Which antibody class is secreted as a dimer into mucus, saliva, tears and breast milk?

1. IgA exists in two forms, a four-chain monomer in the blood and an eight-chain dimer in exocrine gland secretions. 2. Those secretions of the mucous membranes include mucus, saliva and tears. 3. Dimeric IgA is therefore the only antibody to leave the interior of the body to protect body surfaces. 4. IgA is also present in breast milk, or colostrum, which serves to protect the infant from disease. 5. Vaccines against diseases that enter through mucous membranes, such as influenza, are usually formulated to enhance IgA production. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 21 "The Lymphatic and Immune System", section 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies_