K cell activity is low in newborn infants.

K cell activity of 18 mature, healthy newborns was measured against O,Rh(D)-positive erythrocytes, sensitized by anti-D antibodies, in cord and venous blood (later obtained 3-4 days after delivery). 53 healthy women served as controls. Activity was determined as a function of target cell number in the enzyme-like kinetic model of cytotoxicity. It was low in cord blood, increased in venous blood by days 3-4 but it did not reach the level of adults. While cytotoxic activity of male infants increased significantly from birth to the 3rd-4th day of life, it did not change in female infants. No correlation was found between the weight and K cell activity of newborns.