Antigen is a substances usually protein in nature and
sometimes polysaccharide, that generates a specific immune response and induces
the formation of a specific antibody or specially sensitized T cells or both.
Although all antigens are recognized by specific lymphocytes
or by antibodies, only some antigens are capable of activating lymphocytes.
Molecules that stimulate immune responses are called Immunogens.
Epitope
is immunologically active regions of an immunogen (or antigen) that binds to
antigen-specific membrane receptors on lymphocytes or to secreted antibodies.
It is also called antigenic determinants.
Auto-antigens, for example, are a person’s own self
antigens. Examples: Thymoglobulin, DNA, Corneal tissue, etc.
Alloantigens
are antigens found in different members of the same species (the red blood cell
antigens A and B are examples).
Heterophile
antigens are identical antigens found in the cells of different species.
Examples: Forssmann antigen, Cross-reacting microbial antigens, etc.
Adjuvants
are substances that are non-immunogenic alone but enhance the immunogenicity of
any added immunogen.
Chemical nature of Antigens
(Immunogens) –
A. Proteins –
The vast majority of immunogens are proteins. These may be
pure proteins or they may be glycoprotein or lipoproteins. In general, proteins
are usually very good immunogens.
B. Polysaccharides –
Pure polysaccharides and lipo-polysaccharides are good
immunogens.
C. Nucleic Acids –
Nucleic acids are usually poorly immunogenic. However, they
may become immunogenic when single stranded or when complexed with proteins.
D. Lipids –
In general lipids are non-immunogenic, although they may be
haptens.
Types of Antigen on the basis
of order of their class (Origin) –
1. Exogenous antigens –
These antigens enter the body or system and start
circulating in the body fluids and trapped by the APCs (Antigen processing
cells such as macrophages, dendritic cells, etc.)
The uptakes of these exogenous antigens by APCs are mainly
mediated by the phagocytosis. Examples: bacteria, viruses, fungi etc. Some
antigens start out as exogenous antigens, and later become endogenous (for
examples, intracellular viruses).
2. Endogenous antigens –
These are body’s own cells or sub fragments or compounds or
the antigenic products that are produced. The endogenous antigens are processed
by the macrophages which are later accepted by the cytotoxic T-cells.
Endogenous antigens include xenogenic (heterologous), autologous and idiotypic
or allogenic (homologous) antigens.
Examples: Blood group antigens, HLA (Histocompatibility
Leukocyte Antigens), etc.
3. Autoantigens –
An autoantigen is usually a normal protein or complex of
proteins and sometimes DNA or RNA that is recognized by the immune system of
patients suffering from a specific autoimmune disease. These antigens should
not be, under normal conditions, the target of the immune system, but due
mainly to genetic and environmental factors, the normal immunological tolerance
for such an antigen has been lost in these patients.
Examples: Nucleoproteins, Nucleic acids, etc.
On the basis of immune
response –
1. Complete antigen or
immunogen –
Posses antigenic properties denovo, i.e. they are able to generate
an immune response by themselves. It has high molecular weight (more than
10,000). It may be proteins or polysaccharides.
2. Incomplete antigen or
Hapten –
These are the foreign substance, usually non-protein
substances. Unable to induce an immune response by itself, they require carrier
molecule to act as a complete antigen. The carrier molecule is a non-antigenic
component and helps in provoking the immune response. Example: Serum Protein
such as Albumin or Globulin. It has low molecular weight (less than 10,000).
Haptens can react specifically with its corresponding antibody. Examples:
capsular polysaccharide of pneumococcus, polysaccharide ‘C’ of beta hemolytic
streptococci, cardiolipin antigens, etc.
Determinants of Antigenicity
–
The whole antigen does not evoke immune response and only a
small part of it induces B and T cell response. The small area of chemical
grouping on the antigen molecule that determines specific immune response and
reacts specifically with antibody is called an antigenic determinant.
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