The serology of Sda effects of transfusion and pregnancy.

Pre- and posttransfusion antibody titers were performed on 6 patients with anti-Sda transfused with incompatible blood. In 3 of these patients a significant rise in IgG antibody titer was found. The data suggest that in occasional patients the Sda antigen does evoke a secondary immune response. We evaluated 245 pregnant women for the presence of Sda and found that 30% were Sd(a-). This incidence was significantly higher than that found in normal blood donors (4%), but was lower than that described in previous reports. We found that 22% of pregnant women in their first trimester were Sd(a-) whereas at term 36% were Sd(a-). These significantly different incidences of antigen positivity suggest decreased antigen expression with progressing pregnancy, as seen in the Lewis system. No difference was found in the incidence of anti-Sda between pregnant women, either during their first trimester or at term, and normal donors.