From the Guidelines
Palpable purpura is a type III hypersensitivity reaction due to immune complex deposition in small blood vessels, as evidenced by the Gell and Coombs classification of hypersensitivity reactions 1. This classification system categorizes hypersensitivity reactions into four types, with type III being characterized by immune complex-mediated hypersensitivity, which includes conditions such as serum sickness, vasculitis, and palpable purpura.
Key Characteristics of Type III Hypersensitivity
- Immune complex deposition in vessel walls
- Activation of complement and attraction of neutrophils
- Release of inflammatory mediators and enzymes that damage vessel walls
- Leakage of red blood cells into surrounding tissues, resulting in palpable purpura Some common conditions associated with type III hypersensitivity reactions include:
- Leukocytoclastic vasculitis
- Henoch-Schönlein purpura
- Serum sickness As noted in a recent guideline for the diagnosis and management of eosinophilic granulomatosis with polyangiitis, palpable purpura is a vasculitis-specific lesion that can occur in this condition 1.
Diagnosis and Treatment
The diagnosis of palpable purpura is typically confirmed by skin biopsy, which shows immune complex deposition, neutrophilic infiltration, and fibrinoid necrosis of vessel walls. Treatment depends on the underlying cause, but may include:
- Removing the triggering antigen
- Administering corticosteroids to reduce inflammation
- Using immunosuppressive agents in severe or persistent cases It is essential to prioritize the most recent and highest-quality evidence when making clinical decisions, and in this case, the Gell and Coombs classification of hypersensitivity reactions provides a clear framework for understanding the pathophysiology of palpable purpura 1.
From the Research
Hypersensitivity Reactions
- Hypersensitivity reactions are categorized into four subtypes according to the type of immune response and the effector mechanism responsible for cell and tissue injury 2
- Type III hypersensitivity reactions are characterized by IgG/IgM immune complex mediated responses 2, 3
Palpable Purpura
- Palpable purpura is a hallmark of cutaneous vasculitis, which is often associated with immune complex formation and vessel damage 4
- The pathogenic mechanisms of palpable purpura appear to be complex, involving immune complex formation, vessel damage, and altered vessel function mediated by infectious agents or immunologic responses 4
- Cutaneous vasculitis, secondary to drug reaction, infection, or collagen vascular disease, is a common identifiable cause of palpable purpura 5