Practice question

MS4A1 gene on chromosome 11q12.2 encodes CD20 differentiation antigen, 33 to 37 kDa tetra-spanning membrane phosphoprotein with both N and C termini intracellular, four transmembrane helices forming tetrameric oligomer functioning as store-operated calcium channel component modulating B cell receptor induced calcium flux via interaction with calcium release-activated calcium modulator ORAI1. Expression tightly regulated during ontogeny: absent on hematopoietic stem cells and early pro-B, appears at late pro-B stage after productive immunoglobulin heavy chain rearrangement, persists through mature naive, germinal center, memory B cells, but extinguished upon plasma cell differentiation when transcription factors BLIMP1 and XBP1 repress MS4A1 and induce secretory program. Lack on stem cells ensures regenerative capacity after depletion; absence on long-lived plasma cells preserves humoral memory maintaining protective antibody titers against prior vaccines. Rituximab binding extracellular loops between transmembrane domains 3-4 induces translocation to lipid rafts enriched in cholesterol and sphingolipid, enhancing Lyn mediated PLCγ2 activation, calcium mobilization, and Bax mediated mitochondrial apoptosis. Concurrent opsonization promotes phagocytosis by macrophages via FcγRI and complement. Reconstitution from pro-B precursors occurs within six to twelve months, infection risk transient compared to continuous pan-B ablation.