Bone marrow is a spongy material that fills some of the bones and produces stem cells, which develop into the three different types of blood cells:

• red blood cells, which carry oxygen to all cells in the body
• white blood cells, which are essential for fighting infection
• platelets, which help the blood to clot and control bleeding.

All these cells normally stay inside the bone marrow until they are mature enough to perform their functions properly. They are then released into the bloodstream so that they can 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. As some myeloid cells and some lymphocytes only live for a few days, 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 every day.

When they are 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 is made up of a collection of small lymph nodes (sometimes called glands) which 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.