Figure 3.

Equilibrium binding of Pab-S to the BCL₁ cell-surface idiotype. (A) BCL₁ cells were incubated with trace amounts of ¹²⁵I-labeled Pab-S and increasing concentrations of Pab-S (▪), S-peptide (⋄), mAb B1 IgG (○), or Pab-F as control (□). After 2.5 h at 4°C cell bound ¹²⁵I was counted (mean ± SD of triplicates). Broken lines are guidelines for the eyes. Solid lines are calculated from the fitted parameters (see text). (B) Scatchard plot of a representative equilibrium binding experiment. BCL₁ cells were incubated with trace amounts of ¹²⁵I-labeled Pab-S (▪), mAb B1 Fab′ (Δ), or IgG (○) and increasing concentrations of the same unlabeled proteins. Parameters (K_d, binding sites per cell, NSB) were fitted according to a linear model. K_d values are the mean of three independent experiments ± SD. The number of apparent binding sites per cell were 1.17, 0.54, and 0.38 × 10⁶ for mAb B1 Fab, IgG, and Pab-S, respectively.