When your immune system creates its own internal debris
In short: Imagine your body's defense system creating a waste product that it cannot clear. Instead of fighting a single invader, the immune system creates clusters of proteins that settle in your organs like sediment in a pipe, triggering a cycle of inflammation and tissue damage.
Type III hypersensitivity, also known as immune-complex hypersensitivity, occurs when antibodies bind to antigens to form clusters. Unlike Type II reactions, where antibodies target specific cells, the antigens in Type III reactions are free-floating. This allows the resulting complexes to circulate through the bloodstream and deposit in various tissues, including the blood vessels, kidneys, lungs, skin, and joints. The specific classes of antibodies involved in these reactions are IgG and IgM. These are the same antibody types that also participate in type II reactions. However, the way they cause damage is distinct because the antigens are not attached to any cells.
Once these complexes settle in the tissues, they trigger a cascade of destruction. The primary mechanism involves the activation of the complement system, which initiates an inflammatory response. This process releases tissue-damaging substances, such as local enzymes that destroy surrounding cells, and interleukin-1, a signaling protein that can induce systemic symptoms like fever. The inflammatory response is a direct result of the complement activation. This specific sequence of events leads to the release of enzymes that destroy tissues locally.
This mechanism underlies several serious autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis, and most types of glomerulonephritis. It can also be triggered by environmental factors. For example, inhaling fungal spores from moldy hay (farmer’s lung), proteins from pigeon dung (pigeon fancier’s lung), or protozoans from air-conditioning units (humidifier fever) can cause these reactions in sensitized individuals. Occupational hazards also contribute to these conditions in warm climates. Workers handling coffee waste or cotton may develop reactions to fungi growing on the materials. These specific triggers are often found in sugarcane waste as well.
While these reactions often cause acute symptoms like chest tightness and malaise that resolve quickly, repeated exposure can lead to permanent damage. Because the underlying issue is the accumulation of these complexes, the most effective treatment is the prevention of further exposure to the specific antigens that trigger the immune response. The symptoms of these reactions typically last for about a day or two. These issues recur every time the person encounters the antigen again. Because permanent damage is rare, the focus remains on avoiding the source.
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Sources
- Summary by Intellect · source: Hypersensitivity Type III (Immune complex hypersensitivity) - causes, pathophysiology
- Also: Immune system disorder - Type III Hypersensitivity | Britannica
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