Blood is made up of liquid (called plasma) and solid cells. Blood cells are made in the bone marrow. Bone marrow is the soft, spongy substance in the centre of bones.
In adults, the most active bone marrow is found in the pelvic bone, shoulder bones, bones of the spine (vertebrae), ribs, breast bone and skull. Immature blood cells in the bone marrow are called stem cells. Stem cells can also be found in smaller amounts in the bloodstream. These are called peripheral blood stem cells.
All of our blood cells develop from stem cells. The process of blood cell development is called hematopoiesis. In the earliest stage of blood cell development, stem cells begin to develop either along the lymphoid cell line or the myeloid cell line. In both cell lines, the stem cells become blasts, which are still immature cells.
Lymphoid cell line
Lymphoid stem cells develop into lymphoblasts which develop into lymphocytes. Lymphocytes are a type of white blood cell. They are also called leukocytes. Lymphocytes help fight infection and destroy abnormal cells.
Myeloid cell line
Myeloid stem cells develop into red blood cells, platelets and some types of white blood cells. Red blood cells carry oxygen to all tissues of the body. Platelets form clots in damaged blood vessels to stop bleeding.
Myeloid stem cells develop into 2 different types of white blood cells, called granulocytes and monocytes. These white blood cells destroy bacteria and other foreign invaders and help to fight infection.
The main function of blood is to carry nutrients, gases, waste products, cells and hormones. Each type of blood cell has a specific job.
- Red blood cells carry oxygen from the lungs to the rest of the body. They also carry carbon dioxide from the body to the lungs so it can be breathed out.
- Platelets form blood clots in damaged vessels to stop bleeding.
- White blood cells help prevent and fight infection by destroying bacteria, viruses and other foreign cells or substances.
Types of white blood cells
There are many different types of white blood cells. Each type has a different job.
Lymphocytes make antibodies to fight infection. They are found in the lymph nodes, thymus, spleen, tonsils, adenoids and the bone marrow. They are also in lymphatic tissue in other parts of the body such as the appendix, small intestine and other parts of the digestive system and respiratory system.
The 3 main types of lymphocytes are:
- B cells make antibodies to fight bacteria, viruses and other foreign material such as fungi.
- T cells fight infection, destroy abnormal cells and control the immune response.
- Natural killer (NK) cells attack any foreign cells.
Granulocytes fight infection and become active in response to tissue inflammation. The 3 main types of granulocytes are:
- Neutrophils are the main cells that fight infection. They surround and eat, or ingest, bacteria.
- Eosinophils and basophils – both attack and destroy certain parasitic organisms and are activated during an allergic reaction.
Monocytes help fight infection by changing into cells called macrophages, which eat foreign invaders such as bacteria and waste from dying cells.
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