Hypersensitivity reactions like contact dermatitis can be caused by
**Core Concept**
Hypersensitivity reactions, including contact dermatitis, are caused by an exaggerated immune response to specific substances, leading to inflammation and tissue damage. This reaction involves the activation of immune cells, such as T lymphocytes and macrophages, which release various chemical mediators.
**Why the Correct Answer is Right**
The correct answer is **IgE**. In contact dermatitis, the allergen is processed by antigen-presenting cells, which present the allergen to T lymphocytes, leading to their activation. Activated T lymphocytes then release cytokines, which in turn activate eosinophils and basophils, leading to the release of histamine and other chemical mediators. **IgE**, a type of antibody, plays a crucial role in this process by binding to the allergen and facilitating its presentation to T lymphocytes.
**Why Each Wrong Option is Incorrect**
**Option A:** IgG is incorrect because it is a type of antibody that provides long-term immunity, but it is not directly involved in the hypersensitivity reaction of contact dermatitis.
**Option B:** Complement system is incorrect because while it plays a role in the inflammatory response, it is not the primary cause of contact dermatitis.
**Option C:** Neutrophils are incorrect because while they are involved in the inflammatory response, they are not the primary cause of contact dermatitis.
**Clinical Pearl / High-Yield Fact**
The Hanifin-Rajka criteria can be used to diagnose atopic dermatitis, which is a type of contact dermatitis. The criteria include the presence of chronic or relapsing dermatitis, a personal history of atopy, and the absence of other dermatological conditions that could cause the symptoms.
**Correct Answer:** D.