1. Mechanisms of Hypersensitivity
   1. Type I - IgE-mediated
      1. First Exposure
         1. 1. Host responds to an antigen
         2. 2. First response produces IgE
         3. 3. IgE binds to mast cells in body tissues
            1. Mast Cells
            2. GI cells
            3. Skin cells
            4. Respiratory Tract
            5. Mast Cell Function
            6. Mediate healing
            7. Mediate inflammatory response
         4. 4. The bond is at Fc and it is permanently attached
         5. 5. Mast cells become coated with IgE
         6. 6. Sufficient number of mast cells coated = host is sensitized
      2. Second Exposure
         1. 1. Mast cells are stimulated to degenerate
         2. 2. Mast cells release cell mediators (i.e. histamines)
         3. 3. Histamines initiate inflammatory response in the host tissue
      3. Atopic Individuals
         1. 1. Genetically predisposed for allergies
         2. 2. Produce more IgE when stimulated than the gen. pop.
         3. 3. Their mast cells have more Fc sites
   2. Type II - Tissue Specific
      1. Mechanisms by which Type II can affect cells
         1. 1. Cell destruction by antibody or complement
         2. 2. Destruction through phagocytosis
         3. 3. Destroyed by toxic products from neutrophils
         4. 4. cell mediated cytotoxicity
         5. 5. Causes cell to malfunction
   3. Type III - Immune complex-mediated
      1. Mechanism of function
         1. 1. Antibody binds to soluble antigens
         2. 2. Moves into the tissue through blood transport
         3. 3. Complement is released
         4. 4. Chemotaxis for neutrophils
         5. 5. Neutrophils attempt to ingest the immune complex but complex is attached to tissue
         6. 6. Tissue damage ensues
      2. Examples of Type III
         1. 1. Serum Sickness (Reynaud's)
            1. 1. Precipitation of immune complex in blood
            2. 2. Occurs when temperature is cold
            3. 3. Decreases blood flow to fingers and toes
         2. 2. Arthus
            1. 1. Occurs after repeated exposure
            2. 2. Form immune complexes in walls of blood vessels
            3. 3. Results - inflammatory response
   4. Type IV - Cell-mediated
      1. Method of Destruction
         1. 1. T lymphcytes attack cells directly
         2. 2. The attackers are either
            1. Cytotoxic cells
            2. kill cells directly
            3. Cytokine producing T cells
            4. Th cells - recruit and activate phagocytic cells
      2. Examples
         1. 1. Response to poison ivy
            1. a.k.a contact dermititis
         2. 2. Response to metals
         3. 3. Graft rejection (i.e. TB skin test)
         4. 4. Rheumatoid arthritis
2. Antigenic Targets of Hypersensitivity
   1. Allergy
   2. Autoimmunity
   3. Alloimmunity