The binding of radiolabelled 125I-CRP to human neutrophils has been characterised according to pH, temperature and time dependence. The binding of 125I-CRP was saturable, very fast (<2 min at 22°C), and the labelled protein was displaced by unlabelled CRP and aggregated human IgG. The dissociation constant was 3.2 × 10−8 M at pH 7.4, 22°C and 8.8 × 10−8 M at pH 6.0, 22°C. The calculated number of binding sites was 5–20 × 104 per cell at pH 7.4, 22°C. An association with an Fc-type receptor is suggested, since aggregated IgG was able to displace specifically CRP.
