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Cells of the Immune System

Outline

Classification and Description of Leukocytes

A. Polymorphonuclear Leukocytes (PMN)

   Neutrophils                 First wave of attack against infection
                                       (acute inflammation). Capable of phagocytosis

    Eosinophils                 Particularly prevalent in tissues infected with
                                        parasites, limits or modulates the inflammatory                                         
                                        reaction.

    Basophils                   Contain physiologically active substances that
    (mast cells in tissue)   are involved in inflammatory response (heparin,
                                       histamine etc). Can attract neutrophils and
                                                  basophils to area.

B. Mononuclear Leukocytes

   I. Lymphocytes

     a. T Cells (Cellular immunity)

        Helper T Cells (TH)     Contain CD4 receptors on cell surface
                                             membrane. Master modulator of immune response. Two subsets: TH1 (stimulate
                                             macrophages) and TH2 (stimulate B cells)

        Cytotoxic T Cells         Contain CD8 receptors on cell surface. Can
                                             kill cells directly

        Natural Killer Cells    Non specific effector cells, lack CD4 or CD8
                                              receptors . Can kill tumor cells and cells
                                             infected with virus.

  b. B Cells (Humoral immunity

                                         When differentiated = plasma cells. Secrete                                            
                                        receptors which circulate in body as antibodies

  c. Null Cells

II. Monocytes (Macrophages)

                                            Large cells capable of phagocytosis. Can
                                            process antigens for presentation to
                                              lymphocytes.

Discussion  

    Cells can communicate with each other by means of cytokines. Cytokines are small hormnone like polypeptides. Produced by a cell, then secreted by that cell, they can then bind to a receptor on the surface of another cell. The binding of a cytokine to a receptor on the surface of another cell intiates a physiological response. Immune cells make many different kinds of cytokines. Often, cytokines can be categorized according to the type of white blood cell that produces them, For example, cytokines produced by lymphocytes are referred to as lymphokines, cytokines produced by monocytes are called monokines.
Cytokines produced by white blood cells include interleukins (numbered 1 - 17), interferons, tumor necrosis factor (TNF) and colony stimulating factor (CSF). The interleukins can regulate the inflammtory response (e.g. producing fever), activate resting white blood cells, bring about diffrrentiation of wbc's,  induce or inhibit synthesis of other cytokines, act on bone marrow cells to bring about proliferation and differentiation of precursor cells, stimulate antibody production , stimulate liver to produce acute phase reactants and act as chemoattractants for othr wbc's.
    Interferons protect neighboring cells from invasion by parasites, including viruses. some interferons can inhibit viral synthesis. TNF can act as a direct cytotoxin to tumor cells, also it can induce fever, sleep, stimulate production of other cytokines, activate macrophages and mediate inflmaamtion. CSF can promote the growth of other wbc's.

    Clinically  

An elevated total white blood cell count is called leukocytosis, a decreased total wbc count is called leukopenia. The normal range for total wbc in adults is 4, 500 - 11,000/ µl. Leukocytosis occurs during pregnancy, emotional stress, strenuous exercise, in response to all types of infections, collagen disorders, leukemias, transfusion reactions, parasitic infestations and inflammatory disorders. Leukopenia can occur as a result of diurnal rhythms, or in response to bone marrow depression, toxic and antineoplastic drugs, radiation, viral infections
    More specific then just a measurement od total wbc count is a wbc with a differential., a test in which each population of wbc is measured.  Normals for adults are as follows:
    Neutrophils (segs)          54-75% or 3,000/µl
    Neutrophils (bands)        3-8% or 620/ul
    Eosinophils                     1-4% or 200/ul
    Basophils                         0-1% or 40/ul
    Monocytes                     2-8% or 300/ul
    Lymphocytes                 24 - 40%
Segmented neutrophils are mature circulating neutrophils. Any bacterial infection is likely to cause an increase in segmented neutrophils. A really severe infection will stress the bone marrow to produce a vastly increased amount of nuetrophils with the result that many immature forms (bands) will be released. The phase "a shift to the left"   refers to an increase in bands. Viral infections are more likely to cause an increase in monocytes and lymphocytes. a parasitic infection is likely to cause an increase in eosinophils