Which immunoglobulin function is to combine with antigens on cellular surfaces?

The correct choice, IgA, primarily functions by combining with antigens on mucosal surfaces, which is crucial for the immune response in mucosal areas such as the respiratory and gastrointestinal tracts. IgA is found in high concentrations in secretions like saliva, tears, and mucous, playing an essential role in protecting body surfaces from invasion by pathogens.

While other immunoglobulins have significant roles in the immune system, IgA is uniquely positioned to interact with antigens at mucosal surfaces. For example, IgM is associated with the early immune response and prevalent in the bloodstream, typically forming pentamers and effectively agglutinating pathogens but not specifically targeted at cellular surfaces like IgA. IgD, on the other hand, is primarily found on B cells and functions mainly in the initiation of B-cell activation rather than directly binding to antigens on cell surfaces. IgE is mainly involved in allergic reactions and defense against parasitic infections, binding to allergens and mediating responses through mast cells and basophils but not specifically targeting cellular antigens as its primary function.

Understanding these specific roles helps clarify why IgA is the correct answer regarding the combination with antigens on cellular surfaces, especially in mucosal immunity.