Immune Cells

The body’s defenses may be classified into non-specific defense mechanisms and specific defense mechanisms. 4 Both mechanisms work hand-in-hand in protecting the body against threats to one’s well-being. Non-specific defense mechanisms. Non-specific defense mechanisms include induction of fever, release of inflammatory chemicals, and ingestion of invading disease-causing microorganisms. Certain body cells, specifically, white blood cells such as the neutrophils or macrophages may engulf or phagocytize bacteria and other harmful foreign particles for destruction.

Specific defense mechanisms. Specific defense mechanisms involve substances such as antibodies which act specifically towards a particular pathogen. 4 Any subsequent exposures to the same antigen or pathogen in the future would induce increased intensity and effectiveness of response against infection; thus providing long-term protection or immunity against the disease. 1 II. Different Cells Involved in Immune Responses A particular group of leukocytes or white blood cells act as the body’s defenses against bacteria, viruses, parasites, as well as other harmful substances.

These cells may leave the bloodstream through a process called diapedesis and confront the invading particle towards its inactivity or destruction. Five classes of leukocytes have been identified; which includes the neutrophils, eosinophils, basophils, monocytes and lymphocytes. 3 Granulocytes. Granulocytes are white blood cells with distinct granules observable within the cytoplasm of the cell. These cells are the neutrophils, eosinophils and basophils. Neutrophils specialize in ingesting bacteria through phagocytosis.

Once particles are ingested, neutrophilic granules containing digestive enzymes dissolve the foreign particle or destroy the bacterial cell wall leading to death of the organism. Eosinophils function primarily to destroy parasites including parasitic worms. The red eosin granules of these cells contain major basic proteins or MBPs which destroy the parasite. Eosinophils also phagocytize antigen-antibody complexes and decrease severity of allergic reactions. Basophils are involved in allergic reactions through releasing histamine.

Although basophils are not phagocytic cells, basophils help in the recruitment of other phagocytic cells through release of inflammatory chemicals and increasing permeability of blood vessels. 3 Agranulocytes. Agranulocytes include the classes of lymphocytes and monocytes. These cells do not contain distinctive granules in the cytoplasm; yet these cells are essential to particular immune responses. Lymphocytes, according to Marieb and Mallat, are the most important leukocytes of the immune system. There are two classes of lymphocytes, the T cell lymphocyte and B cell lymphocyte.

T cells may be further classified into cytotoxic T cells or helper T cells. Cytotoxic T cells recognize invading microorganisms or foreign particles; or bind antigen-bearing cells and initiate destruction of these infected cells. T helper cells act as antigen-presenting cells or activate macrophages. B lymphocytes, on the other hand, multiply to become plasma cells; produce and secrete antibodies which offer long-lasting immunity against infection or disease specific for such antibody. Monocytes, as well, do not contain granules.

Interestingly, activated monocytes may transform into macrophages which are effective in phagocytosis of foreign cells, harmful molecules, and other particles located outside the bloodstream, such as in tissue spaces. 3 III. Development of Immunity Functions of Cells in Antibody-Mediated Immunity (AMI). B lymphocytes transform into plasma cells which secrete soluble chemical substances known as antibodies into the blood and other body fluids. These antibodies targets specific antigens that induced their production.

In return, memory B cells are formed which ensures an increased and more effective response upon secondary exposure to the antigen. 1 Antibodies may also act as antitoxins; or form complexes that activate another form of destructive pathway called the complement pathway; or antibodies may attach to infected cells to serve as signalling mechanism for ingestion and destruction by phagocytic cells. 2 Cellular Mechanisms in Cellular-Mediated Immunity (CMI). Human body cells infected by a virus, for example, usually express viral proteins on the surface of the cell membrane.

These viral proteins are associated with major histocompatibility complex (MHC) molecules; which in turn, results into recognition of the cell as ‘non-self’ or foreign, Cytotoxic T lymphocytes are excellent in recognizing these viral-infected cells and initiate their destruction. However, the function of T helper cells is necessary to act as antigen-presenting cells for the action of cytotoxic T cells; as well as the function of T helper cells in the release of interferon of which activates macrophages. Activated macrophages produce enormous concentrations of proteolytic enzymes, as well as nitric oxide.

These chemicals cause increased intensity of respiratory bursts to dissolve or destroy harmful foreign particles. Another type of lymphocyte is known as the NK cell or the natural killer cell. These cells do not require antigen-presenting cells; unlike that of the cytotoxic T cells. NK cells also produce chemicals influential to the immune system; act effectively against microorganisms; and act to destroy tumor cells. 2

References 1. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. The Adaptive Immune System. In: Molecular Biology of the Cell. [Electronic Book]. 2002. Available at:

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