What is pannus?
The synovium is the primary site for the inflammatory process in RA. The inflammatory infiltrate consists of mononuclear cells, primarily CD4+ T lymphocytes (30%–50% of cells), as well as activated macrophages, B cells (5% of cells), plasma cells (some making RF and ACPA), and dendritic cells that can lead to an organizational structure that resembles a lymph node. Notably unlike the synovial fluid, few, if any, polymorphonuclear leukocytes (PMNs) are found in the synovium. The inflammatory cytokine milieu causes the synovial lining cells (macrophage-like and fibroblast-like synoviocytes) to proliferate. The inflamed synovium becomes thickened, boggy, and edematous and develops villous projections. This proliferative synovium is called pannus and it is capable of invading bone and cartilage, causing destruction of the joint. One of the most important cells in the pannus contributing to cartilage destruction is the fibroblast-like synoviocyte, which has tumor-like characteristics capable of tissue invasion.