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]