Toll like receptors are expressed in all Except
**Core Concept**
Toll-like receptors (TLRs) are a family of pattern recognition receptors that play a crucial role in the innate immune system. They recognize pathogen-associated molecular patterns (PAMPs) and trigger an inflammatory response to eliminate the invading microbe. TLRs are expressed on various cells, including immune cells, epithelial cells, and endothelial cells.
**Why the Correct Answer is Right**
TLRs are expressed on immune cells such as dendritic cells, macrophages, and neutrophils. They are also expressed on epithelial cells, including those in the respiratory, gastrointestinal, and genitourinary tracts. Additionally, TLRs are expressed on endothelial cells, which line blood vessels. However, TLRs are not typically expressed on platelets.
**Why Each Wrong Option is Incorrect**
**Option A:** While platelets do interact with the immune system, they are not typically known to express TLRs. Platelets primarily function in blood clotting and hemostasis.
**Option B:** This option is not provided in the question.
**Option C:** This option is not provided in the question.
**Option D:** This option is not provided in the question.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that TLRs are key components of the innate immune system and play a crucial role in recognizing pathogens and triggering an inflammatory response. Understanding the expression and function of TLRs can help you appreciate the complex interactions between the immune system and pathogens.
**Correct Answer: A. Platelets.**