Which immunoglobulin can activate the complement system?

IgM is the immunoglobulin that can activate the complement system. This is due to its structure; IgM is typically found as a pentamer, meaning it consists of five monomer units linked together. This pentameric form provides multiple binding sites, allowing it to effectively bind to antigens and initiate the classical pathway of complement activation.

When IgM binds to an antigen, it undergoes a conformational change that facilitates the recruitment and activation of complement proteins. This process is essential for opsonization, inflammation, and lysis of pathogens. The ability of IgM to activate the complement system is crucial for its role in the immune response, particularly during the early stages of infection.

The other immunoglobulins listed do have important roles in the immune response but do not significantly activate the complement system in the way that IgM does. IgD is primarily involved in B cell activation, IgA is important for mucosal immunity, and IgE plays a significant role in allergic reactions and defense against parasitic infections. However, their structures and functions do not include the efficient activation of the complement pathway like IgM.