The bone marrow
Bone marrow is a spongy material that fills some of our bones and produces stem cells.
Stem cells are cells at a very early stage of development, and all the blood cells in the body develop from stem cells. In the bone marrow, the stem cells develop into three different types of blood cells:
red blood cells - carry oxygen to all cells in the body
white blood cells - essential for fighting infection
platelets - help the blood to clot and control bleeding.
All these cells normally stay inside the bone marrow until they are mature and able to function properly. They are then released into the bloodstream, where they move around the body in the blood.
White blood cells
There are two main types of white blood cell: lymphocytes and myeloid cells (also known as granulocytes). These cells work together to fight infection. Some myeloid cells and lymphocytes only live for a few days, so the bone marrow is constantly making new cells to replace the old ones in the blood.
The bone marrow normally makes millions of blood cells everyday. When they’re mature enough to leave the bone marrow, the white blood cells are released into the bloodstream to circulate around the body. Lymphocytes, unlike the myeloid cells, also circulate in the lymphatic system.
The lymphatic system is one of the body’s natural defences against infection. It’s a complex system made up of lymphatic organs such as bone marrow, the tonsils, the spleen and a collection of small lymph nodes (sometimes called glands).
The lymph nodes produce lymph, a milky-looking fluid containing lymphocytes. The lymph nodes are found mainly in the neck, armpit and groin, and are connected by a network of tiny lymphatic vessels.
The tonsils, liver, spleen (which breaks down old blood cells) and bone marrow also contain lymphocytes. There are two main types of lymphocyte: B-lymphocytes and T-lymphocytes.