White Blood Cells (Leukocytes)
White blood cells (WBCs), or leukocytes, differ from red blood cells in that they are usually larger, have a nucleus, lack hemoglobin, and without staining, appear to be white. White blood cells are not as numerous as red blood cells. The white blood cell count is usually 5,000-11,000 per cubic millimeter of blood. White blood cells fight infection in ways, which concerns immunity.
Red blood cells are confined to the blood, but white blood cells are also found in lymph and tissue fluid. They are able to squeeze through pores in the capillary wall). Some are always found in small numbers in the tissues, but their number greatly increases when there is an infection. Many white blood cells live only a few days-they probably die while engaging pathogens. Others live months or even years.
Types of White Blood Cells
White blood cells are classified as granular leukocytes or agranular leukocytes. Both types of cells have granules in the cytoplasm surrounding the nucleus, but the granules are more prominent in granular leukocytes. The granules contain various enzymes and antibiotic-like proteins that help white blood cells defend the body. There are three types of granular leukocytes and two types of agranular leukocytes. They differ somewhat by the size of the cell and the shape of the nucleus.
Granular Leukocytes
eutrophils are the most abundant of the white blood cells. They have a multilobed nucleus joined by nuclear threads; therefore, they are also called polymorphonuclear. Their granules do not significantly take up the stain eosin (a pink to red stain) or a basic stain that is blue to purple. This accounts for the name neutrophil.
Neutrophils are the first type of white blood cells to respond to an infection, and they engulf bacteria and cell debris via phagocytosis. Eosinophils have a bilobed nucleus.
Their granules take up eosin red stain, which accounts for the name eosinophil, meaning red-loving. The function of eosinophils is not completely understood, but they are known to increase in number during a parasitic worm infection or an allergic reaction. Basophils have a U-shaped or lobed nucleus. The granules take up the basic stain and turn dark blue, which accounts for the name basophil. Basophils move to sites of injury and enter the tissues, where they release histamine, a chemical associated with allergic reactions. Histamine dilates blood vessels and causes contraction of smooth muscle in the walls of air passageways.
Agranular Leukocytes
Agranular leukocytes (monocytes and lymphocytes) typically have a spherical or kidney-shaped nucleus. Monocytes are the largest of the white blood cells, and after taking up residence in the tissues, they differentiate into even larger macrophages. Macrophages phagocytize pathogens and stimulate other white blood cells to defend the body.
Lymphocytes are of two types, but both have the same appearance. B lymphocytes are responsible for antibody-mediated immunity-that is, they produce antibodies, proteins that combine with antigens. T lymphocytes are responsible for cellmediated immunity-that is, they directly destroy any cell that bears foreign antigens.
White blood cells are divided into the granular leukocytes and the agranular leukocytes. Each type of vvhite blood cell has a specific role to play in defending the body against disease.
