Toll like receptors are expressed by
**Core Concept**
Toll-like receptors (TLRs) are a family of pattern recognition receptors that play a crucial role in the innate immune response by recognizing conserved pathogen-associated molecular patterns (PAMPs). They are expressed on various cell types, including immune cells, epithelial cells, and endothelial cells.
**Why the Correct Answer is Right**
TLRs recognize PAMPs through their extracellular domain, which is composed of leucine-rich repeats. Upon recognition, TLRs undergo oligomerization and recruit adaptor molecules, such as MyD88, to activate downstream signaling pathways. This leads to the production of pro-inflammatory cytokines and type I interferons, which are essential for the elimination of pathogens. TLRs are expressed on antigen-presenting cells (APCs), such as dendritic cells and macrophages, as well as on epithelial cells and endothelial cells, where they play a critical role in the recognition of pathogens and the initiation of immune responses.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too broad and does not specify the type of cells that express TLRs.
**Option B:** This option is incorrect because it is not a type of cell that is known to express TLRs.
**Option C:** This option is incorrect because it is a different type of receptor that is involved in the recognition of pathogens, but it is not the same as TLRs.
**Clinical Pearl / High-Yield Fact**
TLRs are essential for the recognition of pathogens and the initiation of immune responses, and their dysfunction has been implicated in various inflammatory and autoimmune diseases, such as asthma and rheumatoid arthritis.
**Correct Answer:** D.