B cells in rheumatoid synovium. Tissue biopsies were collected from patients with active synovitis. Infiltrates of B cells and T cells are often arranged in complex microarchitectures. Three patterns can be distinguished. (a) T cells and B cells can form typical germinal centers. (b) Alternatively, they can be clustered in aggregates that lack follicular dendritic cells and have no germinal center reaction. (c) The third pattern is formed by T cells and B cells infiltrating diffusely into the synovial stroma. Reproduced with permission .