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Biology LibreTexts

16.3: Immediate Hypersensitivities: Type III

Skills to Develop

  1. Describe the mechanism for Type III (immune complex-mediated) hypersensitivity and give 2 examples.

Mechanism: This is caused when soluble antigen-antibody (IgG or IgM) complexes, which are normally removed by macrophages in the spleen and liver, form in large amounts and overwhelm the body (see Fig. 1). These small complexes lodge in the capillaries, pass between the endothelial cells of blood vessels - especially those in the skin, joints, and kidneys - and become trapped on the surrounding basement membrane beneath these cells (see Fig. 2). The antigen/antibody complexes then activate the classical complement pathway (see Fig. 3). This may cause:

a. Massive inflammation, due to complement protein C5a triggering mast cells to release inflammatory mediators;

b. Influx of neutrophils, due to complement protein C5a, resulting in neutrophils discharging their lysosomes and causing tissue destruction through extracellular killing and causing further inflammation (see Fig. 4 and Fig. 5);

c. MAC lysis of surrounding tissue cells, due to the membrane attack complex, C5b6789n;

d. Aggregation of platelets, resulting in more inflammation and the formation of microthrombi that block capillaries; and

e. Activation of macrophages, resulting in production of inflammatory cytokines and extracellular killing causing tissue destruction.

This can lead to tissue death and hemorrhage.

 

Examples include:

  • serum sickness, a combination type I and type III hypersensitivity;
  • autoimmune acute glomerulonephritis;
  • rheumatoid arthritis;
  • systemic lupus erythematosus;
  • some cases of chronic viral hepatitis; and
  • the skin lesions of syphilis and leprosy.

Summary

  1. Type III (immune complex-mediated) hypersensitivity is caused when soluble antigen-antibody (IgG or IgM) complexes, which are normally removed by macrophages in the spleen and liver, form in large amounts and overwhelm the body.
  2. These small complexes lodge in the capillaries, pass between the endothelial cells of blood vessels - especially those in the skin, joints, and kidneys - and become trapped on the surrounding basement membrane beneath these cells.
  3. The antigen/antibody complexes then trigger excessive activation of the classical complement pathway leading to a massive inflammatory response, influx of neutrophils with extracellular killing of body tissue, MAC lysis of tissue, and aggregation of platelets and macrophages.
  4. Examples include Serum sickness, autoimmune acute glomerulonephritis, rheumatoid arthritis, and systemic lupus erythematosus.

Questions

Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

  1. Describe the mechanism for Type III (immune complex-mediated) hypersensitivity and give 2 examples. (ans)

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)