In response to posts on the thymus and on herpes I thought some people might be
interested in a bit of basic immunology. The thymus is a gland which is a
primary lymphoid organ, where cells from the bone marrow become T lymphocytes (T
cells). The acquired immune system is made up of 2 major parts:
- HUMORAL: depends on ANTIBODIES (Abs) produced by B cells
 -CELL MEDIATED:  depends on T CELLS (T helper, suppressor and cytotoxic, now
more commonly referred to as CD4 + or CD8+ cells).
HUMORAL is particularly active against microorganisms free in the serum,
through circulating antibodies. Also Abs on mucosal surfaces like the gut bind
to and prevent entry of microorganisms.
CELL MEDIATED is particularly active against viral infections (where the virus
is multiplying inside a host cell, so is not generally accessible to antibodies)
and intracellular bacteria.
In reality an infection will stimulate both humoral and cell mediated
responses, but depending on the microorganism causing the infection one type of
response will predominate, and be more effective.
In regard to infections passing from the mother during breastfeeding, this is
particularly a problem when the mother becomes infected during breastfeeding. If
the mother is infected before the child is born (e.g. with HIV), then she will
have produced Abs that can bind to and neutralize free virus.  However if she is
infected during breastfeeding or in late pregnancy there will be a period when
she has the virus but doesn't have high levels of Ab, so is more likely to pass
on infection through breastfeeding.
Hope this is of interest.
Hilary