Toll-like receptors (TLRs) attach to all of the following except:

• A. Host cell DNA
• B. Lipopolysaccharides
• C. Peptidoglycan
• D. Double-stranded RNA

Answer: (A)

Verified step by step guidance

1. Understand the role of Toll-like receptors (TLRs): TLRs are a class of proteins that play a key role in the innate immune system by recognizing molecules that are broadly shared by pathogens but distinguishable from host molecules.
2. Identify the typical ligands for TLRs: TLRs commonly recognize pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharides (components of Gram-negative bacterial outer membranes), peptidoglycan (found in bacterial cell walls), and double-stranded RNA (a molecular pattern associated with viral infections).
3. Consider the nature of host cell DNA: Since TLRs are designed to detect foreign molecules, they generally do not bind to host cell DNA, as this would trigger an inappropriate immune response against the body's own cells.
4. Compare each option to the known ligands of TLRs: Lipopolysaccharides, peptidoglycan, and double-stranded RNA are all recognized by specific TLRs, whereas host cell DNA is not a target for TLR binding under normal conditions.
5. Conclude that the exception is host cell DNA because TLRs do not attach to self-molecules like host DNA, but rather to foreign microbial components.