Techniques which identify hapten‐specific B cells in tissues have been used to determine the sites of B cell activation in rat spleens in response to T cell‐dependent (TD) antigens and T cell‐independent type‐1 (TI‐1) antigens. Surface‐associated hapten binding by specific memory B cells and. B blasts was distinguished from the strong cytoplasmic hapten binding by specific plasma cells and plasmablasts. Blast cells in S phase were identified in tissue sections by staining cells which had been pulse labeled in vivo with 5‐bromo‐2′‐deoxyuridine.

Hapten‐specific B blast cells are found in three sites: (a) around interdigitating cells in the T cell‐rich zones; (b) in the follicular dendritic cell network and (c) in association with macrophages in the red pulp. Hapten‐binding memory B cells, which are not in cell cycle, accumulate in the marginal zones and to a lesser extent the follicular mantles in response to TD and TI‐1 antigens. The hapten‐specific blast response in T zones is confined to the first few days after antigen is given and is low for primary responses to TD antigens, but massive on secondary challenge, when marginal zone memory B cells migrate to the T zones. Both the primary and secondary T zone responses to TI‐1 antigens are impressive and in these responses hapten‐specific B blasts are also found in the splenic red pulp.

The follicular response to TD antigens starts with a small number of B blasts (fewer than five) entering each follicle. These increase in number exponentially so that by the 4th day after immunization they fill the follicle. The oligoclonality of the response is shown in simultaneous responses to two haptens where 6%–31% of the follicles on day 3 after immunization contain blasts specific for only one of the two haptens. During the 4th day classical zonal pattern of germinal centers develops. The surface immunoglobulin‐positive B blasts are lost from the follicle center, while one pole of the follicular dendritic cell network fills with surface immunoglobulin‐negative centroblasts. Centroblasts do not increase in numbers but divide to give rise to centrocytes, which re‐express slg and migrate into the follicular dendritic cell network. Cell kinetic studies indicate that the centrocyte population is renewed from centroblasts every 7 h. Centrocytes either leave the germinal center within this time or die in situ. It is probable that the centroblasts and centrocytes are derived from the small number of B blasts which initiate the follicular reaction, for the centrocytes show the same oligoclonality observed at the B blast stage. The germinal center reaction declines gradually and 3 weeks after immunization centroblasts and centrocytes are no longer seen. At this time small clusters of B blasts can be found proliferating in the follicular dendritic cell network. These secondary B blasts characterize the third phase of the follicular reaction, which continues throughout the established phase of TD responses. Some follicular response is seen to TI‐1 antigens but this is much less dramatic than that seen during TD responses.