Q.Write notes on B-cells.

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.

Mononuclear phagocytes are white blood cells with a single, un-lobed nucleus that defend the body by phagocytosis — engulfing and digesting pathogens and cellular debris. This group includes circulating monocytes, the macrophages that monocytes mature into once they migrate into tissues, and specialised, tissue-resident macrophages such as Kupffer cells in the liver and microglia in the brain — together forming a widely distributed surveillance-and-clean-up network throughout the body.

The complement system is a group of roughly twenty to thirty proteins that normally circulate in blood plasma in an inactive form. When triggered by the presence of a pathogen (or by an antibody already bound to one), these proteins activate each other in a cascade. Some of the activated complement proteins assemble into a membrane attack complex (MAC), which punches pores directly into the pathogen's membrane and causes it to burst (lyse); others coat the pathogen's surface, making it a better target for phagocytosis (opsonisation), or help trigger local inflammation.