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E I03" Macro ha \ O Immune complaxiQj Macrophagg^—Jh _J

Elimination of Immune Complexes Erythrocyte

Fig. 5.2. Biological activities of complement. Opsonization and Phagocytosis: A. Macrophages and neutrophils express the complement receptor CR1 whose ligands include complement fragments C3b, C3bi, and C4b. When deposited onto microbial surfaces these complement fragments serve as opsonins to facilitate phagocytosis by cells expressing CR1 receptors. B. Osmotic Cell Lysis: Activation of the terminal pathway involves the formation of a membrane attack complex (MAC) on a microbial cell surface, which induces osmotic lysis and microbial destruction. C. Mast Cell or Basophil Degranulation: Mast cells and basophils express the complement receptors CR-3a/4a and CR-5a. Binding of the anaphylatoxins to their cognate receptors on induces degranulation and release of inflammatory mediators. D. Chemotaxis: The anaphyla-toxin C5a is chemotactic for phagocytes. E. Elimination of Immune Complexes: Complement activation generates C3b that may be deposited on immune complexes. Interaction of CR1 present on red blood cells with its ligand, C3b, present on the immune complexes, facilitates delivery of complexes to the spleen where phagocytes, again bearing CR1 receptors, interact with free C3b molecules on the immune complexes inducing phagocytosis of the complexes.

Table 5.1 Complement receptors

Complement Receptor Ligand Cells on Which the

Receptor Is Expressed

Table 5.1 Complement receptors

Complement Receptor Ligand Cells on Which the

Receptor Is Expressed

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