Titration of sensitising antibody and effects of “classical pathway” versus “alternative pathway” condition. Human RBC (E) were used without sensitization or sensitised with 10 μl anti(human RBC) antiserum per ml of 109 cells/ml (EA1) or with 100 μl antiserum per ml of 109 cells/ml (EA2). Two-fold serial dilutions of goat serum were made in either DGHB++ or DGHB-Mg-EGTA, starting at 1:1 (50%). Dilutions (100 μl) were incubated with E, EA1 or EA2 cells (100 μl of 108/ml) in the same buffer for 1hour at 37 °C. Percentage lysis was calculated, and the serum concentration (% serum) required for 50% lysis calculated. CH50 (the number of complement lysis units in 100% serum).