Antibody‐Induced Hemolytic Activity of Human Blood Monocytes

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Abstract
The hemolytic effect of purified human blood monocytes was quantitated as the release of isotope from human erythrocytes labeled with 51Cr‐chromate. We here demonstrate that IgM antibody (anti‐A or cold agglutinin) was unable to confer monocyte‐mediated hemolysis, and that lysis was induced by IgG anti‐A antibody. In addition, IgG anti‐D sera containing antibodies belonging to subclass IgG1 or IgG3 had approximately equal hemolytic efficiency, as calculated from the antigen‐binding capacity of the sera at the concentrations inducing 50% monocyte‐mediated hemolysis. The lytic reaction was suppressed by pooled IgG or by individual myeloma proteins of subclass IgGl or IgG3. The inhibitory effect of heat‐aggregated IgG was not different from that of native IgG. IgG2 or IgG I had no or weak effects. Cross‐inhibition of lysis induced by IgG1 and IgG3 anti‐D antibody showed strong and equal suppression by IgG1 and IgG3 myeloma proteins. Hundredfold higher concentrations of IgG2 or IgG4 was required for inhibition of lysis, which may be an effect of contaminating IgGl or IgG3. Our data strongly suggest that the receptors on human blood monocytes for IgGl and IgG3 that participate in monocyte‐mediated hemolysis are identical.