A type of blood cell that is made in the bone marrow and found in the blood and lymph tissue. WBCs are part of the body’s immune system. They help the body fight infection and other diseases. Types of WBCs are granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes (T cells and B cells). Checking the number of WBCs in the blood is usually part of a complete blood cell (CBC) test. It may be used to look for conditions such as infection, inflammation, allergies, and leukemia. Also called leukocyte and white blood cell.