What are White Blood Cells?
White blood cells, also known as leukocytes, are immune cells that circulate in the blood and lymphatic system to fight infection and defend the body from other foreign substances.
There are five types of white blood cells:
Neutrophils
Eosinophils
Basophils
Lymphocytes
Monocytes
Neutrophils, eosinophils, and basophils are granulocytes since they have cytoplasmic granules that can digest microorganisms. Lymphocytes and monocytes are agranulocytes since they lack granules in their cytoplasm.
Each type of white blood cells has a specific function. Some detect invaders and kill harmful bacteria. Others make antibodies to protect your body from bacteria and viruses.
Neutrophils
Neutrophils are granular leukocytes that develop from the myeloid cell lineage within the bone marrow. They are the most abundant type of white blood cells, making up 40-70% of those found in peripheral blood.
Neutrophils are 9-16µm in diameter and have a multi-lobed nucleus. Their cytoplasm contains granules with degradative enzymes which are released during phagocytosis. Their lifespan is short, only 2-3 days, and they are one of the first responders to invading microbes. These cells are primarily involved in immune responses against bacterial infections, and their presence in tissue is associated with acute inflammation.
Neutrophils are part of the innate immune system and are the first cells to arrive at the site of infection. In response to damage or pathogens, tissues produce chemokines that recruit neutrophils into the site of infection in a process called chemotaxis. Here, neutrophils phagocytose microorganisms and subsequently digest them by releasing granules into the phagosome. Neutrophils can also release substances to attract monocytes which, upon entering the tissue, differentiate into macrophages.
Eosinophils
Eosinophils (so named because of their histological staining with eosin) are granular leukocytes derived from the myeloid cell line within the bone marrow. They make up 1-3% of circulating white blood cells. Eosinophils generally remain in peripheral blood for about an hour and are found primarily in tissues.
Eosinophils have a diameter of 12-17µm and have a bi-lobed, sausage-shaped nucleus. Their granules contain molecules such as major basic protein (MBP), cationic protein and peroxidase, which are toxic to parasites. They also phagocytose antigen-antibody complexes.
Basophils
Basophils are granular leukocytes of myeloid lineage. They are 14-16µm in diameter and have a bi-lobed, S-shaped nucleus. Basophils circulate through the peripheral blood and have a lifespan of approximately 2 weeks.
They are very similar in function and appearance to mast cells, which are found within tissues. Basophils contain histamine granules, which cause local inflammatory responses through interaction with IgE. Their role in the immune system is poorly understood, but they may work with mast cells to mediate type I hypersensitivity reactions. Examples include asthma and anaphylaxis.
Lymphocytes
Lymphocytes are agranular leukocytes formed from the lymphoid cell line within the bone marrow. They respond to viral infections and are the smallest white blood cells, with a diameter of 6-15µm. Lymphocytes have round, densely-staining nuclei and sparse cytoplasm.
They circulate between tissues, peripheral blood and lymphatic system. Their lifespan varies depending on the subtype that they differentiate into.
T cells are formed in the bone marrow but mature in the thymus. They are part of the adaptive immune system and participate in cell-mediated immunity. Once activated, cytotoxic T cells can directly attack infected cells. In addition, helper T cells have many functions including activating B cells and forming memory T cells in respond to reinfection.
B cells form and mature in the bone marrow. They are part of the adaptive immune system and participate in humoral immunity by secreting antibodies. Once active, B cells mature into plasma cells and memory B cells.
NK cells provide non-specific immunity against cells displaying foreign proteins such as cancer cells and virally-infected cells. They account for less than 5% of circulating white blood cells. Although they function more effectively together with T helper cells, they can independently detect and kill pathogens as part of the innate immune system.
Monocytes
Monocytes belong to the myeloid cell lineage within the bone marrow. They are primarily involved in the immune response against bacterial infection and makeup approximately 5-10% of all circulating white blood cells. They have a kidney bean-shaped nucleus.
Monocytes are circulating leukocytes that typically remain in the blood for about 8 hours before migrating into tissue where they differentiate into macrophages. Macrophages then form the major population of phagocytes within tissues and have a much longer lifespan than neutrophils, lasting for months or even years. In some tissues, resident macrophages have specific names, such as Kupffer cells in the liver and osteoclasts in the bone. They are much larger than neutrophils, with a diameter of 25-50µm and have a single-lobed, round nucleus. Macrophages phagocytose microorganisms and digest them by releasing granules into the phagosome. They also secrete cytokines which modulate the immune response.
In certain situations, monocytes can also differentiate into dendritic cells. These form an important link between the innate and adaptive immune systems. They assist in T cell activation during the adaptive immune response and are the only cell type that can activate naïve T cells.
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