Type-2 pulmonary epithelial cells secrete
**Core Concept**
Type-2 pulmonary epithelial cells, also known as Clara cells, are a subtype of alveolar epithelial cells found in the lung. They play a crucial role in maintaining lung homeostasis and are involved in the secretion of various substances, including surfactant proteins and enzymes.
**Why the Correct Answer is Right**
Type-2 cells are responsible for the secretion of Surfactant Protein D (SP-D). SP-D is a collectin protein that plays a key role in the innate immune response of the lung, particularly in the recognition and clearance of pathogens and other foreign particles. It also helps to regulate the inflammatory response and maintain the integrity of the alveolar epithelium. The secretion of SP-D by type-2 cells helps to protect the lung from infection and injury.
**Why Each Wrong Option is Incorrect**
**Option A:** Type-2 cells do not secrete surfactant proteins B and C, which are produced by type-1 cells.
**Option B:** Type-2 cells do not produce cytokines, which are typically secreted by immune cells such as macrophages and T-lymphocytes.
**Option C:** Type-2 cells do not secrete mucins, which are glycoproteins produced by goblet cells in the airway epithelium.
**Clinical Pearl / High-Yield Fact**
Surfactant proteins, including SP-D, play a critical role in maintaining lung function and preventing respiratory distress syndrome. Deficiencies in surfactant proteins have been implicated in various lung diseases, including chronic obstructive pulmonary disease (COPD) and acute respiratory distress syndrome (ARDS).
**Correct Answer: D. Surfactant Protein D.**