Antigens are usually exogenous substances (cells, proteins, and polysaccharides) which are recognized by receptors on lymphocytes, thereby eliciting the immune response. The re-ceptor molecules located on the membrane of lymphocytes interact with small portions of those foreign cells or proteins, designated as antigenic determinants or epitopes. An adult human being has the capability to recognize millions of different antigens, some of micro-bial origin, others present in the environment, and even some artificially synthesized.
Antibodies are proteins that appear in circulation after infection or immunization and that have the ability to react specifically with epitopes of the antigen introduced in the or-ganism. Because antibodies are soluble and are present in virtually all body fluids (“hu-mors”), the term humoral immunity was introduced to designate the immune responses in which antibodies play the principal roles as effector mechanism. Antibodies are also gener-ically designated as immunoglobulins. This term derives from the fact that antibody molecules structurally belong to the family of proteins known as globulins (globular pro-teins) and from their involvement in immunity.
The knowledge that the serum of an immunized animal contained protein molecules able to bind specifically to the antigen led to exhaustive investigations of the characteris-tics and consequences of the antigen-antibody reactions. At a morphological level, two types of reactions were defined:
1. If the antigen is soluble, the reaction with specific antibody under appropriate conditions results in precipitation of large antigen-antibody aggregates.
2. If the antigen is expressed on a cell membrane, the cell will be cross-linked by antibody and form visible clumps (agglutination).
Functionally, antigen-antibody reactions can be classified by their biological conse-quences:
Viruses and soluble toxins released by bacteria lose their infectivity or pathogenic properties after reaction with the corresponding antibodies (neutralization).
Antibodies complexed with antigens can activate the complement system. Nine ma-jor proteins or components that are sequentially activated constitute this sys-tem. Some of the complement components are able to promote ingestion of mi-croorganisms by phagocytic cells, while others are inserted into cytoplasmic membranes and cause their disruption, leading to lysis of the offending micro-bial cell.
Antibodies can cause the destruction of microorganisms by promoting their ingestion by phagocytic cells or their destruction by cells mediating ADCC. Phagocyto-sis is particularly important for the elimination of bacteria and involves the binding of antibodies and complement components to the outer surface of the infectious agent (opsonization) and recognition of the bound antibody and/or complement components as a signal for ingestion by the phagocytic cell.
Antigen-antibody reactions are the basis of certain pathological conditions, such as allergic reactions. Antibody-mediated allergic reactions have a very rapid on-set—a matter of minutes—and are known as immediate hypersensitivity reac-tions.