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4 Cards in this Set
- Front
- Back
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Define the term hypersensitivity and describe the main properties of the 4 types of hypersensitivity
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Hypersensitivity: pathological immune response to self-antigens or foreign antigens
Type I: "Immediate" --> occurs through B cell activation which releases IgE that attaches to mast cells and, upon attachment of IgE to an antigen, will stimulate mast cells to release damaging molecules (histamine, cytokines, etc); activation of eosinophils Type II: "Antibody-mediated" --> antibodies bind to antigen thereby stimulating opsonization and recruitment of leukocytes and macrophages Type III: "Antigen Complexes" Complement and Fc receptor-mediated recruitment and activation of leukocytes; ab and ag form complexes if Ag are in circulation --> get stuck in vascular walls and tissues. inflammation and damage of self tissues Type IV: "T cell-mediated (two types)" Delayed: CD4+ helper T cells secrete cytokines to recruit macrophages and other inflammatory leukocytes Cytolytic: CD8+ cytotoxic T cells kill cells expressing antigen directly |
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Describe the mechanisms by which antibodies cause tissue injury
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Only 3 of the 4 types of hypersensitive reactions are affected by antibodies.
Type I: IgE activates mast cells which release damaging histamines, cytokines, and proteases. Type II: Ab facilitate phagocytosis and damage of self cells and tissues by recruiting complement, macrophages, and inflammatory cells. If deposited on cells: combined action of complement and Fc receptors results in phagocytosis If deposited on solid surface: phagocytes may be activated that release toxic substances that induce inflammation and damage tissue. Ab can cause disease by interfering with normal molecules (example = Graves' disease when bound to thyroid receptor) Type III: Ab complex with circulating antigen and recruit neutrophils and macrophages which can damage surrounding tissue. |
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Describe the mechanisms by which helper T lymphcytes and cytotoxic T lymphocytes cause tissue injury
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CD4 T cell: involved in immediate and delayed hypersensitivity reactions.
Immediate: Th2 cells react strongly to antigen and release IL-4 --> stimulates B cell secretion of IgE --> tissue damage due to mast cell activation, also activates eosinophils, stimulates mucus secretion Delayed: Th1 and/or Th17 cells react to an antigen and release cytokines that recruit and activate macrophages (IFN-g released by Th1) and inflammatory cells (TNF released by Th1; IL-17 and IL-22 released by Th17. - also stimulates fibroblasts leading to fibrosis/granulomas CD8 T cell: directly kill cells expressing antigen (i.e.. infected liver cells in liver hepatitis) - can recruit macrophages and stimulate fibroblasts leading to fibrosis/granulomas |
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Distinguish between hypersensitivity and autoimmunity
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Hypersensitivity: pathological immunological response to a self antigen or foreign antigen
Autoimmunity: pathological immunological response to a self antigen. Thus, all autoimmune responses are hypersensitivity, but not vice versa! |
