Immunoglobulin A (IgA) is the main immunoglobulin in mucous secretions, including tears, saliva, and colostrum as well as respiratory, intestinal, prostatic, and vaginal secretions. It is also found in small amounts in blood. Because it is resistant to degradation by enzymes, secretory IgA provides protection against microbes proliferating in body secretions, especially those of the digestive and respiratory tracts.