Types and Function of White Blood Cells (WBCs)

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

Different types of white blood cells have different jobs. Some are involved in recognizing intruders. Some kill harmful bacteria. Others make antibodies to protect your body against exposure to bacteria and viruses.

This article discusses the different types of white blood cells and their various functions.

Types of WBC

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

Some of the cells are part of our innate immune system, meaning they know from birth to attack foreigners. Others are part of our humoral or learned immune system. Humoral immune cells manufacture antibodies after exposure to a germ. This way, the body can be prepared for another attack by that germ.

Neutrophils

Neutrophils make up roughly half of the white blood cell population. They are usually the first cells of the immune system 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 to the scene.

Neutrophils are the main cells found in pus. Once released from the bone marrow, these cells live for only around eight hours. Your body produces roughly 100 billion of these cells every day.

Eosinophils

Eosinophils also play a role in fighting off bacteria. They are very important in responding to parasitic infections (such as worms) as well.

They are perhaps best known for their role in triggering allergy symptoms. Eosinophils can go overboard in mounting an immune response against something harmless. For example, eosinophils mistake pollen for a foreign invader.

Eosinophils account for no more than 5% of the white blood cells in your bloodstream. However, there are high concentrations of eosinophils in the digestive tract.

Basophils

Basophils account for only around 1% of white blood cells. These cells are perhaps best known for their role in asthma. However, they are important in mounting a non-specific immune response to pathogens, organisms that can cause disease.

When stimulated, these cells release histamine, among other chemicals. This can result in inflammation and narrowing of the airways.

Lymphocytes (B and T)

Lymphocytes are also essential in the immune system. They come in two forms: B cells and T cells. Unlike other white blood cells that provide non-specific immunity, B and T cells have specific purposes.

B lymphocytes (B cells) are responsible for humoral immunity, which is the immune response that involves antibodies. B cells produce the antibodies that "remember" an infection. They stand ready in case your body is exposed to that pathogen again.

T cells recognize specific foreign invaders and are responsible for directly killing them. "Memory" T cells also remember an invader after an infection and respond quickly if it is seen again.

B lymphocytes play a key role in the effectiveness of many current vaccines. In some cases, such as tuberculosis and pertussis vaccines, T lymphocytes are the main players.

Monocytes

Monocytes are the garbage trucks of the immune system. Around 5% to 12% of white blood cells in your bloodstream are monocytes. Their most important function is to clean up dead cells in the body.

How WBCs Are Formed

White blood cells begin in the bone marrow in a process called hematopoiesis. All blood cells descend from a common hematopoietic stem cell (HSC). This is also called a "pluripotent" stem cell. These stem cells differentiate—or specialize—in different stages. 

The HSC cell first separates into either a lymphoid or myeloid stem cell.

The lymphoid stem cell gives rise to the lymphoid cell line. This is the family of cells that produces B cells and T cells.

The myeloid stem cells give rise to cells called myeloblasts. These further evolve into macrophages, monocytes, neutrophils, basophils, and eosinophils. Myeloblasts can also turn into red blood cells and platelets.

Elevated White Blood Cell Counts

Infections usually cause an elevated white blood cell count, but there are also other possible causes. WBC counts can be increased by overproduction. In other words, the body may release white blood cells early from the bone marrow.

Stress of any form can result in this release of white blood cells. Some causes of an increased white blood cell count include:

• Infections
• Cancers such as leukemias, lymphomas, and myelomas, in which a greater number of white blood cells are manufactured
• Inflammation, such as inflammatory bowel disease and autoimmune disorders
• Trauma, ranging from fractures to emotional stress
• Pregnancy
• Asthma and allergies
• Exercise

Low White Blood Cell Counts

Conditions that may result in a low white blood cell count include:

• Severe infections
• Bone marrow damage or disorders including aplastic anemia, bone marrow "takeover" by blood cancers or metastatic cancer, or drug or chemical-related damage to the bone marrow
• Autoimmune diseases such as lupus
• Splenic "sequestration," where white blood cells are accumulated in the spleen.

Symptoms

On its own, a low WBC count doesn't have symptoms. But a low count will often lead to an infection, because not enough white cells are present to fight off the invader. Symptoms of infection may include:

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

Chemotherapy

One of chemotherapy's most common and dangerous side effects is its effect on white blood cells known as neutrophils. Recall that neutrophils are the first responders of our immune system.

A decrease in neutrophils during chemotherapy, known as chemotherapy-induced neutropenia, increases the risk of 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.

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.