Which type of hypersensitivity reactions is seen in myasthenia gravis
**Core Concept**
Myasthenia gravis is an autoimmune disease characterized by weakness and rapid fatigue of the muscles under voluntary control. It occurs due to the production of autoantibodies against the acetylcholine receptor (AChR) at the neuromuscular junction.
**Why the Correct Answer is Right**
The autoantibodies in myasthenia gravis bind to the nicotinic acetylcholine receptors on the postsynaptic membrane of the neuromuscular junction, blocking the acetylcholine from binding and initiating muscle contraction. This leads to a decrease in the number of functional AChR, resulting in impaired neuromuscular transmission and muscle weakness. The type of hypersensitivity reaction involved in myasthenia gravis is an example of **Type II hypersensitivity**, where antibodies target and destroy specific cell surface receptors.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Type I hypersensitivity reactions involve IgE antibodies and are typically associated with allergic reactions, such as anaphylaxis or asthma. Myasthenia gravis is not an allergic reaction.
**Option B:** This option is incorrect because Type III hypersensitivity reactions involve immune complexes and are typically associated with diseases like systemic lupus erythematosus (SLE). Myasthenia gravis is not caused by immune complexes.
**Option C:** This option is incorrect because Type IV hypersensitivity reactions involve T-cell mediated responses and are typically associated with diseases like contact dermatitis or tuberculin skin test. Myasthenia gravis is not a T-cell mediated response.
**Clinical Pearl / High-Yield Fact**
The presence of thymoma (a tumor of the thymus gland) in patients with myasthenia gravis is a significant clinical correlation, as it is associated with a higher risk of thymectomy (surgical removal of the thymus gland) to improve symptoms.
**Correct Answer: D. Type II hypersensitivity.**