Lecture 6 transport circulation_part 2
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Transcript of Lecture 6 transport circulation_part 2
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• lymphatic organs: red bone marrow, thymus gland, tonsils, spleen, lymph vessels, and lymph nodes
• lymphatic capillaries take up and return excess fluid to the bloodstream
• lacteals receive lipoproteins and transport them to the bloodstream
• helps defend body against disease
Lymphatic System
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Lymphatic System
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Lymphatic System
Spleen• Found in all vertebrates
• Mechanical filtration of red blood cells to remove old red blood cells
• Active immune response through humoral and cell-mediated pathways
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Lymphatic System
Thymus• Also an organ of the immune system
• Develops T-lymphocytes from hematopoietic progenitor cells
• Active immune response through humoral and cell-mediated pathways
• Thymus begins to atrophy during early teens as its stroma begins to get filled with adipose tissues
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Lymphatic System
Appendix• Blind-ended tube connecting to the caecum
• Shrunken remnant of the part of the caecum
• Vestigial organ in humans
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Lymphatic System
Appendix• found in the digestive tracts of many extant
herbivores• house mutualistic bacteria which help animals
digest the cellulose molecules that are found in plants
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Lymphatic System
Appendix• may harbour and protect bacteria that are beneficial in
the function of the human colon
• Appendicitis – inflammation of the appendix
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Villi of small intestine, showing blood vessels and lymphatic vessels
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Immune System
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Formed elements (45 %) – produced by bone marrow
Recall: BLOOD
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– in humans, secretions from sebaceous and sweat glands give the skin a pH ranging from 3 to 5, which is acidic enough to prevent colonization by many microbes
– microbial colonization is also inhibited by the washing action of saliva, tears, and mucous secretions
– these secretions contain antimicrobial proteins.• Lysozyme - digests the cell walls of many bacteria
First Line of Defense
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• microbes that penetrate the first line of defense face the second line of defense, which depends mainly on phagocytosis
• phagocytic cells called neutrophils constitute about 60%-70% of all white blood cells
• monocytes, about 5% of leukocytes, provide an even more effective phagocytic defense
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Second Line of Defense
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macrophage – develop from monocytes (5% of white blood cells)
– attack foreign microbes by phagocytosis– major component of the vertebrate lymphatic system
Second Line of Defense
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• eosinophils (1.5% of all leukocytes) - for defense against large parasitic invaders, such as the blood fluke, Schistosoma mansoni– position themselves against the external wall of a
parasite and discharge destructive enzymes from cytoplasmic granules
Second Line of Defense
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Schistosoma mansoni
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• Natural killer (NK) cells do not attack microorganisms directly but destroy virus-infected body cells– also attack abnormal body cells that could become
cancerous– mount an attack on the cell’s membrane, causing the
cell to lyse
Second Line of Defense
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• some microbes have evolved mechanisms for evading phagocytic destruction– outer capsules– Mycobacterium tuberculosis are readily engulfed but are
resistant to lysosomal destruction and can even reproduce inside a macrophage
Second Line of Defense
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• damage to tissue by physical injury or entry of microorganisms triggers a localized inflammatory response
Second Line of Defense
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– histamine is released by basophils and mast cells in connective tissue
– leukocytes and damaged tissue cells also discharge prostaglandins and other substances that promote blood flow to the site of injury
Second Line of Defense
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• chemokines secreted by blood vessel endothelial cells and monocytes, attract phagocytes to the area
– a group of about 50 different proteins
– bind to receptors on many types of leukocytes
– induce the production of toxic forms of oxygen in phagocyte lysosomes and the release of histamine from basophils
Second Line of Defense
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– fever, another systemic response to infection, can be triggered by toxins from pathogens or by pyrogens released by certain leukocytes
– resets the body’s thermostat
– higher temperature contributes to defense by inhibiting growth of some microbes, facilitating phagocytosis, and speeding up repair of tissues
Second Line of Defense
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– other antimicrobial agents include about 20 serum proteins, known collectively as the complement system.• carry out a cascade of steps that lead to lysis of
microbes• some complement components work with
chemokines to attract phagocytic cells to sites of infection
Second Line of Defense
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• interferons, - proteins secreted by virus-infected cells– diffuse to neighboring cells and induce them to produce
other chemicals that inhibit viral reproduction– limits cell-to-cell spread of viruses
Second Line of Defense
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• Lymphocytes are the key cells of the immune system
• two main types of lymphocytes: B lymphocytes (B cells) and T lymphocytes (T cells)
Third Line of Defense
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• a foreign molecule that elicits a specific response by lymphocytes is called an antigen
• Antigens react to specific antibodies that are either attached to lymphocytes or are secreted
Third Line of Defense
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• Antibodies constitute a group of globular serum proteins called immunoglobins (Igs)– a typical antibody molecule has two identical antigen-
binding sites specific for the epitope that provokes its production
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– an antibody interacts with a small, accessible portion of the antigen called an epitope or antigenic determinant
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• antigen receptors on a B cell are transmembrane versions of antibodies and are often referred to as membrane antibodies (or membrane immunoglobins)
• antigen receptors on a T cell, called T cell receptors, are structurally related to membrane antibodies but are never produced in a secreted form
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• there is an enormous variety of B and T cells in the body, each bearing antigen receptors of particular specificity
• this allows the immune system to respond to millions of antigens, and thus millions of potential pathogens
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• although a microorganism encounters a large repertoire of B cells and T cells, it interacts only with lymphocytes bearing receptors specific for its various antigenic molecules
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• the “selection” of a lymphocyte by one of the microbe’s antigens activates the lymphocyte
• stimulated to divide and differentiate• produce two clones of cells
– effector cells– memory cells
• clonal selection
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• the selective proliferation and differentiation of lymphocytes that occur the first time the body is exposed to an antigen is the primary immune response– selected B cells and T cells generate antibody-producing
effector B cells, called plasma cells, and effector T cells, respectively
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• a second exposure to the same antigen at some later time elicits the secondary immune response– faster (only 2 to 7 days), of greater magnitude, and more
prolonged– antibodies produced tend to have greater affinity for the
antigen than those secreted in the primary response
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• While B cells and T cells are maturing in the bone marrow and thymus, their antigen receptors are tested for potential self-reactivity– capacity to distinguish self from nonself continues to
develop as the cells migrate to lymphatic organs– autoimmune diseases
• Caused when the immune system mistakes its own cells as pathogens and attack them
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• T cells interact with one important group of native molecules
– collections of cell surface glycoproteins encoded by a family of genes called the major histocompatibility complex (MHC)
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– Two main classes of MHC molecules mark body cells as self:
• Class I MHC molecules, found on almost all nucleated cells
• Class II MHC molecules, restricted to macrophages, B cells, activated T cells, and those inside the thymus
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• two main types of T cells, each responds to one class of MHC molecule:
– Cytotoxic T cells (TC) have antigen receptors that bind to protein fragments displayed by the body’s class I MHC molecules
– Helper T cells (TH) have receptors that bind to peptides displayed by the body’s class II MHC molecules
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– Cytotoxic T cells respond by killing the infected cells– helper T cells send out chemical signals that incite other
cell types to fight the pathogen
(APC)
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Helper T-cell Signal Pathway
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• If the cell contains a replicating virus, class I MHC molecules expose foreign proteins that are synthesized in infected or abnormal cells to cytotoxic T cells– this interaction is greatly enhanced by a T surface
protein CD8 which helps keep the cells together while the TC cell is activated
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Complement System
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Five Classes of Immunoglobulins
• first to be produced after initial exposure to antigen
• promotes neutralization and cross-linking of antigens
• very effective in complement system
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Five Classes of Immunoglobulins
• most abundant Ig in blood
• promotes opsonization, neutralization and cross-linking of antigens
• only Ig that crosses placenta
• present in tissue fluids
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Five Classes of Immunoglobulins
• present in tears, saliva, mucus, and breast milk
• provides localized defense of mucous membranes by neutralization and cross-linking of antigens
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Five Classes of Immunoglobulins
• present on surface of B cells that have not been exposed to antigens
• acts as antigen receptor in the antigen-stimulated proliferation and differentiation of B cells
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Five Classes of Immunoglobulins
• present in blood at low concentrations
• triggers release from mast cells and basophils of histamine and other chemicals that cause allergic reactions
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Blood Transfusions
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Hemolytic Disease of the Newborn (Erythroblastosis fetalis)
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Summary of Immune Response