A sequential study of human B lymphocyte function from birth to two years of age.

The immunoglobulin synthesizing capacity of individual lymphocytes from newborns and infants aged 2, 6, 12 and 24 months respectively has been studied. The technique has been to expose purified lymphocytes to a T lymphocyte dependent (pokeweed mitogen) or a T lymphocyte independent (Epstein-Barr virus) activator of B lymphocytes. Activation has been measured as immunoglobulin secretion of individual B lymphocytes using a hemolytic plaque assay. B lymphocytes from newborns can be made to synthesize IgM at adult levels, but not IgG and IgA. Within 24 months from birth the secretion of IgG has reached adult capacity whereas IgA formation is still diminished. Lymphocytes synthesizing IgG subclasses appear at different times insofar as IgG1 and IgG3 are well demonstrable within 12 months from birth whereas IgG2 and IgG4 has not at all reached adult levels even 24 months after birth. The T lymphocyte dependent activator (pokeweed mitogen) fails to induce immunoglobulin synthesis in peripheral blood lymphocytes from newborns because of a defect helper T cell function. Such T cell capacity appears at the age of 6 months. The data unequivocally demonstrate restricted but definite B lymphocyte functional capacity already at birth and a gradual but partial acquisition of adult competence until the age of 2 years.