A serum factor which inhibits serologic reactions with enzyme-treated red blood cells.

Two patients' sera inhibited agglutination of enzyme-treated red blood cells by antibodies. This blocking effect was detected by using a mixture of antibodies as an enzyme test reagent for the routine control of a two-stage papain test. The blocking factor appeared to be IgG and its effect was complement dependent. It blocked reactions with red blood cells treated with papain, bromelin, or ficin. Reactions with both IgM and IgG antibodies of various specificities were blocked. The presence of the blocking factor in a patient's serum may result in failure to detect clinically significant antibodies unless a control system is used which will confirm that the cells in each test can be agglutinated by enzyme reacting antibodies.