The B cell hypothesis of rheumatoid arthritis pathogenesis - Part Two. (A) A cellular infiltrate containing B and T lymphocytes, plasma cells, and macrophages forms within the subchondral bone. Activation of osteoclasts occurs with resorption of bony trabeculae, leading to bone erosion. (B) Foci of osteitis (circle) on histology of bone from a patient with rheumatoid arthritis (stain: hematoxylin and eosin; magnification: ×400). (C) post-contrast T1-weighted coronal magnetic resonance imaging scan of the wrist of a 52-year-old woman with 1 year of rheumatoid arthritis. Osteitis appears as bone marrow edema involving the lunate (circle) and other carpal bones (arrows point to hamate and 2nd metacarpal base).