B cell
The immune cells that make antibodies. They matter in cancer twice over: as the source of every therapeutic antibody, and as the cells that go wrong in most lymphomas, leukaemias and myeloma.
Each B cell makes one antibody; when it meets its antigen it multiplies and matures into plasma cells that pump out that antibody in bulk, which is the response the hybridoma technique captured to make monoclonal antibodies. B cells carry surface markers (CD19, CD20, CD22) throughout their lives and plasma cells carry BCMA, and these markers make B-cell cancers (most lymphomas, CLL, ALL, myeloma) the most successful targets of rituximab, CAR-T and bispecific antibodies; since normal B cells are also wiped out, patients need antibody replacement and are prone to infection. B cells inside solid tumours, organised into structures resembling lymph nodes, are associated with better responses to immunotherapy.