Observations on the Rare Phenomenon of Anti‐A Prozone and the Non‐Specific1 Blocking of Haemagglutination due to C1 Complement Fixation by IgG Anti‐A Antibodies

The anti‐A prozone activity of two immune anti‐A (group O) sera is described. It occurred in saline, albumen and enzyme tests at 4, 18 and 37°C but was destroyed by washing the ‘prozoned’ cells. This phenomenon is due to C1 fixation of non‐haemolytic complement by IgG anti‐A antibodies maximally sensitizing A1 and A1B cells. The addition of fresh complement restored the haemolytic activity of the sera. The nature of the blocking action is non‐specific, possibly due to the large C1 macromolecule preventing the second stage of haemagglutination by maintaining large intercellular distances.