Blocking of IgM-mediated single radial haemolysis by IgA and non-complement-fixing IgM anti-type-III pneumococcal antibodies.

The results reported here provide information on the effects of mouse non-complement-fixing (NCF) IgM anti-type-III pneumococcal polysaccharide (SIII) in blocking single radial haemolysis (SRH) mediated by complement-fixing (CF) IgM or CF hybrid IgM/A anti-SIII antibodies. The results consistently showed that highly avid NCF IgM or IgA anti-SIII antibodies could prevent the detection of CF IgM or CF IgM/A anti-SIII antibodies by SRH. The altered haemolysis produced by NCF IgM and IgA antibodies in the primary response is similar to that reported when the SRH test was used to screen for primary rubella infection. The findings indicate that false-negative results can be due to the early production of NCF IgM antibodies which can block the detection of CF IgM antibodies particularly if the erythrocytes are coated suboptimally with antigen.