Which antibody is primarily found in tears, saliva, breast milk, and mucous membranes?

The antibody primarily found in tears, saliva, breast milk, and mucous membranes is IgA. This immunoglobulin plays a crucial role in mucosal immunity, helping to protect mucosal surfaces from pathogens.

In the case of tears and saliva, IgA is secreted in a dimeric form, allowing it to be more effective in binding to and neutralizing a variety of pathogens, including bacteria and viruses. This dimeric structure provides enhanced resistance to degradation by enzymes present in these secretions. In breast milk, IgA serves to protect the infant's gastrointestinal tract while providing passive immunity.

IgA is distinct from other immunoglobulin classes, such as IgG, which is the most abundant antibody in serum and primarily involved in systemic immune responses, and IgE, which is associated with allergic responses and parasitic infections. IgM, while important for early immune responses, is primarily found in circulation and not specifically associated with mucosal surfaces. Thus, IgA's unique localization in secretions and mucosal tissues underscores its role in the first line of defense against infections.