DEFINITIONS AND TYPES OF AUTOIMMUNITY

Autoimmunity versus Autoimmune Disease

The classic studies of Paul Ehrlich in the early twentieth century laid the foundation for our current notions of the concept of autoimmunity. Ehrlich used the term autoimmunity to signify an immune response against self and introduced the phrase horror autotoxicus, suggesting that there are mechanisms to protect against autoimmunity. Over the years, autoimmunity has been recognized as not uncommon and not necessarily detrimental. Thus, an important distinction must be drawn between autoimmunity, which may be asymptomatic, and autoimmune disease, which occurs when autoimmunity leads to an inflammatory response, resulting in tissue injury. An autoimmune response does not necessarily imply the existence of autoimmune disease.

T-Cell versus B-Cell-Mediated Autoimmune Disease

Autoimmune disease may be mediated primarily by T cells, as in multiple sclerosis or the animal model experimental autoimmune encephalomyelitis (EAE). In that case, disease can be transmitted from one animal to another by transferring antigen specific T lymphocytes. Alternatively, autoimmune disease may be caused by B cells that produce autoantibodies, as in the case of systemic lupus erythematosus (SLE). Autoantibodies bind to self-antigens (proteins, nucleic acids, or other molecules from one's own body, also known as autoantigens) and can damage cells either by binding directly to a cell surface or extracellular matrix antigen or through the formation of immune complexes (see the section “Mechanisms of Autoimmune Tissue Injury and Examples”). Autoantibody-mediated auto immune diseases sometimes can be transmitted transplacentally, as in the case of neonatal Graves' disease or congenital complete heart block and neonatal lupus. IgG antibodies/autoantibodies can cross the placenta, whereas IgM cannot.