreviated MALT (Mucosa Associated Lymphoid
Tissue). In this category, that associated with the digestive system (gut) is called GALT (figure it
out!), and that under the respiratory tract is called BALT (B for bronchi). While the epithelia
themselves are a barrier, the barrier is further strengthened by lymphoid tissue. Unlike lymph
nodes this lymphoid tissue is without a capsule. Again, this lymphoid tissue may be a diffuse
collection of lymphocytes and APCs or may be seen organized into follicles like those of lymph
nodes.
Lymphoid tissues associated with epithelia develop to a great extent during childhood and
adolescence, but gradually diminish during adult life. Very little or none is seen in old age. Most
of our DR specimens are from elderly subjects and we do not expect to see tonsils or Peyer’s
patches in these specimens.
Tonsils
Surrounding the common passages of respiratory and digestive systems in the neck is a ring of
lymphoid tissue (MALT) collectively known as the tonsils. The lymphoid tissue is located just
under the epithelium lining these passages. Parts of the ring are located on the surface of the
tongue (lingual tonsil), under the palate (palatine tonsil), near the opening of the
pharyngotympanic (Eustachian) tube (tubal tonsil) and in the posterior wall of the nasopharynx
(“pharyngeal” tonsil or adenoids). The tonsil as it is understood by the lay person is the
palatine tonsil. Here we describe the palatine tonsil.
This lymphoid tissue is strategically located in a region where there is a great deal of exposure
to foreign material from external environment. The epithelium over the lymphoid tissue is
usually rather thin and lymphocytes are often found within the epithelium. It should be
understood that this tissue is a part of the wall of the pharynx which also has skeletal muscle and
some glands, and in a histological slide portions of these are often seen.
Two features of the epithelium covering the palatine tonsil are noteworthy. The epithelial cells
are not compact; they form a kind of network, with lymphocytes among them. Do not worry
about the appearance; the concept here is that the epithelium allows a great degree of contact
between its cells and lymphocytes. The epithelium also shows deep crevices called tonsillar
crypts (this feature is obvious in most slides of tonsil). Under the epithelium we find lymphoid
follicles, many with germinal centres. Between the follicles are areas populated by T
lymphocytes.
Tonsillar tissue does not have afferent lymphatics – its exposure to antigens is from the surface
itself. It does have efferent lymphatics which drain to regional lymph nodes. It also has a rich
blood supply which, as in the lymph nodes, brings nutrition and lymphocytes to the tonsils.
Despite being an organ of specific immunity, the tonsils can be the site of microbial attack and inflammation. We
shall not worry too much about the clinical aspects or the detailed anatomy of the pharynx in this unit, but some
facts are of interest even to the educated lay person. Repeated inflammation of the tonsils during childhood can have
long term effects. Tonsillar inflammation is painful, and repeated infection also results in a chronic state of general
ill-health. Inflammatory swelling of the tubal tonsil leads to a block of the tube connecting the pharynx to the ear.
Normally this tube equalizes pressure in the throat and the ear. A blocked tube can lead to an imbalance of pressure
and the ear-drum cannot vibrate efficiently to conduct sounds. The child may therefore be slow in learning due to
deficient hearing. Swelling of the nasopharyngeal tonsil blocks the air passage and the childs tends to breath through
the mouth. An open mouth disturbs the normal relationship of the tongue, teeth and palate, leading to altered
proportions of facial anatomy. The net result over a period is a sick child with a characteristic face, backward in
learning.
Lymphoid tissue in the digestive tube.
Diffuse collections of lymphocytes and related cells are seen almost all over the hollow
digestive tube. Such diffuse masses are usually confined to the lamina propria. In the ileum
there is a great deal of organized lymphoid tissue. These lymphoid masses are seen in the lamina
propria, but they may be large enough to extend into the submucosa. They are called Peyer’s
patches. The epithelial cells covering these patches are of special importance. Unlike the general
pattern in the digestive tube (where the epithelium is columnar), these cells are flatter, with fewer
and larger microvilli. These epithelial cells (called M cells) are important in handing antigens
over to the APCs underneath.
In a routine H & E preparation Peyer’s patches are easily identified by the huge collection of
lymphoid follicles. Like the tonsil, in between the follicles are T cell zones. The appendix
deserves special mention – it has an almost complete ring of lymphoid tissue around a narrow
lumen.
The Spleen
The spleen is arguably the most intriguing of the lymphoid organs. Much of the mystery is
solved if we first take a good look at the functions of the spleen. As a lymphoid organ it
certainly is involved in both humoral and cellular immunity, so it has areas populated
preferentially by B and T lymphocytes. During an antigenic challenge aggregates of lymphocytes
(follicles or nodules) develop germinal centres. Besides this, the spleen is a site of removal of
ageing red blood cells. RBCs are engulfed by macrophages. The cytoplasmic and cell
membrane components of RBCs, as well as haemoglobin are broken down. In the haemoglobin
molecule, a large portion is protein which broken down into amino acids. The haem part is
further broken down – the iron is separated and the rest is turned into other products.
When we consider the immune function of the spleen, the striking feature is that the spleen is
exposed to antigens in blood, not afferent lymphatics as in lymph nodes, and has no direct
contact with antigens from the external environment.
The lymphatic tissue of the spleen is therefore organized around blood vessels.
We have said elsewhere that the connective tissue framework of an organ governs its
organisation. In the spleen we add “blood vessels” to this statement. Let us carry this concept
into our understanding of the structure of the spleen. We can begin with a few comments on the
gross anatomy of the spleen.
The spleen is a large organ in the upper left part left region of the abdomen – left
hypochondrium, to be precise. It develops in the dorsal mesentery of the stomach and is close to
the upper part of the stomach. The normal spleen is hidden under the diaphragm, behind the
stomach and just above the left kidney and the left colic flexure. The normal spleen is cannot be
felt (is not “palpable”) when we examine a patient. It has to enlarge to a great degree to be
palpable. As it develops in close relationship to the stomach, it draws upon the artery of the
foregut (coeliac) for its blood supply. The breakdown products of RBCs are delivered to the
liver, which explains why its vein joins the portal vein! The spleen is covered by visceral
peritoneum.
The spleen has a fairly thick capsule of fibrous connective tissue. The capsule also has some
smooth muscle and can contract. In some animals (not in humans) the smooth muscle is
significant – in case of blood loss it causes contraction of the capsule, resulting in adding blood
to the circulating volume. In these animals the spleen acts as a reservoir of blood. The connective
tissue capsule sends incomplete partitions, called trabeculae (singular = trabecula) into the
splenic parenchyma. The splenic artery enters the organ at the hilum and sends branches along
the trabeculae (trabecular arteries). Trabecular arteries divide into smaller branches until they
may be called arterioles. As these emerge from the trabeculae they are surrounded by a sheath
of lymphocytes. This is called the periarteriolar lymphatic sheath (PALS). Most of the
lymphocytes in PALS are T lymphocytes. In places the PALS enlarges to form lymphoid
follicles. Lymphoid follicles largely contain B lymphocytes.
Follicles in the spleen therefore are distinguished by the presence of a central arteriole (often
called the central artery). Under antigenic stimulus they show germinal centres. The germinal
centre pushes the arteriole to an eccentric position, though it is still called the central arteriole!
On a cut surface of the fresh, unstained spleen the lymphocyte aggregates appear as white dots
and are collectively known as the white pulp.
The trabeculae, PALS and follicles together occupy just about one-fourth of the total volume of
the spleen. The remaining mass is called the red pulp. It is occupied by thin-walled venous
sinusoids forming a three-dimensional network. These vessels are lined by endothelium which
has slit-like gaps (imagine a wooden barrel!) through which blood cells can come out. The
space outside the sinusoids is filled with reticular fibres, fibroblasts and macrophages. In a
section of the spleen these cellular components are seen as a network of bars called splenic
cords. In a routine, H&E stained section the distinction between cords and sinusoids is not
always easy. Venous sinusoids converge towards the trabeculae as small veins which join to
form veins in the trabeculae and finally end up in the splenic vein.
The boundary area between white and red pulp is called marginal zone. It is the site where
lymphocytes are exchanged between blood and white pulp.
We must understand that this is a complex three-dimensional arrangement. In a two-dimensional
section we see fragments of all these features. Most sections of the spleen do show the capsule at
the periphery. Trabeculae may be seen as continuous with the capsule or as fragments within the
substance of the spleen. In either case, the dense fibrous tissue nature is evident – both these
features are seen as fairly thick pink bands. Trabecular arteries and veins are usually easily
recognizable. White pulp is seen as tiny blue areas under low magnification. Under moderate or
high magnification the lymphoid follicles show the central artery and you may see germinal
centres. Remember that with routine staining we cannot distinguish between T and B
lymphocytes! The red pulp is a mass of cells. With careful observation you may see
macrophages with the RBCs ingested by them, and with a bit of luck, sinusoids lined by
endothelium.
The Thymus
Frankly, we should have studied the thymus before the other lymphoid organs. It is the training
ground for T lymphocytes and therefore the beginning of our story. However, there are some
concepts in the biology of the thymus which are difficult to grasp without understanding the
other lymphoid organs.
The organ does have a fibrous capsule and fibrous partitions which incompletely divide it into
smaller lobules. Each lobule has an outer dark part, the cortex, and an inner pale part, the
medulla. The division into lobules is illusory, because in reality the medulla is like a tree with all
branches continuous with each other, each surrounded by cortex. In a two-dimensional picture it
gives an appearance of lobules.
The cellular and supporting tissue structure is quite different from the other lymphoid organs.
The thymus begins its development as an epithelial structure. Clumps of epithelial cells from the
pharynx separate out and form the framework of the thymus. Unlike all epithelial cells, they do
not form a sheet. The cells develop long fingerlike processes which forms a very loose network.
A network is called reticulum, so these cells are called epithelial reticular cells. This is the first
major difference between the thymus and other lymphoid organs. There are few reticular fibres
or fibroblasts.
During embryonic development, cells destined to form lymphocyte populate the spaces in the
reticular network of the thymus. Macrophages and other cells also reside in the thymus. The
story of lymphocyte specialisation is a one of very complex interactions among cells. Those of
you who will study organs and systems will have interesting insights into the story. Here our
aim is to establish a few elementary facts.
Thymic lymphocytes divide repeatedly and gradually move deeper from the cortex to the
medulla. As they do so, they are first trained to recognise antigens. Then they are trained to
recognise the bodies own proteins – they distinguish between “self” and “non-self”. Their
training is punctuated by tests and failure attracts the severe penalty of death. Successful cells
are called immunocompetent cells – they can recognise any foreign antigen and kill the cells
bearing such antigens by a variety of mechanisms. It is believed that some T cells mature before
they leave the thymus through blood vessels in the medulla, while some attain maturity in the
secondary lymphoid organs.
Macrophages in the thymus have rather complex functions, but one that we can easily
understand at the level of this unit is worth mentioning – they clear the debris of the lymphocytes
that have failed the tests and died!
The epithelial reticular cells are truly versatile – they form the physical supporting
framework, stimulate the lymphocytes to divide, create a nurturing environment for these cells,
and even produce some hormones.
In the lab, sections of the thymus are easy to recognise. It is a lobulated structure with a rather
dark cortex and lighter medulla in each lobule. There are no lymphoid follicles which are so
characteristic of the other organs. You will not be able to recognise the epithelial reticular cells
except where they form concentric masses in the medulla. These concentric masses, called
Hassal’s corpuscles are characteristic of the thymus. Sometimes, under low power (10x) you
may confuse them with blood vessels, 40x magnification will settle the issue. The function of
these corpuscles is not known with certainty.
Lymph nodes and lymphatic drainage
Finally, a brief account of lymphatic drainage of the body. We have mentioned before that lymph
needs to be screend by lymph nodes before it is returned to blood. Infections in any part of the
body are likely to give rise to enlargement of lymph nodes. Mild enlargement is often due to the
response of the lymph node (formation of germinal centres). Sometimes however, despite the
most valiant efforts, the lymph node itself may be the site of inflammation and pus formation. An
even more important fact of medical significance is that cancer cells tend to travel via
lymphatics. Cancer cells do not adhere to each other as most other cells do – they can break
loose from the main cancer mass. When they travel via lymphatics they can be trapped in the
lymph nodes where they begin their business of division again and form masses in the lymph
nodes. Enlargement of lymph nodes in a cancer patient is therefore a serious sign that the disease
is spreading. It is the light of these important phenomena that we consider an outline of
lymphatic drainage of the body. At your level, you are advised not to be bogged down by
detailed textbook descriptions.
The upper limb : The first major group of lymph nodes for the upper limb is in the axilla. These
lymph nodes also drain superficial lymphatic vessels from the front and back of the thoracic wall
including the breast and the part of the abdominal wall above the umbilicus. (The elbow region
has a couple of lymph nodes which are important in certain conditions, but we can ignore them).
You can thus see that the axillary nodes have a very wide territory of drainage. They are further
divided into five groups, but we do not need that detail.
The lower limb : If we disregard a couple of nodes near the knee, the first major group is in the
groin, called inguinal nodes. We shall ignore the distinction between superficial and deep
inguinal nodes. Between the two groups, these nodes also drain the lower part of the abdominal
wall, the external genital organs (except testis in the male) and the lower part of the anal canal.
(The explanation for the testis and the anal canal will be found in the tutorial notes on the pelvis).
The head and neck. Lymph nodes are found under the skin and deeper along the blood vessels of
the neck. Together they drain all of the head and neck.
Thoracic organs. The lymph nodes for the thoracic organs are largely in the mediastinum.
Abdominal organs. Abdominal organs are drained by lymph nodes around the aorta. For the
digestive tube they are in front of the aorta, around the origins of the main arteries (pre-aortic
nodes). The lateral structures (kidneys, adrenal glands and testes or ovaries) are drained by
lateral nodes called para-aortic nodes. The digestive tube deserves special mention. It has a series
of nodes located along the arteries, the pre-aortic nodes are the final nodes in the series. The
pelvic organs are mainly drained by nodes along their arteries.
All the lymph from the lower limbs, pelvic and abdominal organs is collected into a sac in the
abdomen, the cisterna chili from which a visible tube (about 4mm in diameter), the thoracic duct
takes it upwards. It is joined by lymphatic vessels from thoracic organs, left upper limb and the
left side of the head and neck before it finally empties into veins at the root of the neck. The
lymph from the right side of the head and neck and the right upper limb is emptied into veins on
the right side by a ‘right lymphatic duct’.
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