Rituximab targets which B-cell marker?

• A. CD19
• B. CD20
• C. CD3
• D. CD80

Answer: (B)

Rituximab binds to CD20 on B cells. It’s a monoclonal antibody that marks CD20-positive B cells for destruction, mainly through complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity, with some direct pro-apoptotic signaling as well. CD20 is expressed on most mature B cells (from pre-B to memory B cells) but not on stem cells or plasma cells, so depleting CD20-positive cells reduces circulating B cells and autoantibody production while allowing eventual B-cell reconstitution. Among the listed markers, CD20 is the one rituximab targets. CD19 is another B-cell marker but not the rituximab target in standard therapy; CD3 is a T-cell marker; CD80 is a co-stimulatory molecule on antigen-presenting cells and not the target here.