Colostrum actually works as a natural and 100% safe vaccine. It contains large quantities of an antibody called secretory immunoglobulin A(IgA) which is a new substance to the newborn. Before your baby was born, he received the benefit of another antibody, called IgG, through your placenta. IgG worked through the baby's circulatory system, but IgA protects the baby in the places most likely to come under attack from germs, namely the mucous membranes in the throat, lungs, and intestines.