Antisera reactions and detection of antibody components query. - (Mar/24/2013 )

Our prof has set some questions based on standard antibody-antigen detections. A rabbit was immunised with mouse antibody components and the antisera generated was then screened against the mouse antibody components initially administered to the rabbit.

This is the answer table (top = antisera, rows below = mouse components (i.e. Ag)):


I understand all of them but the IgM/Fc fragment antigen - I thought it would be positive with kappa chain only (possibly IgG Fab antisera may react?). There's no gamma chain in IgM so that rules out gamma chain and IgG Fc. The J chain might be positive.

Any enlightenment? I'm SO confused.

Have a look at the structure of the whole IgM and only the Fc part of IgM. What is the difference? The difference is that in the whole IgM the monomers are connected to form a pentamer with a J chain. The testing is supposed to check for possible cross reactivity of the antibodies produced and in this table you have found that the antibodies against gamma chain are cross reacting with mu chain but only in the region where there is a link in the pentamer. Thus, the whole IgM does not react with gamma chain antiserum but the Fc part that is "depolymerized" does react.