Changing patterns of lymphocyte circulation during development.

Despite the absence of antibody, immunoglobulin, and extrinsic antigen, the output of T- and B-lymphocytes from single lymph nodes and the intestines in the sheeep fetus increases exponentially over the last third of gestation. The fetus possesses a huge pool of recirculating lymphocytes which have the same blood to lymph transit time through lymph nodes as adult sheep. A subpopulation of intestinal lymphocytes which migrates preferentially through the small intestines of adult sheep was not found in the fetus. The sheep fetus is immunologically virgin, so fetal recirculating lymphocytes cannot be memory cells nor can they be dependent on antigen for their development.