Three Classes Of Blood Cells
Human blood actually contains three different classes of cells: red blood cells for carrying oxygen, five types of white blood cells for fighting infection and platelet cells for wound clotting. As such, each type of cell has its own lifespan, from a few hours to several decades.
Red Blood Cells
Formally known as "erythrocytes," red blood cells contain a special iron-based protein called hemoglobin that is responsible for carrying oxygen molecules from the lungs to the rest of the body. These cells are produced at an emorous rate: two million per second. In general, red bloods last between 100 and 120 days.
White Blood Cells (Type 1: Neutrophils)
Circulating freely throughout the blood stream, neutrophils are often the first responders when tissue is damaged, contributing to the inflammation process. Their lifespan is short--between six hours and two days--but this could be an immune system defense mechanism; if viruses at the damaged area infect the neutrophil, the neutrophil will die before it can release new viruses.
White Blood Cells (Type 2: Eosinophil)
Eosinophils participate in the body's allergic response, helping identify foreign substances and trigger asthma-related symptoms. They circulate in the blood for eight to 12 hours before settling into tissue, where they live for between eight to 12 days.
White Blood Cells (Type 3: Basophil)
Basophils play a big role in the inflammatory response, especially for allergic reactions, because they contain proteins that promote blood flow (histamine) and prevent clotting (heparin). These cells last between four hours and five days.
White Blood Cells (Type 4: Lymphocyte)
Lymphoctyes fall into two major categories: large (e.g. natural killer cells) and small (e.g. Helper T cells, Cytotoxic T cells, and B cell). Natural killer cells, which destroy both tumors and cells infected with viruses, have a life span of up to two weeks. Helper T cells record information on newly-encountered antigens and release special chemicals to attract cytotoxic T cells when the antigen reappears. Each cell lasts only about 26 weeks, although the antigen memory persists as these cells divide and propagate. Cytotoxic or "killer" T cells destroy tumors and virus-infected cells by recognizing a particular protein signature on their surfaces. Killer T cells last anywhere from 1 year to life. Finally, B cells secret antibodies that stick to antigens, in effect "marking" them for killer cells to destroy. B cells have a widely-variable life span of 10 days to 15 months.
White Blood Cells (Type 5: Monocyte)
Monocytes are precursor cells for macrophages and dendritic cells. Stored in the spleen, monocytes enter the bloodstream at the sign of infection, arrive at the sight and turn into either of these two specialized defensive cells. Their lifespan (without changing) lasts several months.
Platelets are small, sliver-shaped cells that form blood clots, an essential mechanism for sealing broken blood vessels. The average lifespan of these cells is between nine and 12 days.