White blood cells, or leukocytes, are a part of the immune system and help our bodies fight infection. They circulate in the blood so that they can be transported to an area where an infection has developed. Here are the six main types of white blood cells and the average percentage of each type in the blood:

Neutrophils - 58 percent
Eosinophils - 2 percent
Basophils - 1 percent
Bands - 3 percent
Monocytes - 4 percent
Lymphocytes - 4 percent

Neutrophils are the one of the body’s main defenses against bacteria. They kill bacteria by actually ingesting them (this is called phagocytosis). Neutrophils can phagocytize five to 20 bacteria in their lifetime. Neutrophils have a multi-lobed, segmented or polymorphonuclear nucleus and so are also called PMNs, polys or segs.

Bands are immature neutrophils that are seen in the blood. When a bacterial infection is present, an increase of neutrophils and bands are seen.

Eosinophils kill parasites and have a role in allergic reactions.

Basophils are not well understood, but they function in allergic reactions. They release histamine (which causes blood vessels to leak and attracts white blood cells) and heparin (which prevents clotting in the infected area so that the white blood cells can reach the bacteria).

Monocytes enter the tissue, where they become larger and turn into macrophages. There they can phagocytize bacteria (up to 100 in their lifetime) throughout the body. These cells also destroy old, damaged and dead cells in the body. Macrophages are found in the liver, spleen, lungs, lymph nodes, skin and intestine. The system of macrophages scattered throughout the body is called the reticuloendothelial system. Monocytes stay in the blood for an average of 10 to 20 hours and then go into the tissues, where they become tissue macrophages and can live for months or even years.

Lymphocytes are complex cells that direct the body's immune system. T lymphocytes start in the bone marrow from pluripotent hematopoietic stem cells, then travel to and mature in the thymus gland. The thymus is located in the chest between the heart and sternum (breastbone). B lymphocytes mature in the bone marrow.

T lymphocytes (T cells) are responsible for cell-mediated immunity.

B lymphocytes are responsible for humoral immunity (antibody production).

75% of lymphocytes are T cells. Lymphocytes are different from the other white blood cells because they can recognize and have a memory of invading bacteria and viruses. Lymphocytes continually pass back and forth between lymph tissue, lymph fluid and blood. When they are present in the blood, they stay for several hours. Lymphocytes can live for weeks, months or years.

There are many types of T cells that have specific functions, including:

Helper T cells - Helper T cells direct the rest of the immune system by releasing cytokines. Cytokines stimulate B cells to form plasma cells, which form antibodies, stimulate the production of cytotoxic T cells and suppressor T cells and activate macrophages.

Cytotoxic T cells - Cytotoxic T cells release chemicals that break open and kill invading organisms.

Memory T cells - Memory T cells remain afterwards to help the immune system respond more quickly if the same organism is encountered again.

Suppressor T cells - Suppressor T cells suppress the immune response so that it does not get out of control and destroy normal cells once the immune response is no longer need.

B cells become plasma cells when exposed to an invading organism or when activated by helper T cells. B cells produce large numbers of antibodies.

Platelets (thrombocytes) help blood to clot by forming something called a platelet plug. The other way that blood clots is through coagulation factors. Platelets also help to promote other blood clotting mechanisms. There are approximately 150,000 to 400,000 platelets in each microliter of blood.

Platelets are formed in the bone marrow from very large cells called megakaryocytes, which break up into fragments. These cellular fragments are platelets. They do not have a nucleus and do not reproduce. Instead, megakaryocytes produce more platelets when necessary. Platelets generally last for an average of 10 days.

Platelets contain many chemicals that assist clotting. These include:

-Actin and myosin, to help them contract
-Chemicals that help the coagulation process to begin
-Chemicals that attract other platelets
-Chemicals that stimulate blood vessel repair
-Chemicals that stabilize a blood clot

A scanning electron microscope image of normal circulating human blood showing red blood cells, several types of white blood cells including lymphocytes, a monocyte, a neutrophil and many small disc-shaped platelets.

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