Interpretation of blocking activity in maternal serum depends on the equation used for calculation of mixed lymphocyte culture results.

Immunosuppressive blocking factors in maternal serum are usually determined by inhibition of mixed lymphocyte cultures (MLC), but reports on the importance of these factors for successful pregnancy are conflicting. Here we measured serum blocking activity in men, non-pregnant nulliparous women, non-pregnant multiparous women, women with normal pregnancies, and in women who had had recurrent spontaneous abortions and were treated with leucocyte immunizations. Three different equations were used for calculation of blocking activity: blocking effect index (BEI); stimulation index (SI); and blocking index (BI). By all three methods of calculation, significantly lower levels of blocking activity were noted for men and women compared with pregnant women and multiparae. In the patients with a history of recurrent spontaneous abortions blocking activity as determined by BEI and BI increased into the positive range after treatment with infusions of third-party donor leucocytes in a statistically significant number of women (P less than 0.05). However, blocking activity as determined by BEI had a higher predictive value for successful pregnancy than did that determined by BI or SI. Our data suggest that the equation used for calculating BEI is superior to other methods for the determination of blocking activity when monitoring the response to leucocyte immunization in women with recurrent spontaneous abortion. However, these results also cast doubt on the importance of blocking antibodies in histories of recurrent abortion, since pregnancies occurred in the absence and spontaneous abortions occurred in the presence of blocking activity.