White Blood Cells

White Blood Cells White blood cells, or leukocytes, are immune cells that are present in the blood. The detailed mechanisms of immune function are covered in the immunity unit, but we will discuss the classes here. There are five common types of leukocytes (and some of those types have subgroups). One way of categorizing these five is by whether or not they contain granules in their cytoplasm when the cells are stained. If they do, they are a type of granulocyte. If not, they fall into the category of agranulocytes. Another way of distinguishing leukocytes from one another is by the morphology (shape and patten) of their nuclei. Granulocytes have multilobulated nuclei and agranulocytes have a spherical (nonlobulated) nucleus. The lifespan of a granulocyte ranges from about 12 hours to 4 days, but the agranulocyte survives for approximately 100 to 300 days. Leukocytes mainly function to protect the body from microbial invasion and toxins. WBCs readily travel through the bloodstream. However, certain leukocytes have the ability to move into the interstitial spaces of the body’s tissues in order to attack foreign agents to protect the body from infection. They leave the capillalry by squeezing through the pores in the capillary walls via a process called diapedesis. This is due to the ability of these leukocytes to rearrange cytoskeletal matrix in a manner that changes their shape. Granulocytes Granulocytes contain many granules within the cytoplasm, and they have a multilobular, irregularly shaped nucleus. There are three types of granulocytes: neutrophils, eosinophils, and basophils. Specific stains used to reveal the granules in their cytoplasm identify each granulocyte. Granulocytes are responders of the nonspecific immune system, which means that the cells respond to injury and build up an inflammatory response against microbes or toxins regardless of whether the body has been previously invaded by the foreign agent. That is to say, granulocytes do not possess memory when mounting an immune response. Nonspecific Immune Response Leukocytes Neutroph il Eosinoph il Basophil Monocyt e Neutrophils Neutrophils are the most abundant type of granulocyte (50 to 70 percent of circulating WBCs). The S- or C-shaped nucleus of these cells has three to six lobules that are connected via strands of chromatin. The multi-lobular shape of the nucleus often classifies neutrophils as polymorphonuclear leukocytes (PMNs). During a bacterial infection, neutrophils are the first responders. The PMNs arrive at the site of infection through a process known as chemotaxis in which chemicals released by damaged cells draw neutrophils. The neutrophils attack bacteria by engulfing them via the process of phagocytosis. Once the bacteria are contained within the neutrophils, the granules release antimicrobial compounds called defensins andlysozymes to destroy the microbes. Pus is the culmination of destroyed neutrophils, bacteria, and dead tissue. Eosinophils Eosinophils only possess a bilobed nucleus linked by a thin strand of chromatin. Unlike neutrophils, eosinophils stain red with an eosin dye; the dye is acidic and is picked up by the cytoplasmic granules that contain digestive enzymes. Eosinophils also engulf foreign molecules attacking the body, particularly in parasitic infection and allergic reactions. They also play a role in modulating the activity of basophils. Basophils The nucleus of basophils can contain two to five U- or S-shaped lobules. These leukocytes pick up basic dyes that cause their granules to have a blue-purple appearance. The granules of basophils contain histamine, serotonin, and heparin. The cells release histamine during an inflammatory response to tissue damage and microbial invasion. Basophils are similar in appearance and function with mast cells, which are strictly localized in connective tissue. Agranulocytes Unlike granulocytes, agranulocytes do not have granules within their cytoplasm and lack a lobulated nucleus. Under a microscope agranulocytes are observed to have a nucleus that composes the bulk of its cellular volume. Agranulocytes include lymphocytes, which are responsible for the specific immune response, meaning that they have memory and build up a vigorous response against toxins or microorganisms that the body has encountered before. The other agranulocyte is the monocyte, which is an immature form of the nonspecific macrophage. Lymphocytes Lymphocytes can be categorized by their size (small, medium, and large). These cells have a round nucleus that virtually fills up the cell leaving very little blue-staining cytoplasm surrounding it (when stained in a standard manner). Lymphocytes are the only WBCs that reenter the bloodstream after moving into tissue to attack foreign agents. There are two types of lymphocytes: T lymphocytes (T cells) and B lymphocytes (B cells). They both originate in the bone marrow, but T cells grow and mature within the thymus gland while B cells typically mature in bone marrow then move to lymph nodes and similar tissue. T cells attack abnormal cells in the body, such as virally infected cells, tumor cells, and donor transplant cells. B cells sense and attack invading antigens (foreign substance in the body) such as toxins, viruses, or bacteria. In response to antigens, B cells divide into plasma cells that make antibodies aimed at neutralizing and destroying these antigens. Monocytes The nuclei of monocytes are also large, but have a kidney shape. The cytoplasm of monocytes is greater in quantity compared to lymphocytes, and it stains in a bluish-gray color. Monocytes have the capacity to leave the bloodstream and enter tissue, where they mature into cells called macrophages. Macrophages phagocytose (phagocytize) microbes and tissue debris. .

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