I think that our antibodies get passed through the milk, which gives the bub a headstart if a bug attacks? ETA: Well, yeah, according to that info, you should pass on the ability to fight that particular bug.
As Sherie said though, there are other variables that come into play too.
Our story (so far) is that Riv was breastfed, had a bad cold pretty early on, but now, he never gets sick. Who knows why, I guess that b/fing helped, perhaps keeping him away from sick kids helped, perhaps him eating/licking anything he could and building immunity that way helped, perhaps him not being immunised made a difference...who knows?