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Epitopes recognised by a panel of 23 anti-Fc gamma monoclonal antibodies (McAbs) have been subdivided into three groups each having a distinct topographical distribution. One group of mutually inhibitory McAbs are reactive with epitopes expressed on the fy "surface" of the C gamma 2 domain. A second group recognises epitopes in the region of arginine 355 of the C gamma 3 domain whilst the third group recognises epitopes expressed in the inter C gamma 2/C gamma 3 domain region--as evidenced by inhibition of Staphylococcus aureus protein A binding. An antibody of the latter group reactive with IgG1, 2, 4 and IgG3m(15,16) proteins but not IgG3m(5) or IgG3m(21) proteins allows histidine 435 to be identified as a critical residue for expression of the epitope recognised by this antibody.

The specificity of a panel of 27 monoclonal antibodies reactive with human Fc gamma has been further defined through reactivity profiles with sub-fragments of Fc gamma and IgG paraproteins having deletions within C gamma 2 or C gamma 3. Antibodies are identified that are directed to discontinuous epitopes requiring native Fc gamma for expression. Other antibodies are reactive with isolated C gamma 2 or C gamma 3 domains. Sub-class specific and sub-class restricted antibodies allow correlation of epitope expression with primary structure.

A monoclonal antibody has been raised against a cross-reactive idiotypic determinant expressed on human autoantibodies with anti-I and -i specificities. The McAb is directed against a conformational epitope requiring interaction of H- and L-chains for maximal expression. This epitope was strongly expressed on the monoclonal protein of one out of 100 patients with paraproteinaemia and the peripheral blood lymphocytes of one out of 50 cases of B-cell leukaemia. A small amount of the epitope is detectable among immunoglobulins in normal serum.