Immune Interferon Induces

Receptors for the Fc domain of IgG (FcR)' are present on the membranes of cells of the myeloid and monocytic lineages (1) as well as on other unrelated cell types (2-4). Myelomonocytic cells interact with IgG antibody-sensitized cells or pathogenic organisms through their FcR: this interaction results in the activation of cellular defense mechanisms, such as antibody-dependent cell-mediated cytotoxicity (Ab-CMC), phagocytosis, and the extracellular release of lysosomal enzymes (5, 6) . FcR are first detected at the promyelocyte stage of myeloid differentiation, and their expression increases at later stages of maturation (3, 7) . An FcR that binds human monomeric immunoglobulins of the IgGI class (812) has been purified by immunoabsorption to human IgGI from both freshly isolated human monocytes and the macrophage cell line U937 (12) . This 73,000dalton molecule is not detectable on normal peripheral blood polymorphonuclear cells (PMN) using the same technique . Two antibodies, 3G8 (13) and B73.1 (14, 15), precipitate a molecule of 50,000-70,000 daltons from PMN and from natural killer/killer cells respectively : both antibodies inhibit binding of immune complexes to PMN (13-15). Antibody B73.1 also induces modulation of FcR on natural killer cells and inhibits antibody-dependent cytotoxicity (Ab-CMC) mediated by killer cells and PMN probably by reaction with the FcR itself or a closely related structure (14) . Binding of immune complexes to FcR of both monocytes and PMN is preferentially inhibited by human IgGI and IgG3 (11, 16). The available data suggest that, as in the mouse, there are at least two human FcR for IgG ; the human monocyte FcR is analogous to the mouse macrophage FcRl (trypsin-sensitive, specifically binding to monomeric mouse IgG2a) (17, 18), whereas the PMN and natural killer cell FcR correspond structurally and in binding properties to the mouse macrophage FcR2 (trypsin-