Germinal center reaction during the T cell-dependent immune response is unaltered in DPP IV knock out animals. Spleens were extracted on day 9 from animals (n = 3) immunized with NP-CGG in alum. Spleens were weighed and divided into two parts. One part was used for FACS analysis and other for immunohistochemistry. PNA+ GC structures were counted from three random 10× fields/animals. The total numbers of GL-7+ B220+IgD- GC cells/spleen were enumerated as described in Materials and Methods. These data are representative of 2 independent experiments.