Download Adaptive immunity

Immunology
L1
Dr. Hayder Naji
Immunity :- is defined as resistance to disease, specifically
infectious disease.
Immune system :-The collection of cells, tissues , and
molecules that mediate resistance to infections .
Immune response :- the coordinated reaction of immune cells
and molecules to infectious microbes.
Immunology :- is the study of the immune system and its
responses to invading pathogens.
The physiologic function of the immune system is to prevent
infections and to eradicate established infections.
Innate and adaptive immunity /
innate immunity (also called natural or native immunity), which
mediates the initial protection against infections, this type of
host defense is always present in healthy individuals, prepared
to block the entry of microbes and to rapidly eliminate microbes
that do succeed in entering host tissues.
The first line of defense in innate immunity is provided by
epithelial barriers and by specialized cells and natural antibiotics
present in epithelia, all of which function to block the entry of
microbes.
If microbes do breach epithelia and enter the tissues or
circulation, they are attacked By phagocytes, specialized
lymphocytes called natural killer (NK) cells, and several plasma
proteins, including the proteins of the complement system. All
these mechanisms of innate immunity specifically recognize and
react against microbes but do not react against noninfectious
foreign substances. innate immune responses enhance adaptive
immune responses against the infectious agents.
1
Immunology
L1
Dr. Hayder Naji
Adaptive immunity ( also called specific or acquired
immunity), which develop more slowly and mediates later,
even more effective, defense against infections . it stimulated by
microbes that invade tissues.
The adaptive immune system consists of lymphocytes and their
products, such as antibodies.
Whereas the mechanisms of innate immunity recognize
structures shared by classes of microbes, the cells of adaptive
immunity, namely, lymphocytes, express receptors that
specifically recognize different substances produced by
microbes as well as noninfectious molecules. These substances
are called antigens. Adaptive immune responses generate
mechanisms that are specialized to combat different types of
infections. For example, antibodies function to eliminate
microbes in extracellular fluids, and activated T lymphocytes
eliminate microbes living inside cells.
Types of Adaptive Immunity :- There are two types of
adaptive immunity, called
Humoral and cell mediated immunity, that are mediated by
different cells and molecules and are designed to provide
defense against extracellular microbes and intracellular
microbes, respectively.
Humoral immunity is mediated by proteins called antibodies,
which are produced by cells called B lymphocytes. Antibodies
neutralize and eliminate microbes and microbial toxins that are
present in the blood and in the lumens of mucosal organs.
Antibodies do not have access to microbes that live and divide
inside infected cells. Defense against such intracellular
microbes is called cell-mediated immunity because it is
mediated by cells called T lymphocytes Some T lymphocytes
2
Immunology
L1
Dr. Hayder Naji
activate phagocytes to destroy microbes that have been ingested
by the phagocytes into phagocytic vesicles. Other T
lymphocytes kill any type of host cells that are having
infectious microbes in the cytoplasm.
The antibodies produced by B lymphocytes are designed to
specifically recognize extracellular microbial antigens, whereas
T lymphocyte recognize antigens produced by intracellular
microbes. Another important difference between B and T
lymphocytes is that most T cells recognize only microbial
protein antigens, whereas antibodies are able to recognize many
different types of microbial molecules, including proteins,
carbohydrate, and lipids.
Immunity may be induced in an individual by infection or
vaccination (active immunity) or conferred on an individual by
transfer of antibodies or lymphocytes from an actively
immunized individual (passive immunity). Passive immunity is
therefore useful for rapidly conferring immunity, but it does not
induce long-lived resistance to the infection. An excellent
example of passive immunity is acquiring antibodies from
mothers through the placenta and milk.
Cells of the Immune System
I. Agranular Leukocytes
White blood cells with a single, unlobed nucleus and cytoplasm
that contains few or no granules are known as agranular
leukocytes.
Agranular leukocytes derive from lymphoid or myeloid lineage
precursors and account for approximately 35% to 38% of the
leukocytes in circulation.
3
Immunology
L1
Dr. Hayder Naji
A. Lymphoid lineage cells
B lymphocytes or B cells / reside in the bone marrow and are
able to synthesize immunoglobulin molecules. In fact, B cells
and plasma cells, are the only cells that are capable of
immunoglobulin synthesis.
T lymphocytes / lymphoid lineage cells of bone marrow origin
migrate to, differentiate in, and are vetted within the thymus.
natural killer (NK) cell / These large, nonphagocytic, granular
lymphocytes are named for their ability to kill abnormal (e.g.,
infected or malignant) host cells . They account for 5% to 10%
of all lymphocytes in the circulation.
B. Monocytic lineage cells
Mononuclear cells that differentiate from myeloid precursors are
known as monocytes in the circulation or macrophages once
they leave the circulation and enter the tissues. Monocytes are
large mononuclear cells and account for approximately 5% to
7% of the leukocytes in the peripheral blood .Monocytes spend
1 to 2 days in the circulation (their half-life is approximately 8.4
hours), then cross the endothelium to enter tissues throughout
the body, where they reside for up to several months as
macrophages. They phagocytize, or pick up cellular debris,
foreign cells, and particles and degrade them enzymatically.
Another group of phagocytic cells with both myeloid and
lymphoid origins is collectively known as dendritic cells, so
named for their branchlike cytoplasmic projections
II. Granular Leukocytes
These cells have multilobed nuclei and cytoplasmic granules
that contain amines (stained by basic dyes), basic proteins
4
Immunology
L1
Dr. Hayder Naji
(stained with acidophilic or eosinophilic dyes), or both (neutral
staining).
A. Neutrophils
Comprising approximately 60% of the peripheral blood
leukocytes, neutrophils are the most numerous leukocyte
population. They are also called polymorphonuclear (PMN)
cells because of their variable number of nuclear segments (two
to five). With a half-life of approximately 7 hours, over 100
billion neutrophils enter the circulation daily in normal adults.
Neutrophils are very effective at killing bacteria. An increase in
the number of peripheral blood neutrophils is often an indication
of acute infection.
B. Basophils and mast cells
The acidic cytoplasmic granules of basophils contain vasoactive
amines (e.g., histamine) that cause smooth muscle contraction .
These bilobed cells are found in low numbers in the peripheral
blood (0% to 1%) or in their tissue resident form, known as mast
cells. Both basophils and mast cells are important in allergic
reactions of the adaptive immune response.
C. Eosinophils
So named because of their “eosin-loving” granules (eosin is a
dye used in histology), eosinophils are bilobed granulocytes
with cytoplasmic granules that contain basic proteins. Although
they comprise 0% to 5% of the peripheral blood leukocytes,
eosinophils are active participants in innate and adaptive
immune responses to parasitic helminth (worm) infections .
5
Immunology
L1
Dr. Hayder Naji
Recognition of Microbes by the Innate lmmune System
The components of innate munity recognize structures that are
shad by various classes of microbes and are not present on host
cells
Phagocytes express receptors for bacterial lipopolysaccharide
(LPS, also called endotaxin), which is present in many bacterial
species but is not produced by mammalian cells.
phagocytes reccgnize terminal mannose residues on
glycoproteins; many bacterial glycoproteins have terminal
mannose, unlike mammalian glycoproteins, which end with
sialic acid or N-acetylgalactosamine.
Phagocytes recognize and respond to double-stranded RNA,
which is found in many viruses but not in mammalian cells.
unmethylated CpG nucleotides, which are common in bacterial
DNA but are not found in mammalian DNA.
The microbial molecules that are the targets of innate immunity
are sometimes called molecular patterns, to indicate that they are
shared by microbes of the same type. The receptors of innate
immunity that recognize these shared structures are called
pattern recognition receptors
In contrast to innate immunity, the adaptive immune system is
specific for structures, called antigens, that may be microbial or
nonmicrobial.
The receptors of the innate immune system are encoded in the
germline and are not produced by somatic recombination of
genes. Therefore, the specificity of adaptive immunity is much
more diverse than that of innate immunity, and the adaptive
immune system is capable of recognizing many more
chemically distinct structures. It is estimated that the total
6
Immunology
L1
Dr. Hayder Naji
population of lymphocytes can recognize over a billion
different antigens; in contrast all the receptors of innate
immunity probably recognize less than a thousand microbial
patterns.
The innate immune system responds in the same way to repeat
encounter with a microbe, whereas the adaptive immune system
responds more efficiently to each successive encounter with a
microbe.
Cellular receptors of innate immune system:- Toll-like
receptors (TLRs).
TLR-2
bacterial lipoglycans
TLR-3, -7,-8
for viral nucleic acid
TLR-4
for bacterial LPS (endotoxin)
TLR-5
for a component of bacterial flagella called flagellin
TLR-9
for unmethylated CG rich (CpG) oligonucleotides
which are more abundant in bacteria than in
mammalian cells
7