An effector cell of the immune system that secretes antibodies specific for a particular target antigen, such as one carried by an invading microorganism. Binding of the antibodies neutralizes the antigens (e.g. virus particles) or enhances the actions of other effector cells, such as phagocytosis by macrophages or neutrophils, in destroying the target. The plasma cells are produced by B cells that become activated in response to encountering their specific antigen, in a process that requires participation by a helper T cell. A single progenitor B cell can produce a clone consisting of thousands of plasma cells, each capable of secreting a trillion or so antibodies during a 4–5-day life.