White blood cells—also called lymphocytes—are a type of blood cell that's made in the bone marrow and found in the blood and lymph node. White blood cells are part of the body’s immune system. They help the body fight infection and other diseases. B cells are a type of white blood cell. Checking the number of white blood cells in the blood is used to look for many kinds of conditions, including lymphoma. This blood test can be performed by your doctor.