Ferritin and ovalbumin binding blood lymphoid cells in fetal sheep

Lymphoid cells from blood of fetal sheep between 70 and 140 days of gestational age were examined for surface immunoglobulin (sIg) and for surface binding of horse spleen ferritin FITC and ovalbumin FITC. Fetuses of all ages had cells which bound ferritin or ovalbumin, the proportion of these cells in the lymphocyte preparations from blood decreasing with increasing age of the fetus. A similar decline in the proportion of sIg cells with age was found. The simultaneous occurrence of fetal lymphoid cells binding ferritin or ovalbumin is discussed in relation to the sIg population and to the property of the sheep fetus to respond sequentially to different antigens by the production of antibody.