Which antibody is best at agglutination and complement fixation?

Which antibody is best at agglutination and complement fixation?

• A. IgA
• B. IgG
• C. IgD
• D. IgM

Answer: (D)

Agglutination and complement fixation depend on how well an antibody can cross-link antigens and recruit the complement system. The antibody best suited for both is IgM. Its pentameric structure gives it ten antigen-binding sites, making it exceptionally good at binding multiple epitopes and linking particles together to form visible clumps. That same multivalency also positions its Fc regions close enough to each other to efficiently recruit C1, triggering the classical complement pathway. In contrast, other isotypes like IgG can activate complement but are less effective for large-scale cross-linking because they are monomeric and have fewer binding sites, while IgA and IgD don’t activate complement as potently or are more specialized for other roles.

Agglutination and complement fixation depend on how well an antibody can cross-link antigens and recruit the complement system. The antibody best suited for both is IgM. Its pentameric structure gives it ten antigen-binding sites, making it exceptionally good at binding multiple epitopes and linking particles together to form visible clumps. That same multivalency also positions its Fc regions close enough to each other to efficiently recruit C1, triggering the classical complement pathway. In contrast, other isotypes like IgG can activate complement but are less effective for large-scale cross-linking because they are monomeric and have fewer binding sites, while IgA and IgD don’t activate complement as potently or are more specialized for other roles.