Phagocytic acitivity of bovine leukocytes during pregnancy.

The phagocytic competence, measured as the total number of polymorphonuclear leukocytes per mm3 which phagocytosed Staphylococcus aureus, strain 321, in vitro, was determined in eight cows during complete pregnancies. Such leukocytes are referred to as "Active PMN'S". There was a gradual decline in the number of these cells from conception to a minimum between the 16th and 20th weeks of pregnancy, followed by a steady increase to the cessation of lactation when a marked drop occurred, after which there was an increase to a maximun during the second week prepartum. From this maximum there was a rapid decrease to an absolute minimum during the first week after parturition. From the second week postpartum there was a gradual increase to conception. The correlation coefficient (r) of number of active PMN'S with time before conception was -0.474 )p-0.01). There were significant differences (p=0.01) in numbers of active PMNS Among the eight cows. It was found that the cows fell into two groups, one whose members had, overall, significantly more active PMNs (p=0.001) than those in the second group. The between cow differences may have been due to 1) age, since the cows with the highest numbers of circulating active PMNs were younger than those in the other group of 2) the combined stress of pregnancy and lactation, as those cows which were both pregnant and milking had the lowest numbers of active PMNs.