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Immune Disorders: Immunodeficiencies, Autoimmunity, and Hypersensitivities

In June 1981, the Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, published a report of an unusual cluster of five patients in Los Angeles, California. All five were diagnosed with a rare pneumonia caused by a fungus called Pneumocystis jirovecii (formerly known as Pneumocystis carinii). Why was this unusual? Although commonly found in the lungs of healthy individuals, this fungus is an opportunistic pathogen that causes disease in individuals with suppressed or underdeveloped immune systems. The very young, whose immune systems have yet to mature, and the elderly, whose immune systems have declined with age, are particularly susceptible.

Figures (2)

Rheumatoid arthritis
Figure 21.1 The Worldwide AIDS Epidemic (a) As of 2008, more than 15 percent of adults were infected with HIV in certain African countries. This grim picture had changed little by 2012. (b) In this scanning electron micrograph, HIV virions (green particles) are budding off the surface of a macrophage (pink structure). (credit b: C. Goldsmith)

Key Points

  • Multiple sclerosis (MS) is a neurodegenerative disease in which the immune system attacks myelin, a protective covering of nerve fibers in the central nervous system, causing communication problems between the brain and the rest of the body.
  • Type I hypersensitivity is also known as immediate hypersensitivity because it occurs within seconds to minutes of exposure.
  • Type II hypersensitivity reaction refers to a reaction in which antibodies (classically IgG or IgM) are directed against cellular or extracellular matrix antigens with the resultant cellular destruction, functional loss, or damage to tissues.
  • Complement levels can often provide a hint that type III hypersensitivity is occurring: a drop in C4 and C3 implies classical pathway activation, often associated with SLE.
  • Type IV hypersensitivity reactions are classically described as delayed-type hypersensitivities (DTH) because onset of symptoms following exposure takes 48–72 hours.
  • Contact dermatitis (IVa) is diagnosed via patch testing, where suspected allergens are applied to the skin and read at 48–72 hours.

Terms

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