Injection of soluble antigen extends GC longevity. A, Immunization schedule for the analysis of GC size. Mice received either 6 (300 μg/ml, 100μg/ml or 20 μg/ml), 2 (300 μg/ml), or no injections of indicated quantities of Ars-KLH in PBS. B, Distribution of GC sizes following immunization. Sections of spleens were stained with PNA to identify GC, which were measured (see experimental procedures), enumerated and plotted as % of total for each class size. C, Vanishing GC response on day 21 in mice without soluble antigen injection (0b). Arrows point to residual GCs. D, Prolonged GC reaction on day 21 in mice after 6 injections with 300 μg Ars-KLH (6b300). Spleen section shows large germinal centers (PNA = blue) with anti-idiotypic staining (mAbE4 = brown).