Neutrophils

• Neutrophils are the most common form of leukocyte. They usually make up 40 to 75 percent of all leukocytes. A higher percentage of neutrophils often indicates an acute infection. After being produced in the bone marrow (where all five types of leukocytes form), neutrophils move into the extravascular tissue, where they survive for a few days as they wait for an infection to occur. Neutrophils fight bacteria by ingesting them, and then releasing enzymes that destroy them. Neutrophils are classified as granulocytes, because granules are present in their cytoplasm.
  
  

Eosinophils

• Eosinophils comprise 1 to 6 percent of the total leukocyte count. Like neutrophils, eosinophils migrate from the bone marrow to the extravascular tissue, where they wait for an infection to occur. Eosinophils can survive in this tissue for several weeks. These cells work in a similar manner to neutrophils; their digestive enzymes are particularly effective against parasitic worms. Eosinophils are also granulocytes.
  
  

Basophils

• Basophils are the least common of all leukocytes, making up less than 1 percent of the leukocyte count. Like eosinophils and neutrophils, basophils are granulocytes. They are easy to identify, because they have more granules in their cytoplasm than the other types of granulocytes. Basophils discharge these granules, which contain histamine and heparin, directly into damaged tissue. These cells promote blood flow and inflammation in damaged areas.
  
  

Monocytes

• Monocytes account for 2 to 10 percent of all leukocytes. They are agranulocytes, meaning that their cytoplasm lacks granules. These cells spend up to a day in the bloodstream before moving into tissues (generally the liver, lungs and lymph nodes). Once they arrive at their destination, they may stay there for years. Monocytes are phagocytic, meaning they digest bacteria or other harmful particles. They also produce antigens that are important to the immune response.
  
  

Lymphocytes

• Lymphocytes are agranulocytes that make up 20 to 45 percent of the total amount of leukocytes. These cells live for several years. B-cells and T-cells are the two main types of lymphocytes. The primary function of B-cells is to produce antibodies. T-cells are important for fighting tumor cells and virus-infected cells. A high number of T-cells can indicate the presence of the AIDS virus.