Colocalization between MCs and IgA-secreted plasma cell in inflamed mucosal tissue. (A-B) In the gut mucosa of patients with inflammatory bowel disease, MCs (A-B blue signal) accumulate at sites of inflammation where they colocalize with CD20-expressing B cells (A red signal) and IgA-secreting plasma cells (B, red signal). (C-D) In lymph nodes with reactive follicular hyperplasia, in which IgG class switch is prominent, the distribution of IgG-expressing plasma cells (C red signal) appears to be unrelated to that of MCs (C-D blue signal). By contrast, the few IgA-expressing plasma cells (D red signal) localize mainly in close proximity of MCs and even show signs of cell-cell contact (inset). (STREPT-ABC and APAAP methods; original magnifications: panels A-B, ×200; panels B-C, ×100; inset, ×400.)