White blood cell

White blood cells, scientifically known as leukocytes, are essential components of the immune system.

These cells protect the body against infectious diseases and other foreign entities. While they make up only about 1% of total blood volume in a healthy adult, they are vital for survival. In contrast, red blood cells make up 40% to 45% of blood volume. Every leukocyte contains a nucleus, which distinguishes them from anucleated red blood cells and platelets.

All white blood cells originate in the bone marrow. They derive from multipotent cells called hematopoietic stem cells. From the marrow, they travel through the blood and the lymphatic system to defend the body.

Leukocytes are categorized using different classification systems. One method uses physical characteristics, such as the presence of granules in the cytoplasm. This divides them into granulocytes and agranulocytes.

Another method uses cell lineage, which refers to how the cells develop. This classifies them as myeloid cells or lymphoid cells. Myeloid cells include neutrophils, eosinophils, basophils, and monocytes. Lymphoid cells consist of lymphocytes. This dual classification helps scientists understand both the structure and the biological origin of each cell type.

Neutrophils are the most abundant type of leukocyte, making up 60% to 70% of the total count.

They are often called polymorphonuclear leukocytes because they have a multi-lobed nucleus. These cells are usually the first responders to bacterial or fungal infections. Neutrophils are phagocytic, meaning they engulf and digest pathogens. After they consume several pathogens, they cannot renew their lysosomes and they die. This process of cell death and activity often forms pus in a wound. Their lifespan in circulation typically ranges from 5 to 135 hours.

Other granulocytes include eosinophils and basophils.

Eosinophils make up about 2.3% of white blood cells. They primarily target large parasites that are too big for other cells to engulf. They also play a major role in allergic inflammatory responses. Eosinophils have bi-lobed nuclei and pink-orange granules.

Basophils are the rarest, representing less than 0.5% of the count. They release histamine to dilate blood vessels and increase blood flow. They also release heparin, an anticoagulant that prevents blood from clotting too quickly. This helps other white blood cells reach the site of an injury or infection.

Lymphocytes are a major group of lymphoid cells, accounting for about 30% of white blood cells.

They include B cells, T cells, and natural killer cells. B cells produce antibodies to block pathogens and assist in immune activation. T cells are further divided into several functional types. Helper T cells, or CD4+ T cells, coordinate the immune response by releasing cytokines. Cytotoxic T cells, or CD8+ T cells, identify and kill virus-infected or tumor cells. Natural killer cells also target cells that show signs of stress or viral infection. Some memory lymphocytes can live for many years to provide long-term protection.

Monocytes are the largest type of white blood cell, comprising about 5.3% of the total.

Like neutrophils, monocytes are phagocytic. However, they are much longer-lived and serve an extra role in the immune system. They present pieces of captured pathogens to T cells to trigger an antibody response. Eventually, monocytes leave the bloodstream to become tissue-resident macrophages.

These macrophages act like vacuum cleaners, removing dead cell debris and attacking microorganisms that neutrophils cannot handle effectively. Some monocytes also become dendritic cells to help signal the rest of the immune system.

Medical professionals monitor white blood cell counts as part of a complete blood count.

A normal leukocyte count is generally between 4 billion/L and 11 billion/L. In the United States, this is often expressed as 4,000 to 11,000 cells per microliter. An increase above these limits is called leukocytosis, which often indicates infection or inflammation. A decrease below the lower limit is known as leukopenia, which suggests a weakened immune system. While high counts are often healthy responses to germs, they can sometimes indicate bone marrow disorders or blood cancers.