Download Histology of the mucosal lymphatic tissues The lymphoid system is

‫بسم هللا الرحمن الرحيم‬
Histology of the mucosal
lymphatic tissues
MUCOSO-LYMPHOID ORGANS
Peyer’s patch
V. APPENDIX
ILEUM

The lymphoid system is divided into a series
of functional anatomical compartments, of
which the 2 most important are:
The peripheral lymphoid system (made up of the
lymphatic organs e.g. spleen & lymph nodes): this is
the compartment that responds to antigens that
have entered the tissues or spread into the blood.
 The mucosal lymphoid system located near the
surfaces where most pathogens invade: is the
mucosal immune system (commonly described as
MALT).


2 further distinct compartments are:
The body cavities (peritoneum and pleura).
 Skin associated lymphatic tissue (SALT).

The Mucosal Immune System
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The mucous membranes
are one of the largest
organs of the body.
They comprise the
linings of the gastrointestinal, urogenital &
respiratory tracts.
The mucosal surfaces of the body are highly vulnerable to
infection. They are thin and permeable barriers to the
interior of the body because of their physiological activities
in gas exchange (the lungs), food absorption (the gut),
sensory activities (eyes, nose, mouth, and throat), and
reproduction (uterus and vagina).

The immune system of the MALT differs from that of
the rest of the peripheral lymphoid system in several
respects:

The types and distribution of T cells differ, with significantly
greater numbers of intraepithelial T lymphocytes (IEL) in
and lamina propria (LPL) in the gut mucosa compared with
peripheral L.Ns & blood.

The major antibody type
secreted across the epithelial
cells lining mucosal surfaces
is secretory IgA. This is
synthesized by plasma cells
in the lamina propria and
transported into the lumen of
the gut through epithelial
cells (enterocytes) at the
base of the intestinal crypts
by transcytosis. IgA binds to
the mucus layer overlying the
gut epithelium and acts as an
antigen-specific barrier to
pathogens and toxins in the
gut lumen.

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The mucosal lymphoid
system is exposed to
foreign antigens from
foods, commensal bacteria
of gut & from pathogenic
microorganisms and
parasites. No immune
response can normally be
detected to food antigens.
Indeed, soluble antigens
taken by mouth may induce
antigen-specific tolerance
or antigen-specific
suppression. In contrast,
pathogenic microorganisms
induce strong protective
response.
Gut-associated lymphoid tissue (GALT)

This is comprised of:
A) Mucoso-lymphoid organs
Tonsils (palatine, lingual, pharyngeal).
 Peyer's patches in ileum.
 Lymphoid aggregates in the appendix and large intestine.
 Lymphoid tissue accumulating with age in the stomach.
 Small lymphoid aggregates in the esophagus.

B) Diffusely distributed immune cells
Lymphocytes and plasma cells.
 M (microfold) cells.
 Mucosal mast cells.
 Antigen Presenting Cells (APC).

A) Mucoso-lymphoid organs
1. The tonsils are incompletely encapsulated
lymphatic organs at the entrance of the gut
and airways:
 Palatine tonsils: possess crypts, deep
invaginations of stratified squamous epithelium of
the covering tonsils; the crypts frequently contain
debris. Some of lymphoid nodules in lamina
propria have germinal centers (secondary
nodules). The tonsils are separated from
subjacent structure by a C.T. capsule.
 Pharyngeal tonsils: is a single tonsil located
in the posterior wall of the nasopharynx covered
by pseudostratified columnar epithelium.
 Lingual
tonsils: are located on the dorsum
of the posterior third of the tongue and are
covered by a stratified non keratinized
squamous epithelium. Ducts of the mucous
lingual glands often open into the base of
their crypts.
MUCOSO-LYMPHOID ORGANS
Tonsils
branching crypt
SSE Stratified Squamous Epithelium
TONGUE: Lingual tonsils
Lingual tonsil
PALATINE
deep
Lingual
mucous gland
capsule
cleft
pseudostratified columnar epithelium
PSCE
Connective
tissue
PHARYNGEAL
LINGUAL tonsil has SSE, but is
shallow, wedged intermittently
between the muscular &
glandular tongue core and the
dorsal posterior epithelium
Muscular core
LINGUAL tonsil has SSE, but is shallow,
wedged intermittently between the
muscular & glandular tongue core and
the dorsal posterior epithelium
2. Peyer's patches: of the
ileum have a distinctive
structure, forming
domelike structures
extending into the
lumen of the intestine.
The bulk of the Peyer`s
patches is B cells,
organized in a large and
highly active domed
follicle. T cells occupy
the areas between
follicles. The antigen
enters across a
specialized epithelium
made up of M cells.
MUCOSO-LYMPHOID ORGANS
Peyer’s patch
V. APPENDIX
ILEUM
Peyer`s Patches
Dome (M-cell) epithelium
Villus
Mantle zone
Germinal center
Solitary nodule/follicle
Diffuse lymphoid tissue - T & B lymphocytes, Plasma cells, APCs, Ms
3. The appendix:
The appendix is a short diverticulum arising from the blind terminus of the
cecum. It has a narrow, irregular lumen, which often contains debris. The
wall is thickened due to the presence of large aggregates of lymphoid
nodules in the mucosa and even in submucosa (in middle aged and older
individuals).
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Mucosa:
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VERMIFORM APPENDIX
Simple columnar (contains surface
columnar cells and goblet cells).
Lamina propria displays numerous
lymphoid nodules (capped by M cells)
and lymphoid cells. It does not form
villi but possess shallow crypts with
some goblet cells, surface columnar
cells and numerous enteroendocrine
cells.
Muscularis mucosa is composed of an
inner circular and outer longitudinal
layer of smooth muscle.
Submucosa is composed of
fibroelastic C.T. containing confluent
lymphoid nodules and associated
cells.
Glands - thinned out
Lamina propria
Muscularis mucosae
Tunica submucosa
Lymphoid nodules in
lamina propria and submucosa,
often erasing the muscularis
mucosae
T muscularis externa - thin
Tunica serosa


& No villi
Muscularis externa is
composed of an inner circular
and an outer longitudinal layer
of smooth muscle.
Serosa completely surrounds
the appendix.
B) Immune cells

Lymphocytes
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In addition to the organized lymphoid tissue in which induction of immune
responses occurs within the mucosal immune system, small foci of
lymphocytes and plasma cells are scattered widely throughout the
lamina propria of the gut wall. These represent the effector cells of the
gut mucosal immune system.
M (Microfold) cells:

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Site: they are specialized epithelial cells in
the epithelium overlying the Peyer's patches.
They are much less prominent than the
absorptive gut epithelial cells (enterocytes).
Shape and structure: they have microfolds
on their luminal surface, instead of the
microvilli present on the absorptive epithelial
cells of the intestine. M cells lack a thick
surface glycocalyx and do not secrete
mucus. Hence they are adapted to interact
directly with molecules and particles within
the lumen of the gut.

Function: they take up molecules and particles from the gut lumen by
phagocytosis, then transported them through the interior of the cell to the
basal cell membrane, where it is released into the extracellular space. This
process is known as transcytosis. At their basal surface, the cell membrane
of M cells is extensively folded around underlying lymphocytes and antigenpresenting cells, which take up the transported material, released from the M
cells and process it for antigen presentation.

Because M cells are
much more accessible
than enterocytes to
particles within the
gut, a number of
pathogens target M
cells to gain access to
the subepithelial
space, even though
such pathogens then
find themselves in the
heart of the adaptive
immune system of the
intestine, the Peyer's
patches.
MUCOSAL IMMUNE DEFENSE
Antigen
Antigen
M CELL
APC
Y

M cells take up
antigens from
the lumen of the
gut by
endocytosis.
The cell
membrane at the
base of these
cells is folded
around
lymphocytes and
dendritic cells
within the Peyer's
patches.
T
Y
B
Y

B
PROBLEM - Bugs in lumen, defensive cells
below epithelium. Epithelium includes M cells to
deliverantigen to APCs, & the main epithelial
cells transport IgA from Plasma cells up to the
lumen.
P
M (Microfold) cells
Antigens are transported through M cells by the process of
transcytosis and delivered directly to antigen-presenting cells
and lymphocytes of the mucosal immune system.

3. Mast cell
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Two types of mast cells are recognized; those from
connective tissue and a distinct set of mucosal mast cells
(MMC). The activities of the latter are dependent on T-cells.
Site: Mast cells are present in most tissues in the vicinity of
blood vessels, and are especially prominent near the
boundaries between the outside world and the internal
milieu, such as the skin, mucosa of the lungs and digestive
tract, as well as in the mouth, conjunctiva and nose.
Structure: it possess a small oval pale staining nucleus and
a cytoplasm filled with coarse deeply stained metachromatic
granules. It has a well-developed golgi complex and RER.
Function: Although best known for their role in allergy and
anaphylaxis, mast cells play an important protective role as
well, being intimately involved in wound healing and defense
against pathogens.
4. Antigen Presenting Cells:

Macrophages, Dendritic cells and B cells:
Appendix
Appendix
Palatine
Tonsil
Lingual
Tonsil