CHARACTERIZATION OF THE IGG-FC RECEPTOR ON HUMAN-PLATELETS

  • 1 January 1982
    • journal article
    • research article
    • Vol. 60  (6) , 1277-1282
Abstract
To determine quantitatively the number and avidity of receptors for the Fc portion of IgG on human platelets, the binding to platelets of human monomeric monoclonal IgG1 and of small covalently crosslinked polymers of IgG1 labeled with 125I was studied. The binding of labeled IgG1 monomers to platelets is too weak to permit quantitation. The binding of dimers or larger polymers of IgG1 is much more avid (greater at 4.degree. C than 37.degree. C), is readily reversible and is saturable. The number of receptor sites ranges from 400-2000/platelet and the mean equilibrium association constant (Ka) for the binding of dimers at 4.degree. C is 2.2 .times. 107 M-1 .+-. 0.9 .times. 107 M-1. The binding is specific for the Fc portion of IgG, and IgG1 and IgG3 bind to the receptors much more avidly than IgG2 or IgG4. Unlabeled IgG1 dimers are about 7-8-fold more potent in inhibiting binding than are IgG1 monomers, and larger polymers are even more potent than dimers. The Fc receptors on platelets bind human IgG1 with the same specificity and similar avidity as Fc receptors on polymorphonuclear leukocytes (PMN), but PMN have about 300-fold more receptors per unit of surface area than platelets.

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