Charcterization and measurement of normal platelet-associated immunoglobulin G by fluorospectrophotometry

Platelet-associated immunoglobulin G of normal healthy subjects was measured and its binding characteristics studied by use of a newly developed assay which is rapid, quantitative, sensitive and inexpensive. The assay is based on fluorospectrophotometry. Fluorescein isothiocyanate was used as a fluorescent marker to label IgG. Measurement of platelet-associated IgG by this method showed that normal platelets have at least two types of binding sites for IgG of normal healthy subjects. High- and low-affinity binding sites numbering 410 +/- 200 and 1800 +/- 500, respectively, were identified. Specificity of binding was shown by competition between fluorescein isothiocyanate-labeled IgG and nonlabeled IgG. The effect of pH and temperature and the kinetics of binding were also investigated.