Humoral immunity involves antibodies. B cells are lymphocyte precursors of antibody-producing plasma cells. In humoral immunity, B cells with receptors (BCR) specific for an antigen bind to the antigen and divide. Dividing B cells produce plasma cells and memory cells. Plasma cells produce antibodies, then die by apoptosis when the infection has cleared.
Memory cells remain viable for the lifetime of the organism. They are activated whenever the same infection is again encountered, rapidly differentiating into plasma calls that produce the appropriate antibody.
The first encounter with antigen elicits a slowly developing primary response. The rapid, secondary response is due to proliferation of existing memory cells (active immunity). Passive immunity is due to antibodies provided by the mother (natural) or injected (artificial). Vaccination is an immune response primed by a relatively harmless form of the antigen.
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