IgM, unlike other heavy chain isotypes, appears in the serum asa pentamer of the basic four-chain Ig monomer; thus making itdecavalent rather than bivalent in terms of combining sites. Oneintriguing result of this is an increase in the likelihood - allother things being equal - that the IgM will be bound (versusfree). Another way of saying this is that even though the affinityconstant (Ka) of each individual combining site is the sameregardless of the overall molecule's valency, this decavalentversion will have more whole Ig molecules bound (to antigen) atequilibrium than would a bivalent version (given equimolar levelsof combining sites). This phenomenon is sometimes called increased\"avidity\" (to avoid using the rigorously defined chemical term\"affinity\" incorrectly). I'd like you to discuss a few issuesraised by this:
- First, bearing in mind that IgM is the first isotype madeduring primary responses,why does this higher \"avidity\" makeintuitive sense?
- Second, can you foresee ways that this added \"stickiness\" mightalso engender problems?
- Finally, does this mean that IgM molecules are more or less\"specific\" than their IgG counterparts? (watch it, that last one istricky)
