An antigen is a substance which when brought parenterally into the body stimulates the production of an antibody.

An antigen reacts with antibodies mainly in an observable way.

The part of an antigen, which determines the specificity of the antigen-antibody reaction, is referred to as an epitope which is made of 6 to 10 amino acids.

Antibodies apprehend the epitope as a gift on the floor of antigens and bind to them.

The size of the epitope is 27 to 35 Å.


Triggers the immune system

Provoke the immune system to produce antibodies

Foreignness: The foreignness is directly proportional to antigenicity. That is more foreign substance is more antigenic

Size: Antigenicity is related with the molecular size large molecules are more antigenic than small ones

Chemical nature: An aromatic radical is a must for antigenicity. Most naturally occurring antigens are proteins and polysaccharides. Not all proteins are antigens. An antigen is present on the capsule of the cell, in the nucleic acid or in cytoplasm etc., for example, Salmonella typhi has the antigen on the surface whereas in Streptococcus pneumoniae it is present on the capsule.

Susceptibility to tissue enzyme: Only substances, which are metabolized and are susceptible to action of tissue enzymes, behave like antigens, example polypeptides composed of element states are antigenic

Antigenic specificity: Antigenic determinants are the active sites present on the antigen. The remaining portion of the antigen molecule is antigenically inert. In an antigen-antibody reaction, antigen molecules react specifically at determinants site with the complementary antibody. Antigenic specificity is of the following types –

  • Species specificity
  • Isospecificity
  • Auto specificity
  • Organ specificity
  • Heterogenetic specificity


There are two attributes of antigenicity as follows –

  • Immunogenicity i.e. the induction of an immune response.
  • Immunological reactivity i.e. the specific reaction with antibodies or sensitive cells.

Based on the ability to carry out these two functions, antigens may be classified into two types i.e. a complete antigen and Haptens

  • Complete antigens are the substances which can induce antibody formation by themselves and can react specifically with these antibodies.
  • Haptens, on the other hand, are the substances unable to induce antibody formation on its own but can become immunogenic i.e. capable of inducing antibodies, when covalently linked to proteins, called carrier proteins. Haptens can react specifically with induced antibodies. These antibodies are produced not only against to the Hapten but also against the carrier proteins.


Depending on their ability to induce antibody formation, antigens are classified into

  • T-cell-dependent antigens
  • T-cell-independent antigen

Most of the natural proteins are T dependent antigens and B cells cannot respond to these antigens without a Co-stimulatory signal from the T-helper (TH) cells.

Some antigens can directly stimulate antibody production by B cells, without the apparent participation of T cells. Such antigens are called T-cell independent antigens. Most microbial Sugars, lipids and certain nucleic acid are T-cell independent antigens. These are of two types Type 1 antigen e.g. Endotoxin, lipopolysaccharide (LPS) etc and Type 2 antigens which are polymeric compounds like proteins (flagellar protein) or polysaccharides (cell wall LPS or pneumococcal capsular polysaccharide).

There is another class of antigens which is called as Superantigen. Superantigens are certain protein molecules such as staphylococcal enterotoxin that activates a very large number of T-cells irrespective of their antigenic specificity. This is because they cause the release of cytokines which results in massive proliferation of T-lymphocytes.

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