Which is the immunoglobulin which activates complement by alternate pathway?
**Core Concept**
The complement system is a part of the innate immune response that helps eliminate pathogens from the body. It can be activated through three main pathways: the classical pathway, the alternative pathway, and the lectin pathway. Each pathway has a specific mechanism of activation, and understanding these mechanisms is crucial for identifying the correct answer.
**Why the Correct Answer is Right**
IgA is the primary immunoglobulin responsible for activating the complement system through the alternative pathway. This occurs when IgA antibodies bind to the surface of pathogens, leading to the formation of C3 convertase, a key enzyme in the complement cascade. The alternative pathway is unique in that it can be activated without the presence of antibodies, but IgA antibodies can enhance this activation. The alternative pathway is also activated by the presence of mannose-binding lectin on the surface of pathogens.
**Why Each Wrong Option is Incorrect**
**Option A:** IgE is primarily involved in the activation of mast cells and basophils during allergic reactions, and it does not play a significant role in complement activation. While IgE can bind to pathogens, it does not activate the complement system through the alternative pathway.
**Option C:** IgM is the primary immunoglobulin responsible for activating the classical pathway of complement activation. IgM antibodies can bind to pathogens and form a large complex that activates the classical pathway, but it does not activate the alternative pathway.
**Option D:** IgG is also involved in the activation of the classical pathway of complement activation, similar to IgM. IgG antibodies can bind to pathogens and activate the classical pathway, but they do not activate the alternative pathway.
**Clinical Pearl / High-Yield Fact**
IgA is the primary immunoglobulin responsible for mucosal immunity, and it plays a crucial role in protecting the body from pathogens that enter through the mucous membranes. Understanding the role of IgA in complement activation is essential for appreciating its importance in mucosal immunity.
**✓ Correct Answer: B. Ig A**