Types and Function of White Blood Cells (WBCs)

White blood cells (WBCs) are a part of your immune system that fight infection and defend the body against other foreign materials.

There are five types of white blood cells:

• Neutrophils
• Eosinophils
• Basophils
• Lymphocytes
• Monophils
  

Each type of white blood cell has a specific job. Some spot intruders and kill harmful bacteria. Others make antibodies to protect your body from bacteria and viruses.

This article discusses the different types of white blood cells and their functions, as well as conditions that can affect your white blood cell count. 

Types of WBC

White blood cells are also known as leukocytes. They are the body's defense against infections. There are several different types with different purposes.

Some white blood cells are part of our innate immune system, meaning they know how to attack foreigners from the moment we’re born. Other white blood cells are part of our humoral or learned immune system. Humoral immune cells make antibodies after exposure to a germ to help prepare the body for a future attack. 

Neutrophils

Neutrophils make up about half of the white blood cells in the body. They are usually the first immune system cells to respond to invaders such as bacteria or viruses.

As first responders, they also send out signals alerting other cells in the immune system to come help. Neutrophils are the main cells found in pus, the white stuff you may see inside a wound or pimple. 

Once released from the bone marrow, the cells only live for about eight hours. Your body makes roughly 100 billion of these cells every day.

Eosinophils

Eosinophils also play a role in fighting off bacteria, as well as parasitic infections (such as worms).

Eosinophils are best known for their role in triggering allergy symptoms. These white blood cells can go overboard in mounting an immune response against something that’s actually harmless. For example, eosinophils can mistake pollen for a foreign invader and give you seasonal allergy symptoms. Eosinophilia symptoms can also include fever, night sweats, fatigue, and weight loss.

Eosinophils only make up about 5% of the white blood cells in your bloodstream. However, the digestive tract actually has high concentrations of eosinophils.

Basophils

Basophils only make up about 1% of white blood cells. These cells are very involved in asthma and are part of your body’s basic immune response to organisms that can make you sick. 

When stimulated, basophils release histamine and other chemicals, which cause inflammation in the body and narrowing of the airways.

Lymphocytes (B and T)

Lymphocytes come in two forms: B cells and T cells. Unlike other white blood cells that provide general (non-specific) immunity, B and T cells have specific roles.

B lymphocytes (B cells) are responsible for humoral immunity, which is the immune response with antibodies. B cells make antibodies that "remember" an infection. These antibodies are always ready in case your body is exposed to a specific pathogen again.

T cells recognize specific foreign invaders and kill them. "Memory" T cells also remember an invader after an infection and respond quickly if it comes back.

Lymphocytes are part of what makes vaccines effective. When you get a vaccine, your body will make B and T cells that will be able to recognize the pathogen the vaccine protects you against as well as be ready to fight it off should you be exposed.

Monocytes

Monocytes make up about 5% to 12% of white blood cells in your bloodstream. Their most important job is to clean up dead cells in the body—they’re like the “garbage trucks” of your immune system.

How WBCs Are Formed

White blood cells start out in the bone marrow in a process called hematopoiesis. All blood cells come from a common hematopoietic stem cell (HSC). This is also called a "pluripotent" stem cell. These stem cells specialize—or differentiate—in different stages. The HSC cell first separates into either a lymphoid or myeloid stem cell.

The lymphoid stem cell leads to the lymphoid cell line, which is the family of cells that produces B cells and T cells.

Myeloid stem cells lead to myeloblasts, which evolve into macrophages, monocytes, neutrophils, basophils, and eosinophils. Myeloblasts can also turn into red blood cells and platelets.

Symptoms of WBC Disorders

A low WBC count does not have symptoms on its own, but having low WBCs can make you more likely to get an infection.

Common symptoms of infections include:

• Fever
• Cough
• Painful or frequent urination
• Blood in the stools
• Diarrhea
• Redness, swelling, or warmth near the infection

Elevated White Blood Cell Counts

Leukocytosis means a high white blood cell count. Infections usually cause a higher-than-normal (elevated) white blood cell count, but there are also other causes of high WBCs. 

WBC counts can also be increased when the body releases white blood cells too early from the bone marrow (overproduction).

Common causes of elevated WBCs include:

• Infections (immature WBCs called blasts may appear in severe infections because the body is trying to fight off pathogens as quickly as possible)
• Asthma and allergies
• Stress (e.g., from illness, injuries, exercise)
• Pregnancy (high WBCs can be normal because the body is under stress)
• Inflammation (e.g., from chronic conditions like inflammatory bowel disease or autoimmune disorders)
• Cancer (e.g., leukemias, lymphomas, myelomas)

The treatment for high WBCs depends on what’s causing the levels to be elevated. For example, bacterial infections can cause high white blood cells and can often be treated with antibiotics.

Myeloproliferative Disorders

Myeloproliferative disorders are a specific group of conditions that can affect blood cells. Some of these conditions can lead to a higher-than-normal white blood cell count, including:

• Polycythemia vera is a rare type of blood cancer that usually affects red blood cells and platelets but may also lead to high WBCs. It can cause general symptoms like fatigue and shortness of breath, as well as bleeding problems like heavy menstrual periods and nose bleeds. It can also cause itchy skin after a hot shower or bath. Polycythemia vera can be treated with medications that suppress the bone marrow and prevent blood clots, as well as having blood taken often to lower the volume of cells (therapeutic phlebotomy).
• Chronic myelogenous leukemia (CML) is a blood cancer where the bone marrow makes too many WBCs. This type of cancer is slow and takes years to develop, so the symptoms can be general like fatigue, or an enlarged spleen. Treatment for CML includes medications and in some cases, a bone marrow transplant.
• Essential thrombocythemia is a rare blood disorder that causes too many platelets but it can also lead to slightly higher than normal WBCs. It can result in blood clots and complications like heart attacks and strokes and may turn into other blood cancers. Symptoms include fatigue, headaches, and visual changes. It can be treated with drugs to help prevent blood clots (like aspirin and warfarin) and medications that reduce the number of cells in the bone marrow).
• Primary myelofibrosis is a rare disease that causes bone marrow to be replaced with scar-like tissue that eventually leads the tissue to fail. It can cause higher than normal WBCs, but over time, the cells made by the bone marrow usually decrease because the tissue can no longer work as it should. Symptoms of primary myelofibrosis include fatigue, itching, night sweats, and weight loss. It can be treated with certain cancer drugs called JAK inhibitors and, in some cases, stem cell transplants.

Myelodysplastic Syndrome

Myelodysplastic syndrome refers to cancers that affect the bone marrow and lead to problems with blood cells. Usually, these cancers cause low blood cell counts but some of them can cause high white blood cell counts.

• Chronic myelomonocytic leukemia (CMML) can cause a higher-than-normal level of monocytes, a specific white blood cell. The total WBC count may also be very high. Symptoms include fatigue, fever, and an enlarged liver. It’s treated with chemo drugs and in some cases, stem cell transplants.
• Atypical chronic myeloid leukemia (aCML) is a rare cancer that can cause higher than normal WBCs. Symptoms include weight loss and fatigue. Treatment can include JAK inhibitors cancer drugs and, in some cases, stem cell transplants.
• Juvenile myelomonocytic leukemia (JMML) is a blood cancer that affects young people and can cause high WBCs (specifically, monocytes). Symptoms include paleness, shortness of breath, and fatigue. It can be treated with bone marrow transplants, chemo, and sometimes, stem cell transplants.

Medications

Some medications can raise WBC, including:

• Epinephrine 
• Allopurinol
• Aspirin
• Heparin 
• Lithium 
• Granulocyte colony-stimulating factors (used to stimulate bone marrow) 
• TNF inhibitors 
• Beta-agonists (e.g., albuterol) 
• Quinine
• Interferons 
• Chloroform
• Triamterene
• Steroids

The rise in WBC might be temporary and go back to normal once these medications are stopped. 

Low White Blood Cell Counts

Leukopenia means a low white blood cell count. A low white blood cell count can be caused by:

• Severe infections
• Bone marrow damage or disorders (e.g., aplastic anemia, bone marrow "takeover" by blood cancers or metastatic cancer; drug or chemical-related damage to the bone marrow)
• Autoimmune diseases (e.g., lupus, rheumatoid arthritis) 
• Splenic “sequestration” (i.e., white blood cells build up in the spleen) 
• White blood cell disorders (e.g., cyclic neutropenia, autoimmune neutropenia, chronic idiopathic neutropenia)

Like with higher-than-normal WBC counts, treating low white blood cell counts depends on what the cause is. It’s also important that people with low WBCs take steps to prevent infections since they are at a higher risk for them. 

Myeloproliferative Disorders

Most myeloproliferative disorders cause high rather than low WBCs, but primary myelofibrosis can eventually lead to low white blood cell counts because scarring damages the bone marrow. 

Low WBCs can also happen when someone is being treated for cancer, so people with chronic myeloid leukemia or other blood cancers can also get low WBCs.

Myelodysplastic Syndrome

Most myelodysplastic syndromes can cause a decreased WBC count because they damage the bone marrow.

Symptoms can include easy bruising and bleeding, fatigue, and fever but in the early stages, people may have symptoms. Treatment for these syndromes usually includes medications like chemo, stem cell or bone marrow transplants, and drugs that work on the bone marrow specifically. 

While they have similar symptoms and treatments, the subtypes of myelodysplastic syndromes have different names:

• Refractory Cytopenia with Unilineage Dysplasia (RCUD)
• Refractory Anemia with Ringed Sideroblasts (RARS)
• Refractory Cytopenia with Multilineage Dysplasia (RCMD)
• Refractory Anemia with Excess Blasts (RAEB)
• Myelodysplastic Syndrome with Isolated del(5q)
• Myelodysplastic Syndrome, Unclassifiable (MDS-U)
• Myelodysplastic/Myeloproliferative Diseases, Unclassifiable (MDS/MPN-U)

Medications

Certain medications and treatments can also lower WBCs, including:

• Drugs to treat infections (antibiotics, antivirals) 
• Immunosuppressant drugs (e.g., cyclosporine, azathioprine) 
• Antihistamines
• Antipsychotics (e.g., clozapine) 
• Anti-seizure drugs (anticonvulsants)
• Water pills (diuretics) 
• Tyrosine kinase inhibitors (e.g., imatinib, dasatinib) 
• Sulfonamides
• Hydroxyurea
• Colchicine
• Interferon 
• Barbiturates 
• Antithyroid drugs

Chemotherapy

One of chemotherapy's most common and dangerous side effects is its effect on neutrophils. These white blood cells are the first line of defense of the immune system, so not having enough of them while going through cancer treatment (chemotherapy-induced neutropenia) increases a person’s risk for serious infection.

Neutropenia makes it more difficult for the body to fight off infections. As a result, bacteria that are normally not very harmful can cause serious illness. Cancer patients getting radiation treatment can also have low WBCs. 

Summary

White blood cells are an important part of our immune system. Different types of white blood cells perform different functions in the body. Overall, white blood cells help to protect us against bacteria, viruses, and parasites.

A high white blood cell count is usually a sign of an infection or illness. A low white blood cell count can indicate another type of problem. Low white blood cell counts can leave you vulnerable to serious infections. Chemotherapy is a common cause of low white blood cell counts.