I read something once but can't remember where. In essence it explained that they receive antibodies via BM, but only ones for illnesses that you have already had or been immunised against.
Colds and viruses mutate all the time so the strain you have isn't the same as ones you have previously had. You are affected by a cold/virus because your body has no immunity to it, therefore neither does she via your milk. Once your body has fought it off, then the immunity to the strain is there and that will pass on to her.
This is the reason why there is a new flu needle every year because of the new strains of the virus that develop constantly.