FIGURE 4.

Reactivity, avidity, and affinity of IgA₂ b12 for gp120. The relative reactivity, avidity, and affinity of different forms of purified IgA₂ b12 were compared with IgG1 b12. A, ELISA analysis examining the reactivity IgA₂ and IgG1 b12 with gp120_MN. Anti-human κ chain Abs were used as secondary reagents to allow direct comparison of reactivity of different b12 isotypes with gp120. B, Functional avidity of IgA₂ and IgG1 b12 for gp120_MN as determined by ammonium thiocyanate dissociation (see Materials and Methods). The AI (inset) is defined as the amount of ammonium thiocyanate (M) required to elute 50% of the Ab from gp120. C, The relative affinities of IgA₂ and IgG1 b12 for gp120_MN were determined indirectly by competitive inhibition. Abs were mixed with increasing concentrations of soluble gp120_MN before being applied to ELISA plates coated with the same Ag. The amount of soluble gp120 required to reduce Ab binding by 50% was defined as the competitive inhibition index (inset, left). The approximate K_D (inset, right) was determined as described in Materials and Methods.