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 is found in the middle of our bones, particularly in our pelvis, backbone (spine) and breast bone (sternum). 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.
All blood cells are made from stem cells (blood cells at their earliest stage of development). There are two types of blood stem cell:
- lymphoid stem cells, which make a type of white blood cell called lymphocytes
- myeloid stem cells, which make all other types of blood cell, including red blood cells, platelets and other types of white blood cells.
Blood stem cells in the bone marrow divide and grow to make new blood cells. The new, developing blood cells are called blast cells. They don’t look like fully developed cells and they can’t do the jobs that fully developed (mature) cells do. Usually, blast cells stay in the bone marrow until they have developed into red blood cells, platelets or white blood cells.
When the cells develop, they are released into your blood to carry out different functions:
- 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 your 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.
The levels of these cells in your blood are measured in a blood test called a full blood count (FBC).