Microbiology 435 Agranulocytes

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Monocytes/Macrophages These two cells are the same cell type; they are monocytes in the blood, and become macrophages when they migrate into the tissue. Monocytes comprise 3-7% of circulating WBCs (0.6 x 109/L) Developmental stages are monoblast promonocyte monocyte; macrophage in tissue Macrophages may live in tissue for months, even years. Maturational changes of the monocyte The monoblast is similar in appearance to myeloblast. The promonocyte largest of three stages. The N:C ratio decreases from 4:1 to 2:1 or 1:1 as the cell matures. The nuclear shape goes from oval or folded to lobular and irregular. The nucleoli disappear as the cell matures. The chromatin pattern goes from fine to slightly more coarse with a lacy appearance. A mature monocyte has an abundance of cytoplasm. Cytoplasmic color goes from blue to blue-gray as the cell matures. There are no significant obvious granules in the cytoplasm which has a "ground glass" appearance in the mature stage. There are often vacuoles in the cytoplasm, particularly in the mature monocytes. Mature monocytes are the largest peripheral WBC most of the time. Macrophage appearance Normally seen only in tissue. Larger than monocytes. Round nucleus, eccentrically placed. Large amount of cytoplasm containing vacuoles.   Lymphocytes Adult normal value of lymphocytes is 20-40% (1500-3000/uL). Newborns typically have more lymphocytes than neutrophils (16-90% of circulating WBCs). There are two general types of lymphocytes: B cells and T cells. B cells don’t live as long as T cells; B cells live a few days while T cells can live up to 4 years with 1% living up to 20 years. The developmental stages of all lymphocytes can be divided into antigen independent and the antigen dependent stages. 1. Antigen independent developmental stages A stem cell in red bone marrow differentiates into the lymphoid stem cell. The lymphoid stem cells develop into the pre-B cell in the bone marrow and the pre-T (prothymocyte) in the thymus (bone marrow and thymus are called primary lymphoid tissue). "B" comes from bursa equivalent, bone marrow and intestinal lymphoid tissue thought to be the equivalent of bird organ called bursa of Fabricius, a saclike structure attached to the cloaca where lymphocytes become immunocompetent B cells). "T" comes the name of a surface antigen found on mouse T cells called "theta." The pre-B cell & prothymocyte give rise to lymphoblast prolymphocyte mature immunocompetent lymphocytes (small lymphocytes) effector cells and memory cells Most cells produced in primary tissue die but a small percentage migrate to the secondary lymphoid tissue. Secondary lymphoid tissue consists of lymph nodes, spleen, Gut Associated Lymph Tissue (GALT) in the walls of the ileum, and tonsils Proliferation of T and B cells in the secondary tissue is dependent on antigenic stimulation. Lymphocytes travel between blood and lymphoid tissue; return to blood via lymphatic vessels. There are more T cells than B cells. Maturational characteristics of small lymphocytes Overall cell size decreases as the cell matures. N:C ratio stays about the same 4:1. Nuclear shape is round in small lymphocytes. Nucleoli disappear as the cell matures. The chromatin pattern becomes more coarse as the cell matures. The color of cytoplasm goes from darker blue to lighter blue. There are essentially no granules but may seen a few azurophilic granules in prolymphocytes and small lymphocytes. 2. Blast transformation (antigen-dependent lymphopoiesis) During development into immunocompetent T and B lymphocytes, the cells acquire specific antigen receptors (without exposure to these antigens), committing them to antigen specificity. Subsequent contact with their respective antigens begins a complex series of events known as blast transformation. Blast transformation leads to cloning of lymphocytes that will take part in the immune response to the invading antigen. This usually occurs in the lymph nodes. The cells change in appearance. These transformed cells are first called variant lymphocytes, then immunoblasts and finally differentiate into effector cells or into memory cells. Both T or B cells go through these changes. The only way to tell the difference is by doing cell surface marker studies, using monoclonal antibodies, etc. The variant lymphocytes may be seen in the peripheral blood during viral infections. The immunoblasts and effector cells are usually only in the lymph nodes and other secondary lymphoid tissue. Under intense stimulation of the immune system however, the immunoblasts and plasma cells (B effector cells) may be seen in the peripheral blood. A variant lymphocyte is a lymphocyte that is not normal but represents an immune response e.g. mononucleosis. Our book calls them reactive lymphocytes The term variant lymphocyte will include all transitional stages occurring after the mature immunocompetent lymph is stimulated by antigens. Variant lymphocyte will replace all other terms: Downey cells, atypical, reactive, transformed lymphocytoid or plasmacytoid lymphocytes, virocytes, stimulated, activated lymphocytes. Characteristics of variant lymphocytes: Increased size Enlarged nucleus Nucleus may be lobulated or monocytoid, rectangular or flat (2-D) Chromatin patterns vary from fine to coarse (fine indicates increased DNA synthesis and mitosis) Nucleoli may be present indicating RNA synthesis, cell division and proliferation) Abundant cytoplasm, frequently vacuolated Gray to blue cytoplasm (blue indicates RNA and ribosomes, protein production) Granules may be present Variant lymphocytes may be confused with lymphoblasts.  Characteristic of the effector plasma cell (plasmacyte): Deeper blue cytoplasm Eccentric nucleus Perinuclear clear zone indicating the area of the Golgi apparatus Coarse chromatin pattern   Null cells Null cells are lymphocytes that have neither T or B cell markers but do have some of the common lymph markers. Null cells cannot be identified morphologically. Three kinds of null cells: Killer cells (K cells) - lymphocytes that are activated by an antibody bound to a cell (opsonization). These cells are involved with an antibody-dependent cell-mediated cytotoxicity reaction (ADCC). Natural killer cells (NK cells) are not dependent on an antibody to recognize foreign cells. They constantly survey the body for cells that have any surface alterations like cancer cells, some cells of the embryo, some bone marrow cells and thymus cells and microbe-infected cells. Lymphokine-activated killer cells (LAK cells) When activated by IL-2 they lyse tumor target cells that are resistant to NK cells.   Function of monocytes and macrophages Monocytes are found in peripheral blood; macrophages are found in tissue. There is not a large reserve pool of monocytes in the bone marrow. Monocytes are found both in the circulating and the marginal peripheral blood pools. Stimulated by growth factors, monocytes migrate into tissues and are generally called macrophages; some fixed in connective tissue fibers and other sites and some wandering freely through the connective tissue. Macrophages are most numerous in "filter" organs like the spleen, liver, lungs, lymph nodes collectively known as the mononuclear phagocyte system, (formerly the reticuloendothelial system) a system that serves as an important body defense mechanism composed of phagocytic cells. Special names of macrophages are: histiocytes in loose connective tissue. Kupffer cells in the liver osteoclasts in bone (they absorb and remove bone) microglial cells in nervous tissue like the brain Langerhans’ cells in the epidermis - they recognize antigens, ingest them and present them to lymphocytes for eventual destruction glomerular mesangial cells in the kidney pulmonary alveolar macrophages in the lungs macrophages in the spleen and lymph nodes monocytes in the blood Both monocytes and macrophages are phagocytic like neutrophils. Monocyte numbers increase whenever there is increased cell damage. Monocytes are less selective than neutrophils and will ingest more variable material like old or abnormal erythrocytes. Monocytes store released iron from lysed erythrocytes. They act as antigen presenters in that they process ingested material and present the antigen on its (the monocyte’s) surface to a T-helper (CD4+) lymphocyte. Monocytes also produce and respond to interleukins and other cytokines involved in the immune response. Functions of Lymphocytes The acquired (specific) immune system (vs. non-specific, innate) is comprised of the humoral and the cell-mediated immune systems. The lymphocytes found in each system are preprogrammed to respond to a specific antigen. The innate or non-specific immune system involves skin and other mucosal surfaces, inflammatory response, commensal bacteria, and phagocytosis. The acquired (AKA adaptive or specific) immune system involves two systems: humoral: humors in medieval physiology were body fluids that determined a person’s health and temperament. Humors also were proteins that protected against disease. B lymphocytes are primarily involved with this type of immunity. cell-mediated. T lymphocytes are responsible for this type of immunity. B-lymphocytes are responsible for the humoral response. Upon antigenic encounter they transform into plasma cells that produce antibodies specific to the antigen. Production of antibodies is a complex process. Memory B cells are also produced. These memory cells respond more quickly than the initial response to a future encounter with the same antigen. Each activated B cell gives rise to many progeny which together are called a clone of cells because they are all identical. T-lymphocytes are responsible for the cell-mediated response. After antigenic stimulation they proliferate and differentiate into memory cells and effector cells. There are at least two types of effector cells: helper T-cells (CD4+) and cytotoxic T-cells (CD8+). CD = cluster differentiation markers. They are identifying membrane proteins (antigens). It is the CD4+ lymphocytes to which HIV has an affinity. The normal ratio of CD4+:CD8+ is 2:1. In HIV-infected individuals the ratio decreases and reverses as the AIDS virus invades and destroys the CD4+ cells. The ratio is used to monitor the progress of the disease. T-helper cells "help" the B-cells produce antibodies by releasing cytokines. They also affect the action of other cells, such as CD8+. CD8+ cells (cytotoxic T-cells) mediate destruction of cells infected with pathogens such as viruses, bacterial, protozoa, or fungi; and are also responsible for rejection in organ transplants. T-helper cells help B-cells produce antibodies and also influence other T-helpers, T-suppressors and cytotoxic T-cells. (T suppressors help keep the immune system in control).