Before this subject is discussed, it is useful to give some definitions:

Antigen: Any substance that causes formation of antibodies to it. The most common antigens are protein, but certain carbohydrate polysaccharides may act in a similar manner. Lipid may be combined with either. Each antigen has a certain chemical configuration that gives it antibody-provoking ability. This specific chemical group may become detached from its carrier moleculeand temporarily lose antigenic power; it is then called a hapten. Attachment of a hapten to another suitable molecule leads to restoration of antigenic properties.

Antibody: Proteins of the globulin class, most often gamma globulins, produced by lymphocytes and plasma cells in response to antigenic stimulation. They may be specific, combining only with specific antigen molecules, or nonspecific, combining with a variety of antigens. Presumably, nonspecific antibodies attack a variety of molecules because similar hapten groups may be present even though the carrier molecule is different (cross reactivity).

Agglutinogen: Antigen on the surface of a red blood cell (RBC).

Agglutinin: Antibody that attacks RBC antigens and manifests this activity by clumping the RBCs.

Hemolysin: Same as an agglutinin, except that lysis of affected erythrocytes takes place.

Isoantibodies (alloantibodies): Antibodies produced against antigens coming from genetically different individuals of the same species. These “foreign” antigens are usually introduced into the body by transfusion or by pregnancy (if fetal RBCs containing antigens that the mother lacks reach the maternal circulation). When isoantibodies are produced, they do not cause disease unless RBCs containing antigens that the antibodies recognize subsequently come in contact with these antibodies.

Autoantibodies: Antibodies produced by the body against one or more of its own tissues. These antibodies are associated with autoimmune disorders and may cause clinical disease. There are several types of antibodies, depending on their occurrence and laboratory characteristics:

Complete (bivalent) antibodies: These usually will directly agglutinate appropriate RBCs. In vitro tests for these antibodies tend to demonstrate better reaction in saline medium at room temperature (20°C) orlower. They often require complement.

Incomplete (univalent) antibodies: These usually cannot directly agglutinate appropriate RBCs but only coat their surface. In vitro tests for these antibodies tend to show better reaction at higher temperatures, such as 37°C, and in high-protein medium.

Warm antibody: Reacts best in vitro at 37°C.

Cold antibody: Reacts best at 4°C-10°C.