Which of the following describes the structure of IgA?

The structure of Immunoglobulin A (IgA) is characterized by its ability to exist in both monomeric and dimeric forms. In its monomeric form, IgA consists of a single unit, which is typical for serum IgA. However, the dimeric form of IgA, commonly found in secretions such as saliva, tears, and mucosal surfaces, features two monomer units linked together by a component known as the J chain. This dimeric structure is crucial for its role in mucosal immunity, allowing IgA to effectively function in protecting mucosal surfaces by neutralizing pathogens and preventing their adherence to epithelial cells.

The other structural options, such as trimer and polymer forms, do not accurately describe the structure of IgA as it is typically characterized. IgA does not commonly exist as a trimer or a polymer, making the monomer and dimer forms the defining characteristics of this antibody type.