B cells

B cells are lymphocytes responsible for the humoral arm of the adaptive immune response. They develop in the bone marrow and, upon maturation, circulate in blood and lymph. When activated by antigen (often with T cell help), they proliferate and differentiate into antibody-secreting plasma cells or long-lived memory B cells. Their antibodies neutralize pathogens, opsonize them for phagocytosis, and activate the complement system.