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Immune System
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broad range of microbes
Acquired immunity allows the immune system to "remember" pathogens and prevent infection when that pathogen is encountered again INNATE IMMUNITY Rapid responses to a broad range of microbes ACQUIRED IMMUNITY Slower responses to specific microbes External defenses Internal defenses Skin Mucous membranes Secretions Phagocytic cells Antimicrobial proteins Inflammatory response Natural killer cells Humoral response (antibodies) Cell-mediated response (cytotoxic lymphocytes) Invading microbes (pathogens)
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Phagocytic leukocyte Pathogen
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Bacteria are surrounded Microbes
MACROPHAGE Vacuole Lysosome containing enzymes Bacteria are engulfed Vacuole forms around bacteria The vacuole fuses with a lysosome Enzymes break down the bacteria Bacteria fragments exit the cell
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The most numerous phagocytic leukocyte in the human body are neutrophils
When tissues are infected, they send out chemical signals that attract neutrophils
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Two other types of phagocytic leukocytes are eosinophils and dendritic cells
Eosinophils engulf parasites Dendritic cells are important for the development of acquired immunity
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Fluid, antimicrobial proteins, and clotting elements move
Pathogen Pin Macrophage Chemical signals Capillary Phagocytic cells Red blood cell Blood clotting elements Blood clot Phagocytosis Fluid, antimicrobial proteins, and clotting elements move from the blood to the site. Clotting begins. 2 Chemical signals released by activated macrophages and mast cells at the injury site cause nearby capillaries to widen and become more permeable. 1 Chemokines released by various kinds of cells attract more phagocytic cells from the blood to the injury site. 3 Neutrophils and macrophages phagocytose pathogens and cell debris at the site, and the tissue heals. 4
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Blood, lymph, and lymphoid tissues (lymph nodes, spleen, and others)
Bone marrow Lymphoid stem cell Thymus B cell T cell Blood, lymph, and lymphoid tissues (lymph nodes, spleen, and others)
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An antigen may have several different epitopes, all of which are capable of being recognized by a different receptor Antigen- binding sites Antibody A Antigen Antibody B Antibody C Epitopes (antigenic determinants)
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Antigen-binding site Antigen-binding site Variable regions Constant
Plasma membrane B cell Cytoplasm of B cell
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Antigen- Binding site Variable regions Plasma membrane T cell Constant
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Infected cell A fragment of foreign protein (antigen) inside the cell associates with an MHC molecule and is transported to the cell surface. 1 Antigen fragment Class I MHC molecule The combination of MHC molecule and antigen is recognized by a T cell, alerting it to the infection. T cell receptor (a) Killer T cell
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Microbe Antigen- presenting cell A fragment of foreign protein (antigen) inside the cell associates with an MHC molecule and is transported to the cell surface. Antigen fragment Class II MHC molecule The combination of MHC molecule and antigen is recognized by a T cell, alerting it to the infection. T cell receptor Helper T cell
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Antibody concentration
(arbitrary units) 104 103 102 101 100 7 14 21 28 35 42 49 56 Time (days) Antibodies to A to B Primary response to antigen A produces anti- bodies to A 2 Day 1: First exposure to 1 Day 28: Second exposure to antigen A; first antigen B 3 Secondary response to anti- gen A produces antibodies to A; primary response to anti- gen B produces antibodies to B 4
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Humoral immune response Cell-mediated immune response
First exposure to antigen Intact antigens Antigens engulfed and displayed by dendritic cells Antigens displayed by infected cells Activate Gives rise to B cell Helper T cell Cytotoxic T cell Plasma cells Memory B cells Active and memory helper T cells Memory cytotoxic T cells Active cytotoxic T cells Secrete antibodies that defend against pathogens and toxins in extracellular fluid Defend against infected cells, cancer cells, and transplanted tissues Secreted cytokines activate
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A dendritic cell engulfs a bacterium
A dendritic cell engulfs a bacterium. The antigen is displayed on the dendritic cell’s class II MHC surface molecule. This is antigen presentation. The helper T cell binds to the antigen and is activated. 1 Cytotoxic T cell Dendritic cell Peptide antigen Helper T cell Class II MHC molecule Attack on infected cells Bacterium 2 3 1 CD4 Secretion of antibodies by plasma cells Dendritic cell Cytokines B cell The helper T cell makes multiple copies of itself (clonal selection) and releases chemical messengers. 2 3 The chemical messengers target killer (cytotoxic) T cells and B cellls, activating them
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Killer T cells recognize infected cells and bind to them.
The activated killer T cell punches holes in the infected cell’s membrane and injects it with a poison. 1 2 3 The infected cell dies. Killer T cells recognize infected cells and bind to them. Cytotoxic T cell Released cytotoxic T cell Cancer cell 1 CD8 3 Class I MHC molecule 2 Target cell Peptide antigen Cytotoxic T cell
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