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Hypersensitivity Reactions: Immune-Complex Reactions
Type III hypersensitivity reactions occur when antigen–antibody complexes form and activate the complement system. Normally, these complexes help the…
Type III hypersensitivity, or immune complex reactions, result from antibodies binding to soluble antigens in the circulation, forming immune complexes. These circulating complexes then activate the complement system, triggering an inflammatory cascade.
When not efficiently cleared by the immune system, these complexes can deposit in the walls of blood vessels and tissues. This deposition recruits complement proteins and neutrophils, which manifests as serum sickness. It is a systemic immune response characterized by fever, rash, joint pain, and lymph node enlargement. It can happen in response to some commonly used antibiotics, among other things.
Another example of the type III hypersensitivity is the Arthus reaction. In previously sensitized individuals, exposure to antigens, like those from insect bites or fungal spore inhalation, can lead to the formation of large quantities of antigen–antibody complexes at the exposure site. This triggers a localized inflammatory response, resulting in redness, swelling, and tissue damage at the site.
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Q1: What happens when antibodies bind to soluble antigens in the bloodstream?
When antibodies bind to soluble antigens in circulation, they form immune complexes that activate the complement system, triggering an inflammatory cascade. If the immune system fails to clear these complexes efficiently, they deposit in blood vessel walls and tissues, recruiting complement proteins and neutrophils, which causes inflammation and tissue injury.
Q2: What are the main symptoms of serum sickness?
Serum sickness is a systemic immune response characterized by fever, rash, joint pain, and lymph node enlargement. It typically appears 7-14 days after antigen exposure and results from immune complex deposition in tissues. The condition can occur in response to commonly used antibiotics and other substances, resolving once the antigen is eliminated.
Q3: How does the Arthus reaction differ from serum sickness?
The Arthus reaction is a localized Type III hypersensitivity occurring at the exposure site, while serum sickness is systemic. In the Arthus reaction, large antigen-antibody complexes form in previously sensitized individuals, causing redness, swelling, and tissue necrosis within 4-10 hours. Serum sickness develops systemically over days after antigen exposure.
Q4: What triggers the Arthus reaction in sensitized individuals?
The Arthus reaction occurs when antigens are introduced into the skin of sensitized individuals with high circulating IgG antibody levels, such as after repeated vaccination. Large immune complexes form rapidly at the injection site, triggering localized inflammation with redness, swelling, and tissue damage within hours of exposure.
Q5: What role do complement proteins and neutrophils play in immune complex reactions?
Immune complex deposits activate the complement system and recruit neutrophils to affected tissues. These cells trigger an inflammatory response that causes tissue injury and the characteristic symptoms of Type III hypersensitivity reactions. The recruitment of these immune components is responsible for the localized or systemic inflammation observed in both serum sickness and Arthus reactions.
Q6: Can immune complex reactions occur from inhaled antigens?
Yes, localized immune complex reactions can develop in the lungs following repeated exposure to inhaled antigens such as mold or animal proteins. These reactions cause conditions like farmer's lung and pigeon fancier's disease, which result from immune complex formation and deposition in lung tissue, similar to the mechanism of the Arthus reaction.
Q7: How do immune complexes normally help clear antigens from the body?
Normally, antigen-antibody complexes facilitate antigen clearance by enhancing phagocytosis and red blood cell-mediated removal. However, when large numbers of immune complexes are present, they overwhelm the clearance capacity, leading to tissue deposition and inflammation rather than efficient elimination of the antigen.