Background and Objectives  Intravascular haemolytic reactions are reported in red‐cell T‐activated patients after blood transfusion. The relationship between T antigen antibodies present in normal plasma and these reactions remains unclear. In this study, we assessed the haemolytic activity of T antibodies in vitro in comparison with anti‐A/B antibodies.

Materials and Methods  We established a haemolysis assay based on treating target red‐blood‐cells (RBCs) with 2‐aminoethylisothiouronium bromide (AET). Two hundred and seven blood donor sera were analysed for anti‐T, anti‐A/B haemolysins and anti‐T agglutinins.

Results  Anti‐T haemolysins were found in 4 (1·9%) blood donor sera using a standard haemolysis method and in 174 (84%) samples using AET‐treated RBCs. Haemolysis correlated with agglutination titres (P < 10⁻⁷). With both methods, anti‐T haemolysins were much weaker than anti‐A and anti‐B haemolysins. Gradual desialylation of RBCs showed a correlation between sialic acid level as indicated by agglutination with Sambucus nigra lectin and anti‐T mediated haemolysis that was significantly increased (fold 2·4) independently of T antigen expression.

Conclusion  These data indicate that, in vitro, anti‐T‐mediated haemolysis depends primarily on the degree of desialylation of target RBCs. They suggest that the haemolytic activity of T antibodies‐containing human sera is usually weak and may only become significant in the very rare setting of a profound desialylation of RBCs.