Conformation of Immunoglobulin M. III. Structural requirements of antigen for complement fixation by equine IgM.

This research explores the structural requirements of antigens for complement fixation by equine Immunoglobulin M (IgM). The study concludes that large antigen size is necessary for IgM to bind in a multivalent manner, thereby providing the correct conformation for complement fixation.

Experimental Process

The study involved testing complexes of IgM equine anti-dansyl antibodies and varying dansyl substituted carriers for their capability to fix complement (a protein-chain responsible for immune response). Unique carrier types such as:

• dansyl92-Ficoll
• dansyl12-poly-L-lysine

showed effective results. However, carriers such as:

• dansyl13-bovine serum albumin
• dansyl127-keyhole limpet hemocyanin
• reduced and alkylated dansyl10-ribonuclease

proved ineffective. Interestingly, the lack of complement fixation by the dansyl-ribonuclease was not due to a lack of antibody-antigen complex formation.

Key Observations

The research further observed that polymerized dansyl-ribonuclease (a complex with a molecular weight range from 74,000 to 230,000) was extremely effective in inducing complement fixation. This finding indicated that a large-size antigen is necessary for IgM to bind in a multifaceted way to provide the appropriate conformation for complement fixation.

Comparative Analysis

The findings from this study correlated with conclusions drawn from earlier studies on rabbit IgM by Cunniff and Stollar. Optimal complement fixation occurred at lower antigen concentrations than maximal precipitation, implying that complexes where several combining sites within a given IgM molecule might be bound to the same antigenic surface may be the most effective.

Insights from Additional Observations

The team noted that the amount of C1q (a component of the complement system) bound to the antibody remained the same in the presence and absence of antigen. This implies that enhanced complement fixation by antibody-antigen complexes is likely due to additional complement component interactions such as C1r or C1s.