The lymphoid system consists of the lymphoid cells (lymphocytes and plasma cells) and lymphoid organs. Based on different roles they perform, lymphoid organs can be classified into central (primary) and peripheral (secondary) lymphoid organs. The central lymphoid organs are lymphoepithelial structures in which the precursor lymphocytes proliferate, develop and acquire immunological capability. In mammals, the bone marrow, the thymus and the bursa of fabricius in birds represent primary lymphoid organs. After acquiring immunocompetence, the lymphocytes migrate along blood and lymph streams, accumulate in the peripheral lymphoid organs and, following antigenic stimulus, effect the appropriate immune response. The spleen, lymph nodes and mucosa - associated lymphoid tissue (MALT) constitute the major peripheral or secondary lymphoid organs. Lymphoidal tissues in the gut ( peyer's patches), appendix, tonsils, salivary glands, tear glands and also the secretion (colostrums) of the lactating breast of the mother also are included in the immune sysytem.
The mucosa lining the alimentary, respiratory, genitourinary and other lumina and surfaces are constantly exposed to numerous antigens. These areas are endowed with a rich collection of lymphoid cells, either specialized aggregates as the Peyer's patches orScattered isolated lymphoid follicles
- collectively called the Mucosa associated lymphoid tissues (MALT). Such lymphoid tissues in the gut are called Gut associated lymphoid tissue (GALT) and those in the respiratory tract are called Bronchus associated lymphoid tissue (BALT). MALT contains lymphoid cells as well as phagocytic cells. Both B and T cells are present. The mucosa is endowed with secretory IgA. The mucosal regions afford/confer protection from many enteric and respiratory infections.
The terms immunogen and antigen are often used synonymously. However, these terms imply two closely related entities. The first describes a molecule that provokes an immune response(immunogenicity) and hence is called an immunogen. The other describes a molecule which reacts with the antibody produced, or with the activated cellular constituents of cell mediated immunity(antigenicity), and is referred to as an antigen.
In contrast to this is the hapten. Haptens are small well defined chemical groups such as dinitrophenol (DNP) which are not immunogenic on their own but will react with preformed antibodies. To make a hapten immu-nogenic, it must be linked to a carrier molecule which is itself immunogenic.
Antigens are recognized not only by antibodies, but also by antigen specific T cell receptors. In contrast to immunoglobulins, which usually recognize intact antigens, T cell surface receptors recognize processed antigens on the surface of antigen presenting cells.
The part of the antibody molecule which makes contact with the antigen is termed the paratope. Consequently, the part of the antigen molecule that makes contact with the paratope is called the epitope. As most antigens are protein in nature, they exist in a folder, three dimensional, tertiary structure. Hence there may be a cluster of amino acid sequences on the three dimensional structure constituting a series of epitopes. Each of these epitope clusters on the antigen are known as an antigenic determinant.
Generally the substances, which are capable of eliciting an immune response, that is, the synthesis of antibodies are called antigens. However the above definition of antigens do not infer the cell mediated response. Hence the term immunogen is introduced to denote antigenic substances which elicit both humoral and cell mediated immune response or either humoral or cell mediated response.
Antigens may be either natural or artificial/synthetic ones. Natural antigens, fall into two categories viz., particulate antigens and soluble antigens. Bacteria, viruses, erythrocytes and cells represent particulate type antigens. Bacterial toxins, proteins, carbohydrates, glycoproteins and lipoproteins represent the soluble antigens.
In addition, a variety of chemical compounds, biological macromolecules, synthetic polypeptides are regarded as potential antigens. Several polysaccharides, nucleoproteins and lipoproteins are also considered as antigens. Recently antibodies to DNA have been used in immunization.