Specificity of Human Lymphocyte

The first studies of complement (C) receptor specificity concerned the primate erythrocyte immune adherence receptor (1) . The erythrocyte immune adherence receptor was at first thought to be specific for only the C3b fragment of immune complex-bound C3 (2, 3), but later Cooper demonstrated that immune complexbound C4 (C4b) was also bound to erythrocytes (4) . When bone marrow-derived lymphocytes (B cells) were demonstrated to have C3 receptors, these were at first thought to be also specific for the C3b fragment of C3 (5) . Later however, B cells were found to have two different C3 receptors : one which was specific for C3b and antigenically similar to the erythrocyte immune adherence receptor, and another which was specific for C3b inactivator-cleaved C3b (CM) and which was not found on erythrocytes (6) . C3b inactivator is present in serum and has an enzyme-like activity which cleaves complex-bound C3b into the fragments C3c and C3d (7) such that the C3c fragment is released from the immune complex leaving the small C3d fragment complex bound (8) . Recently, it has been shown that C3b inactivator also cleaves complex-bound C4. However, the presence of C2 apparently protects C4 from the action of this inactivator (N . R. Cooper, personal communication) . Studies of lymphocytes with C receptors (CRL)1 have indicated that many but not all possessed surface immunoglobulin (SIg), and that depending on the lymphoid source, some had either one or both types of C3 receptors (6, 9) . Both types of C3 receptors were usually found on normal lymphocytes . However, peripheral lymphocytes from most patients with chronic lymphatic leukemia were found to have a large number of cells with C3d receptors but usually few cells with immune adherence (C3b) receptors . In 20% of the patients studied, lymphocyte SIg was found to be missing (9) . Even though lymphocytes with C receptors and no SIg were rarely found in the peripheral blood of normal