Fetal calf serum augmentation during cell separation procedures accounts for the majority of human autologous mixed leukocyte reactivity.

In the present experiments, we have observed that cells separated using techniques that provide no xenoantigen exposure to stimulator cells do not participate in the autologous MLR. However, exposure of separated cells in general and stimulator cells in particular to fetal calf serum does lead to autologous MLR. These data confirm the previous observations of Huber et al. that the human autologous MLR is primarily due to xenoproteins absorbed to stimulating cells during the separation procedures and indicate that a reassessment of human autologous MLR studies will be needed.