Which immunoglobulin is a dimer?

Immunoglobulin A (IgA) is the correct answer as it is characterized as a dimer in its secretory form. IgA is primarily found in mucosal areas, such as the gut, respiratory tract, and urogenital tract, and is also present in secretions like saliva, tears, and breast milk. In these secretory forms, IgA consists of two antibody monomers that are joined by a J chain, which gives it the dimeric structure.

The dimeric form of IgA is particularly effective in mucosal immunity because it can efficiently neutralize pathogens and prevent their entry through mucosal surfaces. The presence of secretory component also protects the dimeric IgA from degradation by enzymes in secretions, allowing it to function optimally in its protective role.

In contrast, the other immunoglobulins mentioned are typically found in different forms. IgG, for example, is primarily a monomer and accounts for the majority of antibodies in serum, while IgM is a pentamer in its secreted form, consisting of five monomer units joined together. IgE is typically a monomer as well and plays a role in allergic reactions and defense against parasitic infections. Understanding these structural variations is