hypersensitivity reaction
Chapter: 4
- Type I hypersensitivity
- This type of immunologically mediated injury occurs when there is a local or generalized reaction after an exposure to an antigen to which the person has become sensititized. Normally, allergists call the antigen an "allergen", since the most common type of Type I reaction would be what is commonly known as hay fever. The reaction is mediated by the binding of the antigen to specific IgE present on mast cells. The anaphylatoxins, C3a and C5a, are also involved (see Figure 4-3 page 72; Figure 4-5 page 107, big Rubin).
- Type II hypersensitivity
- Type II hypersensitivity involves an antigen-antibody reaction that is directed against cells, or more rarely, against extracellular membrane components such as collagen or basement membrane. IgM and IgG are the isotypes generally involved. When they bind to a cellular antigen, they activate the complement system, which forms a membrane attack complex that kills the cell. Some components of the complement system also serve to attract inflammatory cells to the site, which can cause further damage, through 2 mechanisms. First, complement can act as a opsonin in the process of opsonization, which identifies a cell for phagocytosis via C3b receptors on the PMN or macrophage cell membrane. Second, antibody bound to a cell can identify it for attack by natural killer cells, via Fc receptors. (see Figures 4-4 through 4-7, pages 73-75; Figures 4-6 through 4-9, page 108-110, big Rubin).
- Type III hypersensitivity
- Type III hypersensitivity reactions involve tissue injury mediated by immune complex formation (antigen-antibody complexes)(see Figures 4-8 through 4-9, page 76-77; Figures 4-10 and 4-11, pages 110-111, big Rubin). The antigen-antibody complexes are deposited in tissue, where they activate complement and cause tissue damage in a manner similar to that of type II reactions. The only real difference between type II and type III reactions, then, is in the location of the antigen. Type II reactions are against antigens located on cell surfaces or extracellular matrix components, whereas type III reactions occur against soluble or circulating antigens. Systemic lupus erythematosis and most types of glomerulonephritis are due to Type III immune injury.
- Type IV hypersensitivity
- Type IV hypersensitivity is the only one that does not involve antibody. Therefore, it is often called cell-mediated hypersensitivity, or delayed-type hypersensitivity. It is delayed because it takes the immune cells (macrophages and T-cells) a few hours or days to get there (see Figure 4-10, page 79; Figure 4-11. page 112, big Rubin). The classic example of delayed-type hypersensitivity is the purified protein derivative (PPD) Tb skin test.
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