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http://www.benzenecausescancer.info/home/del-parkinson/immune-system-primer
The human immune system is made up of a surprising variety of different cell types disseminated throughout the body and collectively defining one's capacity to mount an immune response. Lymphocytes are the predominant cell type responsible for immune responses. Other cell types involved in the human immune response are described below:
- Macrophages
- Dendritic cells
- Granulocytes
All blood cells and certain other cells in the body (particularly in the reticuloendothelial system) are continuously regenerated throughout life by the process called hematopoiesis. Most hematopoietic cells are short lived, some surviving for only a day or two, and thus hematopoiesis serves to maintain a steady renewal of these cells on physiological demand.
Hematopoietic cells are highly mobile, moving with the flow of blood in the cardiovascular system or with the flow of lymph in the lymphatic system.
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Macrophages
When monocytes leave the blood and enter the tissue, they become activated and differentiate into macrophages. Those that have recently left the blood during inflammation and move to the site of infection through positive chemotaxis are sometimes referred to as wandering macrophages.
In addition, the body has macrophages already stationed throughout the tissues and organs of the body. These are sometimes referred to as fixed macrophages.
Many fixed macrophages are part of the mononuclear phagocytic (reticuloendothelial) system. They, along with B-lymphocytes and T-lymphocytes, are supported by reticular fibers in lymph nodules and in the spleen where they filter out and phagocytose foreign matter such as microbes.
Similar cells derived from stem cells, monocytes, or macrophages are also found in the:
- Liver (Kupffer cells), The kidneys (mesangial cells), The brain (microglia),
- Bones (osteoclasts), The lungs (alveolar cells), The CNS (microglial cells),
- Histiocytes,
- Osteoclasts,
- Tissue (Langerhans cells),
- Synovial type A cells,
- Interdigitating cells,
- Epithelioid cells, and
- Foreign-body-type multinucleated giant cells.
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Macrophages have a number of very important functions in body defense including:
- Killing microbes, infected cells, and tumor cells by phagocytosis.
Macrophages that have engulfed microorganisms become activated by a subset of T-helper lymphocytes called TH1 cells. Activated macrophages develop a ruffled cytoplasmic membrane and produce increased numbers of lysosomes.
- Processing antigens so they can be recognized by T-lymphocytes during the adaptive immune responses.
Macrophages, as well as the dendritic cells mentioned below, that process antigens through phagocytosis and present them to T-lymphocytes are often termed antigen-presenting cells or APCs. Antigen-presenting macrophages engulf the microorganism and degrade it with their lysosomes.
- Secreting proteins called cytokines
(sometimes called monokines when produced by monocytes or macrophages) that play a variety of roles in nonspecific body defense. For example, macrophage-produced cytokines promote inflammation and induce fever, increase phagocytosis and energy output, promote sleep, activate resting T-lymphocytes, attract and activate neutrophils.
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Dendritic cells
Immature dendritic cells are found under the skin and mucous membranes where they engulf microorganisms and molecules through phagocytosis and pinocytosis. Subsequently, the immature dendritic cells migrate to the follicles of secondary lymphoid organs such as lymph nodules, lymph nodes and the spleen and in the process, mature into professional antigen-presenting cells (APCs).
Dendritic cells are derived from bone marrow progenitor cells and monocytes. Mature dendritic cells have numerous pseudopodia-like projections to increase their surface for antigen presentation. Dendritic cells are considered to be the most potent antigen presenting cells (APCs) in the body.
Like macrophages, dendritic cells engulf the microorganisms and other materials and degrade them with their lysosomes. Peptides from microbial proteins are then bound to a groove of unique molecules called dendritic cells and produce the same cytokines as the macrophages.
Dendritic cells may differentiate from monocytes (in the tissues) or they may have their own line of differentiation directly from granulocyte-monocyte progenitor cells...this has not been settled yet. These cells are covered with long membrane extensions that make them look like dendrites in nervous tissues, hence the name.
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Granulocytes
Granulocytes comprise 41 to 81 percent of blood leukocytes, are 20-30 µm in diameter, and have a multilobed nucleus with loosely packed strands of intensely basophilic chromatin. They are surrounded by a moderate amount of lightly basophilic cytoplasm containing large numbers of granules.
- Neutrophils comprise 40 to 75 percent of blood leukocytes contain small, lightly staining granules phagocytic effectors of antibody-mediated immunity and hypersensitivity.
- Eosinophils comprise 1 to 5 percent of blood leukocytes contain orange-to-red staining granules help regulate inflammatory responses active in antibody-mediated cytolysis of immature forms of intestinal parasites.
- Basophils comprise 0 to 1percent of blood leukocytes contain large, blue-black staining histamine-rich cytoplasmic granules and FcR that bind IgE molecules help generate inflammatory responses mediators of immediate (type I) hypersensitivity.
- Mast cells are tissue cells with histamine-rich cytoplasmic granules and FcR that bind IgE molecules (~10X more of each than found in basophils) help generate inflammatory responses tissue mediators of immediate (type I) hypersensitivity.
- Monocytes comprise 3 to 7percent of blood leukocytes 20-50 µm in diameter nucleus is large and indented: contains loosely packed strands of intensely basophilic chromatin surrounded by a large amount of lightly basophilic cytoplasm cytoplasm contains numerous granules and occasional vacuoles engulf and digest foreign matter phagocytic effectors of cell-mediated immunity and hypersensitivity.
- Circulating dendritic cells constitute 0.1 percent of blood leukocytes and are also found in lymph ("veiled" cells) develop into mature tissue dendritic cells.
- Interdigitating dendritic cells found in T cell rich regions of secondary lymphoid tissue process and present antigen to T cells.
- Interstitial dendritic cells found in most organs lungs, liver, heart, kidney, digestive tract, etc. process and present antigen to T cells.
- Langerhans cells found in epidermis of skin process and present antigen to T cells.
- Lymphoid cells comprise 20 to 45 percent of blood leukocytes, are responsible for immune responses. They are 10-30 µm in diameter, with a nucleus that is nearly round and contains coarse lumps of intensely basophilic chromatin. The cytoplasm is lightly basophilic and variable, with less cytoplasm in "resting" lymphocytes and more in "activity" lymphocytes.
- B Lymphocytes develop in bone marrow differentiate into plasma cells, which synthesize and secrete antibody molecules. They are also capable of antigen processing and presentation.
- T Lymphocytes develop in thymus.
- T helper cells can synthesize and secrete cytokines and function to regulate immune responses (both antibody and cell-mediated) when appropriately stimulated during immune responses.
- T cytotoxic cells are mature precursor cells that, as a result of a cell-mediated immune response, differentiate into cytotoxic T lymphocytes, which mediate cellular immunity vs. virus-infected and tumor cells.
- Natural Killer cells develop in several lymphoid tissues some in thymus, others in bone marrow not antigen-specific, but can recognize "self" cells active in early phases of cell-mediated immune responses synthesize and secrete cytokines that promote these responses.
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