**Question:** Toll-like receptors are expressed in all except

A. Testis
B. Brain
C. Placenta
D. Skin

**Correct Answer:** D. Skin

**Core Concept:** Toll-like receptors (TLRs) are a family of pattern recognition receptors (PRRs) that play a crucial role in the innate immune system by recognizing conserved molecular patterns from pathogens, such as bacteria, viruses, fungi, and parasites, leading to inflammation, immune response activation, and tissue repair. TLRs are expressed on various immune cells, including monocytes, macrophages, dendritic cells, and neutrophils.

**Why the Correct Answer is Right:** Toll-like receptors are primarily expressed on immune cells, and their main function is to recognize pathogen-associated molecular patterns (PAMPs) and initiate immune responses. Hence, TLRs are not expressed on non-immune cells or tissues that do not have a role in immune response.

**Why Each Wrong Option is Incorrect:**

A. Testis: The testis, although containing immune cells, primarily functions in testosterone production and spermatogenesis, not immune response. TLRs are not expressed on testicular cells due to their specialized functions.

B. Brain: The brain is a non-immune system tissue and lacks TLR expression due to its specific functions, such as neural development, cognition, and memory.

C. Placenta: The placenta is an interface between the maternal and fetal circulations, facilitating nutrient and waste exchange during pregnancy. Although it contains immune cells, its primary function does not involve immune responses, explaining the lack of TLR expression.

D. Skin: The skin, as part of the immune system, has immune cells and plays a crucial role in immune responses against pathogens. Unlike the other options, TLR expression on skin cells allows the skin to recognize PAMPs and initiate immune responses against invading pathogens.

**Clinical Pearl:** Understanding the tissue-specific expression of TLRs helps in understanding the immune response mechanisms in different organs and tissues and how they contribute to the overall immune system. This knowledge is essential for interpreting clinical cases involving immune dysregulation or infections in these tissues.