Direct binding of pGCR to mannose receptors on rat alveolar macrophages. (A) Increasing concentrations of¹²⁵I-labeled pGCR were incubated with rat alveolar macrophages at 4°C in the presence (▪) or absence (⧫) of yeast mannan as described in Materials and Methods. Based on these data, specific binding (▴) of pGCR to mannose receptors was calculated. (B) Scatchard analysis of the specific binding data provided a straight line indicative of a single class of binding sites. In this experiment, Scatchard analysis indicated the K_d for the binding of pGCR to the mannose receptor was 14 nmol/L and the number of binding sites per cell was 2.5 × 10⁴.