The secretory component of immunoglobulin molecule is –
**Core Concept**
The secretory component of immunoglobulin is a crucial part of the immunoglobulin molecule that plays a significant role in its transport and secretion. It is a polypeptide chain that associates with the Fc region of immunoglobulins, particularly IgA, and facilitates their transport across epithelial surfaces.
**Why the Correct Answer is Right**
The secretory component is essential for the transport of IgA across mucosal surfaces, such as the gut, respiratory, and genitourinary tracts. This process is mediated by the polymeric immunoglobulin receptor (pIgR), which binds to the J-chain of IgA and facilitates its transcytosis across epithelial cells. The secretory component is then cleaved from the pIgR and remains attached to the IgA molecule, enabling its secretion into the mucosal lumen.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the secretory component is not a part of the Fab region of immunoglobulins. The Fab region is responsible for antigen binding, whereas the secretory component is involved in transport and secretion.
* **Option B:** This option is incorrect because the secretory component is not a type of immunoglobulin light chain. The light chains are either kappa (κ) or lambda (λ), whereas the secretory component is a distinct polypeptide chain that associates with the Fc region of IgA.
* **Option C:** This option is incorrect because the secretory component is not a receptor for immunoglobulins. The receptors for immunoglobulins are Fc receptors, which are expressed on the surface of various immune cells, such as macrophages and natural killer cells.
**Clinical Pearl / High-Yield Fact**
The secretory component is essential for the mucosal immune system, enabling the secretion of IgA into mucosal surfaces to prevent infections. Defects in the secretory component or the pIgR can lead to immunodeficiency disorders, such as IgA deficiency.
**Correct Answer: C. Secretory component.**