Jackie,
I have read that the human infant does not begin to make all of the 5 classes
of antibodies until 9 mos of age, and levels do not reach adult levels until
around age 3.  This would counteract the idea of the immune system being
mature by 6 months.  Also, how long does the thymus persist?  This gland is
active in T cell processing, and regresses somewhere between age 3 and 6
years, if I remember correctly.
Catherine Watson Genna, IBCLC  NY City  [log in to unmask]