Blood cells are made in the bone marrow, the spongy tissue inside the bones. Stem cells are responsible for making all the blood cells in the marrow.
The process of blood cell formation is called hematopoiesis. When blood cells are fully developed, they enter the bloodstream, where they perform their functions of delivering oxygen, fighting infections or clotting injured blood vessels.
Blood is composed of plasma and cells suspended in plasma. The cells include red cells, platelets, neutrophils, eosinophils, basophils, monocytes and lymphocytes. Plasma cells produce specific antibodies that fight bacteria and viruses.
The red cells make up half the volume of the blood. Platelets are small cell fragments that help stop bleeding.
Neutrophils and monocytes are white blood cells. They are called phagocytes, and they ingest bacteria or fungi, helping to cure infection. Eosinophils and basophils are two additional types of white cells that are involved in allergic responses.
Lymphocytes, another type of white blood cell, are located in the lymph nodes and spleen, but some enter the blood. There are three types of lymphocytes: T cells, B cells and natural killer cells.
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