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Resistance and the Immune System: Acquired Immunity
Chapter 19
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• Acquired Immunity responds to, distinguishes between and remembers specific pathogen it has encountered.
• Four Important Attributes:• Specificity
• Antigens are microbes or microbe parts that provoke an immune response
• Immune system recognizes small parts of the antigen called antigenic determinants or epitopes
• Immune deficiency is the loss of the body’s ability to respond to antigens and epitopes
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• Tolerance of Self• Regulatory T cells prevent other T cells from attacking “self”
cells• Autoimmune diseases occur when self-tolerance breaks down
• If nonimmunogenic molecules (haptens) are linked to proteins, they may not be recognized as “self”
• Thus they might provoke an immune response (allergies)
• Minimal “Self” Damage• An immune response must be strong enough to eliminate the
pathogen, yet controlled so as not to cause extensive damage
• Immunological Memory• Immunological memory is the ability to “remember” past
pathogen exposures • The body fights off any subsequent infections
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• The cornerstone of acquired immunity are the lymphocytes• B lymphocytes (B cells) are
involved in producing antibodies against epitopes
• T lymphocytes (T cells) provide resistance through lysis of infected or abnormal cells• Subsets: T helper cells, T
suppressor cells, T cytotoxic cells and T delayed type hypersensitivity
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• Humoral Immune Response• The humoral immune response involves:
• activation of B cells • production of antibodies against the identified antigen• Highly specific, body can generate antibodies against any
antigen or epitope
• Cell Mediated Immune Response• If the microbes enter cells, antibodies are useless
• Then the cell mediated immune response is activated to eliminate “nonself” cells
• T cells have receptor proteins; the highly specific receptor proteins on the lymphocyte surface implies that even before an antigen enters the body, immunocompetent B and T cells are already waiting.
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• In the fetus, lymphocytes arise from hematopoietic stem cells in the yolk sac and bone marrow• They develop into:
• Myeloid progenitors, which become:– red blood cells – most white blood cells
• Lymphoid progenitors, which become lymphocytes
• T lymphocytes are formed in the thymus
• B cells are formed in the bone marrow
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• Maturation of both T and B cells insertion of surface receptor proteins.
• B cells can recognize antigenic determinants on an antigen; T cells depends on surface receptor proteins + MHC proteins
• MHC: embedded in the membranes of all cell bodies• 2 classes (“self”): Class I MHC proteins (nucleated cells) and
Class II MHC (B lymphocytes, macrophages)
• Entry of antigen --- phagocytosis (macrophage) --- MHC-peptide complexes --- ANTIGEN PRESENTING CELLS (dendritic cells)
• Unprocessed antigens stimulate the immune response poorly.
Recognition Commitment
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• Responds to cells infected with pathogens
• Cellular Immunity Relies on T-Lymphocyte Receptors and Recognition• Cytotoxic T cells have T-cell receptors (TCRs) and CD8 coreceptor proteins• Naïve T cells have TCRs and CD4 coreceptor proteins
• Naïve T cells can help with both humoral and cell mediated immunity• HIV attaches to the CD4 receptor and infects the cell
• TCRs and coreceptors allow T cells to recognize and bind to the major histocompatibility complex (MHC)
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• Naïve T Cells Mature into Effector T Cells
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• Host cells infected by viruses can:• degrade viral antigens • present peptide fragments with MHC-1
proteins on the cell surface• Activated cytotoxic T cells recognize and
bind to the MHC-1/peptide complex on infected cells
• They release toxic substances such as perforin and granzymes to:• cause cell death • expose pathogens to antibodies
• T cells can also recognize and kill tumor cells
• Memory T lymphocytes – provide long term immunity
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• Antigen exposure activates only T and B cells with receptors that recognize specific epitopes on that antigen
• B and T cell clones contain lymphocytes that develop into effector cells that which target pathogens
• B cells – activation begins when antigen enter a lymphoid organ and bring antigenic determinants close to the appropriate B cells that can respond
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• TH2 Cells Initiate the Cellular Response to Humoral Immunity
• The Process of Antibody-Mediated Immunity
• T-dependent antigens
• T independent antigens
• Superantigens• Plasma Cells
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• Antibodies are of a class of proteins called immunoglobulins
• Epitope recognition requires antibodies to have a special structure of:• 2 identical heavy (H) chains • 2 identical light (L) chains
• Each light and heavy chain has:• A constant region, which
determines the location and functional class of the antibody
• A variable region, which contains different amino acids for the many antibodies produced
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• The variability allows formation of the specific antigen binding site
• The Fab fragment of an antibody combines with the Epitope
• The Fc fragment performs functions in:• opsonization• activation of the
complement system• allergic reactions
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• IgM is the first (but short-lived) Ig to appear in circulation after B cell stimulation• presence indicates a very recent infection; fetal infections
• IgG (gamma globulin) is the major circulating antibody• It provides immunity to the fetus and newborn (maternal
antibody)• Appears 24-48 h after antigenic stimulation (booster shots)
• IgA provides resistance in the respiratory and gastrointestinal tracts• It is found in colostrum; also located in tears, saliva
• IgE plays a role in allergic reactions by sensitizing cells
• IgD is a cell surface receptor on B cells; also called membrane antibody and is important in the activation of B cells
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• A primary antibody response occurs the first time the body encounters a pathogen
• A secondary antibody response is more powerful and sustained • It occurs with a subsequent infection by the same pathogen
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• Antibody Interactions Mediate the Disposal of Antigens (Pathogens)• Formation of antigen-antibody complexes result in the antigen:
• death• inactivation• increased susceptibility
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The membrane attack complex causes cell lysis
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