Pre- and posttransfusion antibody titers were performed on 6 patients with
anti-Sd^a 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
Sd^a 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-Sd^a between pregnant women, either during their first
trimester or at term, and normal donors.