The Fc piece of which Immunoglobulin fixes C1?
**Core Concept**
The Fc region of immunoglobulins (Ig) is responsible for interacting with various immune effector molecules, including the complement system. In this context, the Fc region of a specific immunoglobulin interacts with the first component of the complement system, C1.
**Why the Correct Answer is Right**
The correct immunoglobulin that fixes C1 is IgM. This is because IgM has a pentameric structure, which allows for multiple binding sites for the C1q component of the C1 complex. The C1q protein has a "collar-like" structure with six globular heads, each of which can bind to the Fc region of an IgM molecule. This binding event initiates the activation of the classical complement pathway, leading to the cascade of immune responses. The IgM molecule's high avidity for C1q is due to its pentameric structure, which provides multiple binding sites for C1q.
**Why Each Wrong Option is Incorrect**
* **Option A:** IgG does not fix C1 efficiently, as it has a monomeric structure and a lower avidity for C1q.
* **Option B:** IgA is primarily involved in mucosal immunity and does not interact with C1.
* **Option D:** IgE is involved in allergic reactions and does not interact with C1.
**Clinical Pearl / High-Yield Fact**
Remember that IgM is the primary immunoglobulin responsible for fixing C1 and initiating the classical complement pathway, which is crucial for eliminating pathogens and preventing infections.
**Correct Answer:** C. IgM.