I know this is a little delayed, but I saw this in a recent LACTNET digest
and I hope I can shed some light on this issue.

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Date:    Tue, 31 Oct 1995 09:57:39 -0500
From:    "Catherine Watson Genna, IBCLC" <[log in to unmask]>
Subject: Re: immune system

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]

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Immune system ontogeny is an ongoing process that begins in utero and
continues through the end of puberty.  Children reach different landmarks
of
immune system "maturation" at different ages.  Before the third trimester,
the fetus is able to produce fully "mature" lymphocytes.  At the other end
of
the spectrum, the thymus does not reach its "mature" dimensions until
puberty
when a certain degree of hormonally-induced involution takes place.  Many
scientists will refer to a child's immune system "maturity" based on the
ability to produce an appropriate immune response to a specific
immunological
challenge.  For example, neonates respond very poorly to certain kinds of
bacterial infections because their immune system has not matured enough to
recognize bacterial polysaccharides.  Secretory immunity "matures" at
different rates depending on if the child has been breastfed or not.
Immunity to parasites is highly variable depending on which parasite is
being
considered.  Thus, one's definition of immune system maturity depends on
what
specific component of the immune system one is describing.

I hope this helps.  For further reading, I can refer you to a chapter I co-
authored on "Nutrition and Immunity" in the upcoming edition of the
Pediatric
Nutrition Handbook.

Paul Zimmer, Ph.D.
Developmental and Clinical Immunology
University of Alabama at Birmingham