Bone marrow is the spongy tissue inside our bones that produces blood cells — red blood cells, platelets, and white blood cells.

White blood cells help the body fight against infection. There are many different types of white blood cells, including lymphocytes, neutrophils, and monocytes. They fight against invading bacteria, viruses or fungi to help destroy infection. 

Three major types of lymphocytes play an important role in the immune system.

B-cell
B-lymphocytes (B-cells) originate in the bone marrow. They make proteins called antibodies, which attach onto the surface of infection-causing microbes. Generally, these are Y or T shaped. Each type of antibody reacts to different microbes by sticking to molecules, called antigens, which sit on the surface of the microbe. It is this antibody-antigen binding that triggers B-cells to grow and produce more antibodies, which fight infection.

T-cell
T-lymphocytes (T-cells) mature in the thymus, which is a small organ in the upper chest, just behind the sternum (breastbone). T-cells help B-cells make antibodies against invading bacteria, viruses or other microbes. Unlike B-cells, T-cells can produce chemicals that kill infected cells after binding to the antigen on the surface of the infected cell. T cells regulatory cells suppress immune responses of other cells. This is an important “self-check” built into the immune system to prevent excessive reactions.

natural-killer-cell
Natural killer (NK) cells are a type of lymphocyte that directly attacks cells which have been infected by a virus.