Antibodies and their functions. Also called immunoglobulin. giving immunity to the host, to fight against antigens.

1. Antibodies Function
Prep: Amen Ullah
Lecturer Surgical
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2. Antibody Structure
 Antibodies belong to a group of glycoproteins called
globulins, and for this reason they are also known as
immunoglobulins (Ig).
 Most antibodies contain four polypeptide chains .
 Two of the chains are identical to each other and are called
heavy (H) chains, each consists of about 400 amino acids.
 Short carbohydrate chains are attached to each heavy
polypeptide chain.
 The two other polypeptide chains, also identical to each
other, are called light (L) chains, and each consists of about
200 amino acids. A disulfide bond (S!S) holds each light
chain to a heavy chain.
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3. Antibody Actions
 Neutralizing antigen. The reaction of antibody with antigen blocks or neutralizes
some bacterial toxins and prevents attachment of some viruses to body cells.
 Immobilizing bacteria. If antibodies form against antigens on the cilia or flagella of
motile bacteria, the antigen– antibody reaction may cause the bacteria to lose
their motility, which limits their spread into nearby tissues.
 Agglutinating and precipitating antigen. Because antibodies have two or mor sites
for binding to antigen, the antigen–antibody reaction may cross-link pathogens to
one another, causing agglutination (clumping together). Phagocytic cells ingest
agglutinated microbes more readily. Likewise, soluble antigens may come out of
solution and form a more-easily phagocytized precipitate when crosslinked by
antibodies.
 Activatin complement. Antigen–antibody complexe initiate the classical pathway
of the complement syste (discussed shortly).
Antibodies Action
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4. • Antibodies protect the body from invading
organisms in
two ways
1. By direct actions
2. Through complement system.
Mechanism of action of antibodies
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5. Direct Actions of Antibodies
Antibodies directly inactivate the invading organism by
any one of the following methods:
i. Agglutination: In this, the foreign bodies like RBCs or
bacteria with antigens on their surfaces are held
together in a clump by the antibodies.
ii. Precipitation: In this, the soluble antigens like
tetanus toxin are converted into insoluble forms and
then precipitated.
iii. Neutralization: During this, the antibodies cover the
toxic sites of antigenic products.
iv. Lysis: It is done by the most potent antibodies.
These antibodies rupture the cell membrane of the
organisms and then destroy them.
Direct action of antibodies
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6. 2. Complement System
Complement system is the one that enhances or accelerates various
activities during the fght against the invading organisms.
It is a system of plasma enzymes, which are identifed by numbers from
C1 to C9.
Including the three subunits of C1 (C1q C1r C1s), there are 11 enzymes
in total.
Normally, these enzymes are in inactive form and are activated in three
ways:
a. Classical pathway
b. Lectin pathway
c. Alternate pathway.
2. Complement System
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7. a. Classical pathway
1) Opsonization: Activation of neutrophils and macrophages to
engulf the bacteria, which are bound with a protein in the plasma
called opsonin.
2) Lysis: Destruction of bacteria by rupturing the cell membrane.
3) Chemotaxis: Attraction of leukocytes to the site of antigen-
antibody reaction.
4) Agglutination: Clumping of foreign bodies like RBCs or bacteria.
5) Neutralization: Covering the toxic sites of antigenic products.
6) Activation of mast cells and basophils, which liberate histamine:
Histamine dilates the blood vessels and increases capillary
permeability. So, plasma proteins from blood enter the tissues
and inactivate the antigenic products.
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8.  b. Lectin pathway
 Lectin pathway occurs when mannose-binding lectin (MBL),
which is a serum protein binds with mannose or fructose
group on wall of bacteria, fungi or virus.
c. Alternate pathway
 Complementary system is also activated by another way,
which is called alternate pathway.
 It is due to a protein in circulation called factor I.
 It binds with polysaccharides present in the cell membrane
of the invading organisms.
 This binding activates C3 and C5, which ultimately attack
the antigenic products of invading organism.
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9. Thank You
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