agree re genetic predisposition. However i also think that if the gut is the first line of defence for the immune system, bm lines the gut in a specific way (bear with me, baby brained and can't remember where i've read half this stuff but some of it on MN).
Most people who eat well have usually had the range of childhood illnesses (ear infections, tonsilitis et al) that are usually treated with anitbiotics.
Antibiotics kill off the good bacteria in the gut along with the bad bacteria causing the infection. Prolonged use of anti-b's (and indeed lots of other drugs, even such as paracetamol) will put additional stress on the immune system due to the reduced number of good bacteria in the gut to help fight infection and stop the body from digesting nutrients etc.
This then means the immune system doesn't function normally and so that person becoes susceptible to colds, flus, coughs etc etc.
wehre cancer etc is concerned, yes I don't know wether a mother would be capable of creating antibodies for her child - but it does protetc against childhood cancers, and also against uterine and breast cancer in the mother.
God i wasn't intending to write an essay !!!! So, my point is i wonder if a bf child who is not exposed to the usual range of drugs and is eating well will be able to 'resist' gentically predisposed illnesses - and bm could indeed perhaps help a child with leukaemia etc purely by helping to boost the immune system as it contains valuable pro & pre biotics, helping the gut to digest foods efficiently and the body to uptake nutrients and use in the fight against the cancer.
phew !! hope i've made a bit of sense ? in my twisted thinking !!!