Blood is made up of different types of blood cells, which float in a liquid called plasma. Blood cells are made in the bone marrow. This is a spongy material that’s found in the middle of our bones, particularly in our pelvis and backbone (spine). Normally, millions of new blood cells are made every day in the bone marrow. These new blood cells replace old and worn-out blood cells in the body.
All our blood cells are made from stem cells (blood cells at their earliest stage of development). Stem cells develop into two types:
- lymphoid stem cells, which make white blood cells called lymphocytes
- myeloid stem cells, which make other blood cells, such as red blood cells, platelets and other types of white blood cells.
In the bone marrow, stem cells divide and grow to form fully developed (mature) red blood cells, platelets and white blood cells.
To begin with, new blood cells are immature. They don’t look like red blood cells, platelets or white blood cells, and they can’t yet do the jobs they’re supposed to do. These immature cells are called blast cells (or blasts). Usually, blast cells stay in the bone marrow until they have matured into red blood cells, platelets or white blood cells. These are then released into the blood to carry out the following jobs:
- Red blood cells contain haemoglobin (Hb), which carries oxygen from your lungs to all the cells in your body.
- Platelets are very small cells that help blood to clot, and prevent bleeding and bruising.
- White blood cells fight and prevent infection. There are several types of white blood cell. The two most important types are neutrophils and lymphocytes.