Antigens
The term antigen (Ag) is used in two
senses, the first to describe a molecule which generates an immune response and
the second, a molecule which reacts with antibodies. In general antigens are
large, complex molecular substances that can induce a detectable immune
response. Thus an antigen is a substance that is specific to an antibody or a
T-cell receptor and is often used as a synonym for immunogen.
An immunogen is a substance capable of
initiating an immune response. Haptens are substance that are
non-immunogenic but can react with the products of a specific immune response.
Substances that can enhance the immune response to an antigen are
called adjuvants. Epitope is an antigenic determinant and is the
active part of an antigen. A paratope is the antigen binding site
and is a part of an antibody which recognizes and binds to an antigen.
On the basis of origin, antigens are classified
into exogenous antigens and endogenous antigens.
The antigens which enter the host from the outside in the form of microorganisms, pollens, drugs, or pollutants are called exogenous antigens. The antigens which are formed within the individual are endogenous antigens. The best examples are blood group antigens.
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