The acquired immune mechanisms depend on the activation of the immune system by an antigen. The immune defences are broadly divided into the production of antibody by the stimulation of B-lymphocytes and the activation of T-lymphocytes to produce a cellular response. Foreign substances that can stimulate the immune system are called antigens. This module will briefly look at the structure of antibodies and antigens and how they interact.

Antigens are usually large molecules such as proteins or polysaccharides but in principle any substance may act as an antigen. Antigenic particles may contain many different epitopes. A single virus or bacterium usually contains several different antigenic molecules on its surface.

Antibodies are proteins that are produced by plasma cells that evolve from B-lymphocytes that have been stimulated by contact with a matching antigen. Antibodies belong to a group of proteins collectively known as immunoglobulins. There are five different classes or isotypes of immunoglobulins namely IgA, IgD, IgE, IgG and IgM. When an animal is immunised with a particular antigen there are usually several epitopes on the antigen that can trigger an immune response. The immune system therefore responds by producing a variety of different antibodies with specificity for different epitopes. This will result in the development of a polyclonal antiserum.

The affinity of the interaction between the antibody and the antigen epitope depends on the closeness of fit between the two structures.  The most important sites where antibody and antigen come into contact and must fit each other well are the parts of the paratope that represent the hyper variable regions.

An antibody generated against a specific antigen may cross react with heterologous molecules containing identical or closely related epitopes. In working with infectious diseases it is not uncommon to get some cross-reactions between closely related species of organisms.

The following topics will be covered in this sub-module:

• Antigen
• Antibody structure
• Polyclonal and monoclonal antibodies
• Antibody and antigen interaction