"As it is true, however, that mothers can confer an immunity to their
infants, would you know how they can do that if "genetic variability is
irrelevant'? Would it be through the colostrum? How is the immune system
primed to identify pathogens the child will encounter once born, if not
through some sort of genetic mechanism? I'm asking because I don't know."
Immunity to disease is brought about by the functions of MHC (Major
Histocompatibility Complex). MHC is part of a macroflage that breaks up
cells it detects as foreign objects (like a bacteria or virus). The MHC
then takes these pieces (called antigens) and gives them to T-Helper cells,
which then make a cure from these antigens (which is exactly how many
vaccines work).
You are born with your own unique MHC type (unless you are an identical
twin), which is like the toolbox that holds your tools. Breast milk works
in the same way that vaccines do, by transmitting the antigens of various
foreign cells that the T-helper cells use to make a cure. This is like
adding tools to your box.
You get sick when you don't have the propper MHC tool and the foreign cells
get overlooked. This is easy to do because bacteria and viruses tend to
mutate, which means that your body can miss a few of them.
So the short answer to your question is that your immune system relies on
both genetics and exposure, which is why some children are sickly and then
"grow out" of it.
Hope that helps out some.
Anne Schwarz