An immune response is triggered by immunogens, macromolecules capable of triggering an adaptive immune response by inducing the formation of antibodies or sensitized T cells in an immunocompetent host (a host capable of recognizing and responding to a foreign antigen). Immunogens can specifically react with corresponding antibodies or sensitized T lymphocytes. In contrast, an antigen is a substance that stimulates anti body formation and has the ability to bind to an antibody or a T lymphocyte antigen receptor but may not be able to evoke an immune response initially. For example, lower molecular weight particles, haptens, can bind to an antibody but must be attached to a macromolecule as a carrier to stimulate a specific immune response. In reality, all immunogens are antigens but not all antigens are immunogens. The two terms, immunogens and antigens, are frequently used interchangeably without making a distinction between the two terms.

Foreign substances can be immunogenic or antigenic (capable of provoking a humoral and/or cell-mediated immune response) if their membrane or molecular components contain(s) structures recognized as foreign by the immune sys tem. These structures are called antigenic determinants, or epi topes. An epitope, as part of an antigen, reacts specifically with an antibody or T lymphocyte receptor. Not all surfaces act as antigenic determinants. Only prominent determinants on the surface of a protein are normally recognized by the immune system and some of these are much more immunogenic than others. An immune response is directed against specific determinants and resultant antibodies will bind to them, with much of the remaining molecule being immunogenic. The cellular membrane of mammalian cells consists chemically of proteins, phospholipids, cholesterol, and traces of poly saccharide. Polysaccharides (carbohydrates) in the form of glycoproteins or glycolipids can be found attached to the lipid and protein molecules of the membrane. When antigen- bearing cells, such as red blood cells (RBCs), from one person, a donor, are transfused into another person, a recipient, they can be immunogenic. Outer surfaces of bacteria, such as the capsule or the cell wall, as well as the surface structures of other microorganisms, can also be immunogenic.

Cellular antigens of importance to immunologists include histocompatibility antigens, autoantigens, and blood group antigens (see later, “ABO Blood Grouping Procedure”). The normal immune system responds to foreignness by producing antibodies. For this reason, microbial antigens are also important to immunologists in the study of the immunologic manifestations  of infectious disease.