What best describes the size characteristics of IgM?

IgM is best described as large and confined to blood due to its unique monomeric and pentameric forms. The pentameric form, which consists of five monomers joined by a J-chain, makes IgM the most substantial immunoglobulin in terms of size. This large structure contributes to its effectiveness in forming complexes with antigens, leading to agglutination and increased avidity for antigens.

The confined presence of IgM primarily within the bloodstream is attributable to its size, which hinders its passage through endothelial barriers of tissues. This contrasts with other immunoglobulin classes, like IgG, which are smaller and can easily diffuse into tissues and across cellular membranes. Thus, IgM plays a crucial role in the initial stages of immune responses, especially in the circulating blood, where it can effectively interact with pathogens.