Q.Name any four mononuclear phagocytes.

B-lymphocytes (B-cells) are the white blood cells responsible for producing antibodies. They originate and develop in the bone marrow (in birds, this maturation happens in a specialised organ called the Bursa of Fabricius, which is in fact where the letter “B” in B-cell comes from historically). A B-cell is called mature once it has finished this developmental process and displays the antigen receptors it needs on its surface — but a mature B-cell is not yet doing anything. It only becomes functional, or immuno-competent, once it has actually encountered its specific antigen (usually with the cooperation of a helper T-cell) and been activated; only then can it proliferate and differentiate into antibody-secreting plasma cells and long-lived memory cells. In short: maturity is about completing development, while being functional is about being activated and able to respond. …