Foreign material, including microorganisms, can contain chemical groups recognizable by the body as foreign. In general terms, molecules of any chemical group that elicit an immune response are termed immunogens. More specifically, a molecule that is capable of generating an antibody is termed an antigen. Antigenicity is determined by areas on the molecule termed antigenic determinants or epitopes. If a microorganism bypasses the body's other defenses, the immune system will produce a specific response that is directed against a particular antigenic epitope of this microorganism.
Most antigens are pure proteins, glycoproteins or lipoproteins. T Cells recognize the small peptides of proteins but not polysaccharides or nucleic acids. An Antigen Presenting Cell (APC) can present one of these antigenic peptides to the T Cell, thereby activating it. The activated T cell will in turn join to a B Cell stimulating it to differentiate into a Plasma Cell, which will produce specific antibodies to that particular antigen. This type of antigen that requires a T Cell to B Cell interaction for antibody production to occur is referred to as a T-dependent antigen. On the other hand, B Cells can express antibodies on their surface membrane, which are called B Cell Receptors (BCR). These receptors recognize not only proteins, but also polysaccharides and nucleic acids. These latter molecules are large and contain several different antigenic epitopes each of which can cross-link the membrane bound cell receptors (antibodies) on different clones of B Cells stimulating each cell to produce an antibody to one of the epitopes. This pathway is referred to Polyclonal B Cell stimulation. Since these antigens do not require a T Cell to B Cell interaction in order to produce antibodies, they are referred to as T-independent antigens. Examples include bacterial polysaccharides such as Lipopolysaccharide (LPS), a virulent product of many gram-negative bacteria.
Immunoglobulins (Ig) are gamma globulin proteins present in bodily fluids (e.g. blood serum) and mucosal secretions (e.g., saliva, tears, vaginal secretions), and may also be found at the site of inflammation within the tissue. They are produced by plasma cells, which are differentiated B lymphocytes or B cells. Based on structure and protein composition, immunoglobulins are divided into five classes, two of which are further sub classified. Each has its own distinct chemical structure and specific biological function.
Typical Immunoglobulin Structure
The immunoglobulin molecule is composed of a Constant region and Variable regions.
The Constant region generally is unique to the Ig Class or Ig Subclass and confers its biologic activity. The Variable regions form a complex, conformational molecular arrangement for the attachment of each specific antigen.