AntIgens
• Antigens are substances which, when introduced into
the body, stimulate the production of antibodies.• Most antigens are proteins but some are carbohydrates,
lipids or nucleic acids also.• Antigenic determinants or epitopes are those
sites on antigens that are recognised by antibodies
and receptors present on T-and B-cells.
• Based upon the ability of antigens to carry out their
functions, antigens are of two types : complete antigens
and incomplete antigens (haptens).
Intext Practice Questions
- Name the primary and secondary lymphoid organs in humans.
- How do neutrophils act as a cellular barrier to pathogens?
- Expand the following abbreviations:
(a) MALT (b) CMIS (c) AIDS (d) HIV (e) SCID (f) APC (g) AMIS - How do the killer T cells work?
- Distinguish between active and passive immunity.
• A complete antigen is able to induce antibody
formation and produce a specific and observable
reaction with the antibody so produced.
• Haptens are substances which are incapable of
inducing antibody formation by themselves, but can
do so on combining with larger molecules (normally
protein) which serve as carriers.
• Red blood corpuscles of all ABO blood groups possess
a common antigen, the H antigen which is a precursor
for the formation of A and B antigens.AntIBodIes or ImmunogLoBuLIns
• Antibodies are glycoproteins, called immunoglobulins,
which have specific amino acid sequences by which
they can interact with specific antigens. Antibodies
form 20% of the plasma proteins.
• Each antibody has a combination of atleast 2 light (L)
and 2 heavy (H) polypeptide chains.
• The heavy chain has larger number of amino acids,
while lighter chain has smaller number of them.
• The four chains in an antibody molecule are held
together by disulphide bonds (–S–S–).
• The polypeptides form a Y-shaped configuration. The
stem of Y is exclusively formed by heavy chains. In the
arms of Y, both light and heavy chains occur parallel to
each other, except for antigen binding sites.
• An antibody has a variable portion in the arms. It is
called V-region or antigen binding fragment, Fab.
• The remainder of the antibody is called constant
portion or crystalline fragment, Fc.
• It determines diffusivity, adherence to tissue component,
attachment to complement system and other biological
properties.