White blood cells (WBCs), also called leukocytes, are an essential part of your body’s immune system. Their main job is to defend you against infections, combat harmful organisms such as bacteria, viruses, and parasites, and help regulate inflammation and allergic reactions. Unlike red blood cells, which carry oxygen throughout the body, white blood cells are focused on protecting you from disease and keeping your immune system healthy and balanced.
White blood cells travel throughout your body in the bloodstream and lymphatic system, including in lymph nodes. They constantly patrol for signs of infection, injury, or abnormal cells. When needed, they can move out of the blood vessels and into tissues to fight infection or mediate inflammation. WBCs are made in the bone marrow, the soft tissue inside bones, and can also be found in immune-related organs such as the spleen, tonsils, and lymph nodes.
There are several main types of white blood cells. Each type has a unique role in protecting your health:
Neutrophils – These are the most common type of white blood cell and are the body’s first responders to bacterial infections. They react quickly, moving to the site of infection and destroying bacteria through a process called phagocytosis, where they engulf and break down harmful microorganisms.
Lymphocytes – These cells are central to the immune system and come in different forms:
B cells – These cells make specialized proteins called antibodies that recognize and neutralize bacteria, viruses, and other foreign substances.
T cells – These cells help regulate the immune system, coordinate the immune response, and directly attack infected or cancerous cells.
Natural killer (NK) cells – These cells target and destroy abnormal cells, including virus-infected cells and certain types of cancer cells.
Monocytes – These large white blood cells circulate in the blood and move into tissues, where they mature into macrophages or histiocytes. They help remove bacteria, dead cells, and other debris by engulfing and digesting them.
Eosinophils – These cells play an essential role in defending against parasitic infections. They are also involved in allergic reactions and certain inflammatory conditions, such as asthma.
Basophils – The rarest type of white blood cell, basophils release chemicals like histamine that help control inflammation and are involved in allergic reactions.
Your pathology report may describe white blood cells in several situations. If a tissue or fluid sample shows increased numbers of WBCs, it may indicate inflammation, infection, or an immune system reaction. The report may also comment on the types of white blood cells present, as this can help narrow down the cause—for example, a high number of neutrophils often indicates a bacterial infection. At the same time, an increase in lymphocytes can be seen in viral infections. In bone marrow or blood samples, abnormal WBC appearance or numbers may suggest a blood disorder or cancer.
Many medical conditions can cause changes in the number, type, or appearance of white blood cells:
Infections: Bacterial infections often increase neutrophils, while viral infections may increase lymphocytes.
Inflammatory and autoimmune diseases: Conditions such as rheumatoid arthritis or lupus can alter WBC counts and function.
Allergic reactions and asthma: These often involve increased eosinophils and basophils.
Bone marrow disorders: Diseases affecting the bone marrow, such as aplastic anemia or myelodysplastic syndromes, can reduce WBC production.
Blood cancers: These cancers, including leukemia, lymphoma, and multiple myeloma, all originate from white blood cells or their precursors and can cause significant changes in counts and cell appearance.
Effects of medication or treatment: Chemotherapy, radiation, and certain drugs can lower WBC counts, increasing infection risk.
White blood cells are typically measured using a complete blood count (CBC), a standard blood test that reports both the total number of WBCs and the differential count—the percentage of each type of white blood cell. In some cases, a peripheral blood smear is examined under a microscope to assess the size, shape, and maturity of the cells. If the results are abnormal or if there is suspicion of a blood cancer or bone marrow disorder, additional tests such as a bone marrow biopsy, flow cytometry, or molecular testing may be performed to look for specific diseases or genetic changes.
Cancers that develop from white blood cells are referred to as hematologic cancers or blood cancers. Common examples include:
Leukemia – A cancer of the blood and bone marrow that leads to the production of large numbers of abnormal white blood cells. There are several types, such as acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML). Each type affects different white blood cell lineages and has other symptoms, treatment approaches, and outlooks.
Lymphoma – A cancer that usually begins in lymph nodes and starts in lymphocytes. The two main types are Hodgkin lymphoma and non-Hodgkin lymphoma. Both involve the abnormal growth of lymphocytes, but they differ in their behavior, response to treatment, and appearance under the microscope.
Multiple myeloma – A cancer of plasma cells, which are a type of B cell that normally produce antibodies. In multiple myeloma, abnormal plasma cells accumulate in the bone marrow, crowding out healthy blood cells and causing bone damage, anemia, and other complications.
Which type of white blood cells are mentioned in my report?
Are my white blood cell levels higher or lower than normal?
Could the changes be due to an infection, inflammation, or a blood cancer?
Do I need additional tests, such as a bone marrow biopsy?
How will these results affect my treatment or follow-up plan?