Leukocytes

There are normally 4000 to 11,000 leukocytes (white blood cells) per microliter of human blood. However, leukocytes act primarily within the tissues; those found in the blood are actually in transit. Leukocytes are also found in lymphoid tissues such as the thymus, spleen, and lymph nodes. These cells are referred to as "white" blood cells because they lack hemoglobin and are essentially colorless. Leukocytes are an important component of the immune system. General inflammatory and immune functions of these cells include:

• Destruction of invading microorganisms (bacteria and viruses)

• Identification and destruction of cancer cells

• Phagocytosis of tissue debris including dead and injured cells

Five types of leukocytes are classified as either granulocytes or agranu-

locytes:

• Granulocytes:

• Neutrophils

• Eosinophils

• Agranulocytes:

• Lymphocytes

The granulocytes are phagocytic cells. Their nuclei tend to be segmented into multiple lobes and the cytoplasm of the cells contains numerous granules. These cells are identified by the staining properties of their granules.

Neutrophils are the most abundant of the leukocytes and account for about 60% of the total number of white blood cells. These cells are usually the first to arrive at a site of injury or inflammation. Their primary function is to attack and destroy invading bacteria. In fact, bacterial infection is typically associated with pronounced neutrophilia (an increase in the number of circulating neutrophils). These leukocytes are also involved in removal of tissue debris and therefore play a role in the healing process.

Neutrophils eliminate bacteria and tissue debris by way of phagocytosis. Small projections of the cell membrane extend outward and engulf the harmful organisms and particles. As a result, these materials are internalized within a cell membrane-bound vesicle. A lysosome — an organelle filled with hydrolytic enzymes — then fuses with the vesicle. In this way, the phagocytized material is degraded by these enzymes without any damage to the rest of the cell. Neutrophils have the capacity to phagocytize 5 to 25 bacteria before they also die.

Eosinophils, which constitute only 1 to 4% of the total number of white blood cells, are only weak phagocytes. These leukocytes are produced in large numbers in individuals with internal parasitic infections. The eosinophils attach to the parasites and secrete substances that kill them, including:

• Hydrolytic enzymes released from eosinophil granules (which are actually modified lysosomes)

• Highly reactive forms of oxygen that are particularly lethal

• Major basic protein — a larvacidal polypeptide also released from granules

Eosinophils also tend to accumulate at the sites of allergic reactions, particularly in the lungs and skin. The functions of the eosinophils in these areas include neutralization of inflammatory mediators released from mast cells as well as phagocytosis of allergen-antibody complexes. In this way, the spread of the inflammatory reaction is limited.

Basophils are the least abundant of the leukocytes and account for less than 1% of the total number of white blood cells. They are similar structurally and functionally to the mast cells found in connective tissues, especially in the lungs, skin, and gastrointestinal tract. Basophils and mast cells play an important role in allergic reactions. The granules of these cells contain many substances, including:

• Heparin, which prevents blood coagulation

• Histamine, which promotes bronchoconstriction as well as the vasodilation and increased capillary permeability that lead to inflammation

The leukocytes classified as agranulocytes contain very few granules in their cytoplasm. In further contrast to granulocytes, these cells have a single, large nonsegmented nucleus.

Monocytes account for about 5% of the total number of white blood cells in the blood. Immature in the blood, these leukocytes leave the vascular compartment and enter the tissues, within which they enlarge, mature, and develop into macrophages. Macrophages are large phagocytic cells that can ingest bacteria, necrotic tissue, and even dead neutrophils. These cells survive much longer than neutrophils and may ingest up to 100 bacteria. The life span of the macrophage may range from months to years until it is ultimately destroyed as a result of phagocytic activity.

Lymphocytes constitute about 30% of the total number of white blood cells. The two types of lymphocytes are:

• B lymphocytes

• T lymphocytes

The primary function of the B lymphocytes is to produce antibodies, which are molecules that identify and lead to the destruction of foreign substances such as bacteria. The B lymphocytes and the antibodies they produce are responsible for humoral immunity. T lymphocytes provide immunity against viruses and cancer cells. These lymphocytes directly attack and destroy their targets by forming holes in the target cell membrane, causing cell lysis. The T lymphocytes are responsible for cell-mediated immunity

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