The antibody in breast milk is mainly secretory IgA. Secretory IgA is a dimer, which mainly exists in secretions, such as colostrum, saliva, tears, sweat and respiratory, digestive and urinary secretions.

Secretory IgA consists of two IgA and a secretory fragment, which seems to protect the hinge region of IgA from degradation by protein hydrolase.

IgA can bind to the corresponding pathogenic microorganisms in the digestive tract mucosa, inhibit their adsorption to susceptible cells, and neutralize the exotoxin produced by the corresponding viruses or bacteria. Maternal women can transmit secretory IgA to their babies through colostrum, which plays an immune role in the digestive tract mucosa of babies. They can combine with a large number of soluble antigens in the diet, and can also combine with pyrogens released by normal intestinal flora or pathogenic microorganisms to prevent them from entering the blood. This is also an important natural passive immunity.