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What Are White Blood Cells?

White blood cells are an important component of your blood, which is also made up of red blood cells, platelets, and plasma.

Illustration of blood components

Although white blood cells make up only about 1 percent of 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.

Illustration of bone marrow

Types of white blood cells

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 (granulocytes). They kill and digest bacteria. They are the most common 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 and other invaders too large to be swallowed up by a single blood cell.

Problems affecting white blood cells

Normally, you produce 100 billion white blood cells a day. 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 doctor 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 AIDS/HIV 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 insufficient production of white blood cells.

  • Cancer of the blood. Cancers including leukemia and lymphoma can stop or slow white blood cell production in the bone marrow, resulting in a greatly increased risk for infection.

  • 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.