The immune system consists of two major elements, an innate nonspecific mechanism which includes the phagocytosis of foreign objects, including pathogens, and a superimposed highly specific system with immunological memory that involves recognition of epitopes expressed by pathogens (the humoral immune system involving antibody) or on the surface of infected or altered cells (cell-mediated immunity). There are also recognition mechanisms based on the toll-like receptor system, the Imd pathway and on carbohydrate recognition (collectins, C-reactive protein, ficolins, mannan-binding lectin) and the antiviral interferon system (see oligoadenylate synthetases; RIG-1). Deficiences in any of the components can cause problems (see Bruton's agammaglobulinaemia; complement deficiencies; hypogammaglobulinaemia) and the regulation of immune responses is complex and susceptible to error (see immunoregulation; immunosuppression; interleukin-10; osteoclast differentiation factor; SLAM family; SLAP; T-helper cells; T-regulatory cells). Defects in the DNA-splicing system that generates diversity in the immunoglobulins and T-cell receptors can cause immunodeficiencies (see cernunnos). The basic phagocytic killing mechanism is defective in chronic granulomatous disease and Chediak–Higashi syndrome, predisposing to infection and indicating the importance of the ancestral immune system. Defects in the more specific recognition systems are immunodeficiency disorders such as severe combined immunodeficiency, and can also be caused by virus infection (see AIDS; feline immunodeficiency virus; human immunodeficiency virus; HTLV). Other immunodeficiencies are immunodeficiency, centromeric instability, and facial anomalies syndrome and Omenn's syndrome. Overactivity of the immune system can lead to autoimmune disorders although this may be a failure to develop immunological tolerance. Many disorders have an immunological component: see allergy; anaphylaxis; anergy; eczema; glomerulonephritis; Goodpasture's syndrome; Griscelli syndrome-2; hypersensitivity; Nezelof's syndrome; sarcoidosis; Timothy's syndrome.
Subjects: Medicine and Health.