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Reazioni di ipersensibilità: reazioni da immunocomplessi
Le reazioni di ipersensibilità di tipo III si verificano quando si formano complessi antigene–anticorpo che attivano il sistema del complemento. Norma…
L'ipersensibilità di tipo III, o reazioni a complessi immunitari, derivano dal lego degli anticorpi ad antigeni solubili nella circolazione, formando complessi immunitari. Questi complessi circolanti attivano quindi il sistema del complemento, scatenando una cascata infiammatoria.
Quando non vengono eliminati efficacemente dal sistema immunitario, questi complessi possono depositarsi nelle pareti dei vasi sanguigni e dei tessuti. Questa deposizione recluta proteine del complemento e neutrofili, che si manifestano come malattia sierica. Si tratta di una risposta immunitaria sistemica caratterizzata da febbre, eruzione cutanea, dolori articolari e ingrossamento dei linfonodi. Può verificarsi in risposta ad alcuni antibiotici comunemente usati, tra le altre cose.
Un altro esempio di ipersensibilità di tipo III è la reazione di Arthus. Negli individui precedentemente sensibilizzati, l'esposizione ad antigeni, come quelli derivanti da punture di insetti o inalazione di spore fungine, può portare alla formazione di grandi quantità di complessi antigene–anticorpi nel sito di esposizione. Questo innesca una risposta infiammatoria localizzata, che provoca arrossamento, gonfiore e danni ai tessuti nel sito.
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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.