White blood cells are an important component of your blood system, which is also made up of red blood cells, platelets, and plasma.
Although your white blood cells account for only about 1 percent of your blood, their impact is significant. White blood cells, also called leukocytes, are essential for good health and protection against illness and disease.
Think of white blood cells as your immunity cells. In a sense, they are continually at war. They flow through your bloodstream to battle viruses, bacteria, and other foreign invaders that threaten your health. When your body is in distress and a particular area is under attack, white blood cells rush in to help destroy the harmful substance and prevent illness.
White blood cells are produced inside the bone marrow and stored in your blood and lymphatic tissues. Because some white blood cells have a short lifespan of one to three days, your bone marrow is constantly producing them.
Among your white blood cells are:
Monocytes. They have a longer lifespan than many white blood cells and help to break down bacteria.
Lymphocytes. They create antibodies to defend against bacteria, viruses, and other potentially harmful invaders.
Neutrophils. They kill and digest bacteria and fungi. They are the most numerous type of white blood cell and your first line of defense when infection strikes.
Basophils. These small cells appear to sound an alarm when infectious agents invade your blood. They secrete chemicals such as histamine, a marker of allergic disease, that help control the body's immune response.
Eosinophils. They attack and kill parasites, destroy cancer cells, and help with allergic responses.
Your white blood cell count can be low for a number of reasons—when something is destroying the cells more quickly than the body can replenish them or when the bone marrow stops making enough white blood cells to keep you healthy. When your white blood cell count is low, you are extremely susceptible to any illness or infection, which can spiral into a serious health threat.
Your health care provider can see whether your white blood cell count is normal through a blood test known as the complete blood count. If your count is too low or too high, you may have a white blood cell disorder.
A number of diseases and conditions may influence white blood cell levels:
Weakened immune system. This is often caused by illnesses such as HIV/AIDS or by treatments related to cancer. Cancer treatments such as chemotherapy or radiation therapy can destroy white blood cells and leave you vulnerable to infection.
Infection. A higher-than-normal white blood cell count usually indicates some type of infection—white blood cells are multiplying to destroy an enemy, such as bacteria or a virus.
Myelodysplastic syndrome. This condition causes abnormal bone marrow cell production.
Cancer of the blood. Cancers including leukemia and lymphoma can cause uncontrolled growth of an abnormal type of blood cell in the bone marrow, resulting in a greatly increased risk for infection and or serious bleeding.
Myeloproliferative disorder. This disorder refers to various conditions that trigger the excessive production of immature blood cells. This can result in an unhealthy balance of all types of blood cells in the bone marrow and too many or too few white blood cells in the blood.
Other conditions, such as extreme physical stress caused by an injury or emotional stress, can trigger high white blood cell levels, as can inflammation, labor or the end of pregnancy, smoking, or even extreme exercise.