The Infection Fighters
Although they make up less than 1 percent of blood volume, white blood cells (also called leukocytes) function as essential players in the immune system and aid in clearing cellular and pathogenic debris from the body.
Leukopoiesis
Stem cells for WBCs are present in the marrow of bones and through a series of developmental stages mature and become specified for their particular function. However, many WBCs, especially lymphocytes (cells that determine the specific nature of a foreign substance), don’t fully mature in the bone marrow, but instead develop in lymphoid tissue, such as the thymus, spleen, and lymph nodes.
During this maturation time, the WBCs distinguish themselves based on the presence (or absence) of specific granules. Those cells with granules (which contain enzymes that destroy microorganisms) are collectively called granulocytes, and those without are agranulocytes.
Granulocytes
The cells in this group are all phagocytic cells, meaning these cells will engulf pathogens and debris with their plasma membrane. Once the cellular membrane captures this material, vesicles fuse with the captured material and its contents. Some of these vesicles, called lysosomes, contain hydrolytic enzymes that break down the pathogen or debris contained within the membrane compartment. These cells have the ability to follow chemical trails to the site of pathogenic attack via a process called chemotaxis. These cells can leave the circulation to mount an immune attack in the tissues of the body.
Anatomy of a Word
diapedesis
Diapedesis is the process of WBCs squeezing out of the endothelial cells of capillaries and gaining access to the tissue compartments.
Neutrophils
Neutrophils, a type of granulocyte, are the most abundant of all WBCs and make up about 60–70 percent of the WBC population in healthy individuals. These cells live, on average, about 12 hours and are always the first on the scene of an infection. They are also the first to die as they engulf a pathogen and destroy it along with themselves. Another distinguishing characteristic of the neutrophils is their multi-lobed nucleus. As the neutrophil ages, more and more lobes will appear on the nucleus, with a maximum reaching approximately five lobes. With this difference in appearance, these cells are also often referred to as polymorphonuclear (PMN) leukocytes.
Eosinophils
Eosinophils represent 2.5 percent of all WBCs. These phagocytic cells destroy parasitic organisms and function during allergic reactions. Although they are the same size overall as neutrophils, these have larger granules. Often, the granules are so large that they obscure observation of the bilobed nucleus of these cells.
Basophils
Basophils are the final member of the granulocytes and make up less than 1 percent of WBCs in the circulating blood. These cells have the largest granules, which prevent the direct observation of the nucleus. Basophils are also the only member of the granulocytes that adopt a different name when they leave circulation. In the tissues of the body, they are called mast cells. Their granules contain many inflammatory mediators, including heparin (which inhibits blood clotting) and histamine (which increases vascular permeability).
Agranulocytes
The remaining WBCs that lack granules make up both the largest and the smallest of the WBCs. Monocytes are the largest of the WBCs, about three times the size of an RBC, and make up about 8–10 percent of the WBC population. These are the vacuum cleaners of the body. Highly phagocytic, they move in and out of the circulation, capture and destroy pathogens, and return to areas populated with other WBCs (lymph nodes, thymus) and present antigens to these immune cells to initiate an immune response. Like the basophils, monocytes also change their name when they leave the circulation.
The smallest and the second most abundant of the WBCs are the lymphocytes. Lymphocytes are organized into two groups. B lymphocytes, which start their maturation in the bone marrow, are the cells responsible for the humoral (antibody-mediated) immunity of the body. T lymphocytes, which mature in the thymus, function in the cell-mediated immune system. Both types of lymphocytes are about the same size as RBCs and are identified as having very little cytoplasm that forms a crescent around a large round nucleus.