Antigen-antibody reactions are most easily studied in vitro using preparations of antigens and antisera. Reactions of antigens and antibodies are highly spe-cific. An antigen will react only with antibodies elicited by itself or by a closely related antigen. Because of the high specificity, reactions between antigens and antibodies are suitable for identifying one by using the other. However, cross-reactions between related antigens can occur, and these can limit the use-fulness of the test. The study of antigen-antibody reactions in vitro is called serology, and is important in clinical diagnostic microbiology.
Immunodiagnostic tests use an antigen-antibody reaction to detect and identify a specific antigen or antibody associated with a disease-causing organism. The antigen-antibody reaction itself is very specific. Consequently, if the correct antibodies can be obtained, immunodiagnostic techniques have the advantage of being able to identify the presence of a specific pathogen directly in the specimens and also can be used to detect the specific antibodies produced as a result of the immune response of the host to the organism. The primary com-ponent in the test is the antibody. The specificity, and to some extent the sensitivity, of the assay depends on the quality of the antibodies used in the reagents.
Antibodies are formed by clonal selection as explained. A large pool of B lymphocytes (B cells) display immunoglobulin (Ig) molecules on their surface. These Ig serve as receptors for a specific antigen, so that each B cell can respond to only one antigen or a closely related group of antigens. An antigen interacts with the B lymphocyte that shows the best “fit” by virtue of its Ig surface receptor. The antigen binds to this receptor, and the B cell is stimulated to divide and form a clone. Such selected B cells soon become plasma cells and secrete antibody. Since each person can make 107-108 differ-ent antibody molecules, there is an antigen-binding site on a B cell to fit almost any antigenic determinant.
Antibodies are immunoglobulins that reacts specifically with the antigen that stimulated their production. Antibodies that arise in an animal in response to a single antigen are heterogeneous because they are formed by several different clones of cells; i.e., they are polyclonal antibodies. Antibodies that arise from a single clone of cells, e.g., in a plasma cell tumor (myeloma), are homogeneous; i.e., they are monoclonal antibodies. Monoclonal antibodies can be made by fusing a myeloma cell with an antibody-producing lymphocyte. Such hybrido-mas produce virtually unlimited quantities of monoclonal antibodies in vitro. Monoclonal antibodies excel in the identification of antigens because cross-reacting antibodies are absent thus they are highly specific.
There are two basic methods for identification of unknown organisms by using sera containing various known antibodies; one is agglutination for particulate antigens and another is precipitation for soluble antigens. Other immunoassay methods include, fluorescent antibody technique (FAT), enzyme-linked immunosorbent assay (ELISA) and Western blot analysis.