Neutralization by Immunoglobulin M

Abstract

The pentameric nature of IgM ensures high avidity (about 10⁷-fold greater than Fab) for structures bearing repeated identical antigenic determinants, but in general IgM is of low specificity since its variable region genes are not subject to hypermutation modification that occurs in cells synthesizing IgG or IgA and increases the affinity of these immunoglobulins. Even if this were not so, there are no memory cells in which mutations favouring increased affinity could be preserved. IgM has long been known to be neutralizing either alone or in the presence of complement (Sect. 15). Complexes of IgM bound to picornaviruses have been visualized by electron microscopy (Svehag and Bloth 1967; Brown and Smale 1970; Almeida and Waterson 1969). When attached by a majority of its binding sites IgM adopts the’ staple’ or ‘crab’ conformation but when only a minority of binding sites are engaged it has a more upright stance (Feinstein et al. 1971, 1986; Armstrong et al. 1990). The upright conformation of IgM does not activate complement.