**Question:** Toll like receptors are present in

A. Innate immune system
B. Adaptive immune system
C. Neurons
D. Endothelial cells

**Core Concept:** Toll-like receptors (TLRs) are a group of pattern recognition receptors (PRRs) that play a crucial role in the immune system. TLRs recognize pathogen-associated molecular patterns (PAMPs) and play a vital role in initiating the immune response against pathogens.

**Why the Correct Answer is Right:** Toll-like receptors are primarily present in the innate immune system, where they recognize PAMPs from various microorganisms and initiate the immune response. In this context, option A is correct as it highlights the involvement of TLRs in the innate immune system.

**Why Each Wrong Option is Incorrect:**

Option B (Adaptive immune system) is incorrect because TLRs are part of the innate immune system, not the adaptive immune system which is responsible for generating a more specific and long-lasting immune response.

Option C (Neurons) is incorrect because TLRs are not found in neurons, which are part of the nervous system and not directly involved in immune responses.

Option D (Endothelial cells) is partially correct, but the focus should be on TLRs' role in innate immunity rather than their presence in endothelial cells alone.

**Why Option D is Partially Correct:** TLRs are expressed on various immune cells, including endothelial cells. However, the primary function of TLRs is in initiating the immune response, not just their presence in endothelial cells.

**Clinical Pearl / High-Yield Fact:** Understanding the location and role of TLRs is essential for understanding the immune response and inflammation. TLRs are involved in recognizing pathogens, initiating immune response, and coordinating immune cells to eliminate pathogens. This knowledge is crucial in understanding infections, inflammation, and autoimmune diseases.