Are fetal cells in maternal blood mainly B lymphocytes?

In 10 blood samples from mothers who were studied after the delivery of a male infant and who had Y-body-containing cells in their circulations (mean value, 0.06%) the proportions of surface-Ig- and Y-body-positive lymphocytes were determined after passage through nylon wool. In each case one part of the sample was treated with pronase before the nylon wool incubation and the other was not. Passage through nylon wool of maternal lymphocytes not treated with pronase resulted in a drastic decrease of fetal cells, since only 1 Y-body-containing cell was found in 1 of the 10 maternal blood samples studied, as compared with 28 Y bodies in the 10 samples not passed through nylon wool. In lymphocytes pretreated with pronase, 7 fetal cells were recovered after passage through nylon wool, although only in 5 of the 10 samples. Thus most fetal cells in maternal blood adhere to nylon wool, as do B lymphocytes, but this adherence is to some extent affected by pronase incubation.