Immune System consists of group of cells, molecules, and organs that
act together to defend the body against foreign invaders that may cause
disease, such as bacteria, viruses, and fungi. The health of the body is
dependent on the immune system's ability to recognize and then repel or
destroy these invaders.
Types Of Immunity
Innate Immunity
i. Innate, immunity is the body's first, generalized line of defense against
all invaders.
ii. The responses of innate components are nonspecific.
iii. Components of innate immunity have no memory.
iv. Innate immunity is furnished by barriers such as skin, tears, mucus
and saliva as well as by the rapid inflammation of tissues that takes
place shortly after injury or infection that involves several
components such as phagocytes, complement system, etc.
v. Innate immunity may hinder entrance of the invadors but can rarely
prevent disease completely.
Adaptive Immunity
i. Adaptive immune responses are known as the second line of
defensive mechanism.
ii. Adaptive immunity consist of specific responses.
iii. Components of adaptive immunity have memory.
iv. B cells and T cells have memory.
v. In response to an antigen B cells get transformed into plasma cells produce the antibodies which can
interact with the structures on the surface of the invading organism
called antigens i.e. they are responsible for humoral immunity
(Occurs in the blood & other body fluids).
vi. T cells are responsible for cell-mediated immunity (provides
protection against intracellular pathogens) .
Components of Immune
Components of Innate Immunity
1. External Barriers
i. Lactic acid & fatty acid in sweat & sebaceous
secretions.
ii. Washing actions of tears, saliva and urine.
iii. Mucus secretions at internal linings at various sites aided
by mechanical stratagems (ciliary movement, coughing &
sneezing).
iv. Body fluids: Eg. Acid in gastric juice, Spermine & zinc
in semen, Lactoperoxidase in milk, Lysozyme in tears, saliva &
nasal secretions.
v. Suppression of growth of many potentially pathogenic bacteria
& fungi by normal bacterial flora, due to competition for
essential nutrients or by production of microbicidal substances (Eg. Lactic acid produced by commensal
bacteria which metabolize glycogen secreted by the vaginal
epithelium).
2. Phagocytes
The polymorphonuclear neutrophils
i. Azurophil granules with myeloperoxidase, non-oxidative ant
microbicidal- defensins , cathepsin G etc.
ii. Secondary specific granules with lactoferrin, lysozyme,
alkaline phosphatase, membrane bound cyt b558.
Killing By Phagocytosis
Steps Of Phagocytosis
i. Adherence by primitive recognition mechanism.
ii. Activation of actin-myosin contractile system. Which extend
pseudopods around the microbes.
iii. As adjacent receptors sequentially attach to the
surface of the microbe. Plasma membrane is pulled around the microbe
like a zipper until it is complementally enclosed in a
vacuole (phagosome).
iv. Within a minute cytoplasmic granules fuse with the
phagosome and discharge their contents.
v. This finally results in killing.
Killing By Reactive Oxygen
3. The Macrophage
Located throughout the connective tissue & around the basement
membrane of small blood vessels, in lungs (alveolar macrophage),
liver (Kupffer cells), lining of spleen sinusoids & lymph node
medullary sinuses, mesangial cells in kidney glomerulus, brain
microglia & osteoclasts in bones.
4. Complement System
i. Name given to complex series of 20 proteins.
ii. Characteristically produce a rapid, highly amplified response
to a cascade phenomenon , where the product of one reaction is the
enzymatic catalysts of the next.
iii. They are designated by letter 'C' followed by a number.
(Related to chronology of its discovery rather than to position in
the reaction sequence).
iv. Two pathways: (a). Alternative pathway, activated by microbes
& (b) Classical pathway, activated by Antibodies.
a. Alternative Complement Pathway
Activation of C3
cleavage-
i. A no. of microbes can activate enzyme C3bBb convertase to generate large amount of C3 cleavage" products(C3b) by stabilizing the enzyme on their "surfaces.
ii.
Recruitment of these generates C5 convertase, which cleaves C5 into C5a & C5b.
iii. C5a is released and C5b remain loosely bound to C3b.
iv. Then, C6 & C7 join to this complex. It has affinity for β-peptide chain of C8.
v. C9
gets activated and changed into amphipathic molecule capable of
insertion into lipid bilayer.
vi. Polymerization occurs and annular Membrane Attack Complex (MAC)
is formed.
vii. It is a Trans-membrane Channel fully permeable to electrolytes
and water, bringing about lysis.
The defensive strategy of acute inflammatory reaction initiated by bacterial activation of alternative complement pathway.
b. Classical Pathway
The classical pathway (CP) is activated primarily by immune complexes (ICs) composed of antigen and specific antibody, although other proteins, such as C-Reactive Protein, Serum Amyloid Protein, and amyloid fibrils, as well as apoptotic bodies can also activate the CP ( Bohlson et al., 2007; Cooper, 1985).The proteins of this pathway are C1, C2, C4, C1 inhibitor (C1-Inh), and C4 binding protein (C4bp). Some of their basic characteristics are summarized in Table below.
Protein MW Subunits Plasma Conc.(ug/ml)C1q 410,000 6A,6B,6C 70
C1r 85,000 1 35
C1s 85,000 1 35
C2 102,000 1 25
C4 200,000 α, β, γ 600
C1-Inh 104,000 1 200
C4bp 570,000 8 230
Cooperation In Innate/ Adaptive
We have learnt that components of innate immunity are not specific
and do not develop memory cells as do the components of adaptive
immunity.
So innate immunity does not improve by repeated exposure
to the same antigens. But they play a vital role since they are
intimately linked to the acquired systems by two different pathways
which encapsulate the whole of immunology.
Antinoay, complement and poiymorpns give protection against most
extra-cellular organisms; while T-cells, soluble cytokines,
macrophages and NK cells deal with intracellular
infections.
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