Cytophilic immunoglobulins revisited via natural killer cells.

Cytophilia is one of the biologic properties of the Fc region of immunoglobulins operationally defined as the ability of an antibody molecule to attach to certain cell types before combination with antigen. By attachment of cytophilic antibody to membrane Fc receptors, cells become "armed" with antibodies and able to interact specifically with soluble or particulate antigens. In contrast to this cytophilia, the so-called opsonic antibody binds to FcR of various cell types only after it has complexed with antigen. In spite of knowledge of cytophilic properties of immunoglobulins for more than 30 years and an impressive accumulation of facts regarding the structure and function of cell-surface FcR binding IgG, IgE, or IgA molecules, less progress has been made in understanding the molecular basis of cytophilia. Our extensive studies of the structural and functional aspects of the interaction of IgG in monomeric form with human natural killer cells, via the type IIIA IgG Fc receptor (Fc gamma RIIIA), provide a focal point for revisiting this major pathway of direct and continuous communication between the humoral and cellular compartments of the immune system.