Colloidal gold, a useful marker for antigen localization on follicular dendritic cells.

Bovine serum albumin (BSA) bound to colloid gold particles (BSA-gold; 20 nm diameter) and injected into preimmunized mice was found at ultrastructural level in different locations of the lymph nodes. It was detected particularly in the secondary lysosomes of macrophages and between the cytoplasmic processes of the follicular dendritic cells. Between these processes the gold particles were isolated or grouped in clusters; they were in close contact with cell membranes or embedded in dense material. Colloidal gold injected alone was not retained on these cells. The presence of anti-BSA antibodies in the serum was necessary for trapping of BSA-gold particles on follicular dendritic cells. Injections of BSA alone after BSA-gold had been administered to preimmunized mice eliminated most of the BSA-gold from the dendritic processes. BSA-gold is thus trapped in the form of immune complexes which behave characteristically. BSA-gold is thus a suitable marker for antigen localization. Being small and electron dense it permits more precise location than radioactive markers.