[Development of the immune system]

The development of immune defense mechanisms begins early during fetal life but is not yet completed at birth. The earliest hematopoietic stem cells, which give rise to the lymphocytic and myelomonocytic cell lines, appear between the fourth and eight weeks postconception. T cells, responsible for cellular immune defense mechanisms, are first detectable at 12 weeks and seem to acquire their functional capabilities at the 16th week. B lymphocytes, responsible for humoral immunity, can be identified in the fetal liver at eight weeks and are functional at the 12-th-13th week, with the ability to produce specific IgM antibodies. Thus, in theory, fetuses of more than 16 weeks can produce a full immune response. Neonates still have incomplete development of a number of mechanisms involved in nonspecific immunity (natural killer, polynuclear and macrophage chemo-attraction, complement system).