Which of the following is not targeted by a Toll-like receptor?
a. Lipid A
b. Eukaryotic flagellar protein
c. Single-stranded RNA
d. Lipoteichoic acid

Verified step by step guidance

1. Step 1: Understand the role of Toll-like receptors (TLRs). TLRs are a class of proteins that recognize pathogen-associated molecular patterns (PAMPs) found on microbes, triggering an immune response.
2. Step 2: Identify the common ligands for TLRs. For example, lipid A (a component of lipopolysaccharide) is recognized by TLR4, single-stranded RNA is recognized by TLR7 and TLR8, and lipoteichoic acid (from Gram-positive bacteria) is recognized by TLR2.
3. Step 3: Consider the nature of eukaryotic flagellar proteins. Since flagella in eukaryotes are structurally different from bacterial flagella and are not typically recognized as PAMPs by TLRs, they are generally not targeted by TLRs.
4. Step 4: Compare each option to known TLR targets: lipid A (a), single-stranded RNA (c), and lipoteichoic acid (d) are all known TLR ligands, while eukaryotic flagellar protein (b) is not.
5. Step 5: Conclude that the correct answer is the option that is not a target of TLRs, which is the eukaryotic flagellar protein.

Key Concepts

Toll-like Receptors (TLRs) and Their Ligands

TLRs are pattern recognition receptors that detect conserved microbial molecules called pathogen-associated molecular patterns (PAMPs). Each TLR recognizes specific PAMPs such as bacterial lipids, nucleic acids, or proteins, triggering immune responses. Understanding which molecules are recognized by TLRs is key to answering the question.

Lipid A and Lipoteichoic Acid as PAMPs

Lipid A is a component of lipopolysaccharide (LPS) found in Gram-negative bacteria and is recognized by TLR4. Lipoteichoic acid is a component of Gram-positive bacterial cell walls and is recognized by TLR2. Both are common bacterial molecules targeted by TLRs to detect bacterial infections.

Recognition of Nucleic Acids and Flagellar Proteins by TLRs

Single-stranded RNA (ssRNA) from viruses is recognized by TLR7 and TLR8, triggering antiviral responses. Bacterial flagellin, a protein, is recognized by TLR5. However, eukaryotic flagellar proteins differ structurally and are not recognized by TLRs, making them an exception among the options.