In this immunoradiometric assay (IRMA) of parathyrin (PTH) a polyclonal anti-amino-PTH(1-34) is the capture antibody and a radiolabeled monoclonal anti-hPTH(44-68) is the second antibody. Gel filtration of serum from a hyperparathyroid patient yielded only a single peak of PTH, corresponding to the elution position of synthetic PTH(1-84). Healthy elderly individuals (ages 78 +/- 5 y, mean +/- SD, n = 45) had PTH concentrations (21 +/- 13 ng/L) not significantly higher than those from healthy younger (38 +/- 11 y) adults (20 +/- 8 ng/L, n = 94). Assay results agreed well with those obtained with a carboxyl-terminal PTH assay both in normal subjects (r = 0.63, P less than 0.001) and in patients with primary hyperparathyroidism (r = 0.59, P less than 0.001). Both assays equally discriminated patients with surgically confirmed primary hyperparathyroidism from normal individuals, but the PTH(1-84) IRMA also allowed a nearly absolute discrimination between normal subjects and patients with primary hypoparathyroidism (undetectable serum PTH in 18 of 21 cases) and secondary hypoparathyroidism (caused by hypercalcemia that was caused by a malignant tumor, PTH 1.3 +/- 1.3 ng/L, n = 32). Moreover, the PTH(1-84) IRMA is more sensitive (detection limit in serum, 0.8 ng/L) and easier and quicker to perform than the carboxyl-terminal assay.

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Follicular dendritic cells (FDC) are located inside lymph follicles and are mainly characterized by their capacity to retain antigens. We investigated this aspect in mice lymph nodes by using bovine serum albumin (BSA) labelled with 5-nm colloidal gold particles and homologous anti-BSA antibodies bound to 20-nm gold particles. Gold-labelled BSA injected alone in non-immunized mice was only rarely found in FDC cytoplasmic interdigitations. Injected in the form of immune complexes, it was retained by FDC. Antigen-free anti-BSA antibodies injected under similar conditions as immune complexes were always found in draining lymph nodes in the same locations as BSA-anti-BSA immune complexes. F(ab')2 from mouse immunoglobulins linked to colloidal gold particles were very rarely found between the FDC extensions, whereas it was intensely phagocytosed by macrophages. Our study permitted precise ultrastructural localization between FDC cytoplasmic extensions or inside macrophages and other cells of the lymph nodes, but it also pointed out that homologous antibodies linked to colloidal gold particles might be retained by FDC in the absence of antigens. These observations, carried out with colloidal gold, were checked by using 125I-labelled anti-BSA antibodies. Complement activation determinations of gold-labelled antibodies or immune complexes showed that antibodies or immune complexes fixed on colloidal gold particles do not activate the complement. This observation enabled us to conclude that Fc receptors play a significant part in the retention of gold-labelled antibodies or immune complexes by FDC of lymph nodes.