Leukocytes, also known as white blood cells, are an important component of blood and a key player in the body's immune system. There are a number of different types of leukocytes, each with specific functions. An elevation in leukocytes in the blood can indicate the presence of infection or underlying disease, while leukopenia, in which these cells are reduced in number, can also be an indicator of a medical problem.

These cells can be broken into two major types: granulocytes and agranulocytes. Granulocytes have small granules of material inside their cell membranes, which play an important role in their function, as the cells can release the granules to kill bacteria, fungi, and other invaders. Agranulocytes lack granules in their cell membranes.

There are three types of granulocytes: eosinophils, neutrophils, and basophils. Eosinophils are designed to attack parasites, and they also play a role in allergic reactions. Neutrophils target bacteria and fungi, while basophils play a role in immune response. In someone with normal leukocyte levels, around 50-60% percent of the leukocytes are neutrophils, with one to four percent being esoinophils, and less than two percent being basophils.

Agranulocytes can be broken up into lymphocytes, which make up around 20-40% of the total leukocyte count, and include B cells, T cells, and Natural Killer cells, all part of the immune system, and monocytes. Monocytes make up two to nine percent of the white blood cell count, and they are designed to present antigens to lymphocytes to stimulate immune responses. These cells eventually mature into macrophages, specialized leukocytes which engulf foreign material to neutralize it.

Some leukocytes become fixed in place, such as mast cells, a type of granulocyte involved in immune response. All leukocytes arise from the same pluripotent stem cells generated in the bone marrow, with the body determining which leukocytes are needed and directing the stem cells to mature accordingly. These cells are also constantly being replaced, as many have a very short life span in the body.

When a patient goes to the doctor, the doctor may order blood tests as part of the patient's workup to determine what condition the patient has, and to get a general idea of the patient's health. Part of this bloodwork includes a count of the red and white blood cells, with specific attention to the numbers of different types of cells which might provide clues about a patient's condition; low levels of T cells, for example could indicate that a patient has HIV.