The Univeristy of Melbourne The Royal Melbourne Hopspital

A joint venture between The University of Melbourne and The Royal Melbourne Hospital


31 Aug 2020

Setting it Straight - Affinity, avidity, IgM, IgG, serological surveys and ‘antibody passports’

Setting it Straight - Issue #22

Affinity we’ve met before when talking about the specific binding of Ig receptors to their complementary (cognate) antigens, but avidity, what’s that about?

Perhaps you recall the unique entertainment invented in New Zealand pubs where, after a glass or two, patrons wearing Velcro ‘sticky’ suits would run and jump to ‘attach’ as high as possible up a Velcro-covered wall. Compare that spontaneous, slightly boozy, leap of faith to the elegant ascent of a stone-cold-sober, experienced mountaineer moving fast up a practice climbing wall.

As we all know from using, or misusing, Velcro fasteners, Velcro works by multiple, ‘low-affinity’ attachment points. On the other hand, the placement of a skilled climber’s hands and feet represents a precise, limited, ‘high-affinity’ interaction. Both achieve the same end point, or ‘avidity of binding’, that allows a heavy human being to hang off a vertical wall! The analogy in the immune response to SARS-CoV-2 is that the Velcro model represents the less-specific, ‘naïve’ IgM antibodies made first in any infection, while our climbers are the ‘educated’ IgGs and IgAs we’ve encountered previously.