lysed EA


Figure 2 Immune adherence of sheep erythrocytes to guinea pig platelets. (Photograph courtesy Dr RA Nelson.).

now known that adherence is through the CR1 receptor through C3b, iC3b, and C4b. Complement activation in I-A may be through the classical or alternate pathway. Thus, I-A of zymosan by the alternate pathway of complement can occur in the absence of antibody. I-A to erythrocytes and platelets is part of a wider phenomenon including immune adherence to the CR1 receptor also present on neutrophils and macrophages. I-A to red cells and platelets is also involved in the clearance of complement-coated particles by neutrophils and cells of the reticuloendothelial system, acting as a means of presentation of antigenic particles or immune complexes to phagocytic cells in these tissues.

Originally described with bacteria and protozoa, I-A also involves the surface of helminths and viruses, as well as soluble antigens such as ovalbumin. It has been used as a diagnostic test in the clinical laboratory, particularly in syphilis serology, where the TPI-A (Treponema pallidum I-A) test has had varying popularity. I-A serology may be used by observing the adherence of particulate antigens such as bacteria to the red-cell surface under the microscope or by observing the hemagglutination patterns at the bottom of the wells of a titration dish (Figure 3). An I-A hemagglutination assay has been applied to the study of herpes simplex and varicella-virus infections, and also used for the quantitative estimation of infantile gastroenteritis antigens in the stools.

Other uses of I-A have been to demonstrate activation of C3 on the surface of macrophages which have ingested asbestos in vivo, detected by I-A of human, but not sheep, red cells. Moreover, in studying the complement reactivity of soluble immune complexes, I-A has been a useful marker of the ability of the complexes to activate C3 and to bind to the CR1 receptor. The I-A phenomenon has been used to demonstrate the CR1 receptor sites in solid tissues, using the adherence of EAC (erythrocyte antibody complement) complexes to tissue sections. The demonstration of CR1 sites in glomeruli using this technique has suggested an alternative arrangement for immune complex deposition in the kidnev. The use of EAC complexes to quantitatc B cells in the blood or exudates is also an example of the I-A phenomenon, as B cells are known carriers of the CR1 receptor.

See also: Agglutination; Complement, alternative

Figure 3 Hemagglutination patterns caused by immune adherence. The samples on the left show the greatest degree of aggluti-

pathway; Complement, classical pathway; Complement receptors; Erythrocytes; Immune complexes; Opsonization; Phagocytosis; Platelets; Rosetting techniques.

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