Step by Step
PRR
TLRs are pattern recognition receptors
Unlike the antigen-specific receptors of adaptive immunity (which take days to develop), TLRs are pre-formed receptors on innate immune cells that recognize broad, conserved microbial patterns immediately — no prior exposure needed.
PAMP
What TLRs detect: PAMPs
PAMPs (Pathogen-Associated Molecular Patterns) are structural features shared broadly across classes of pathogens — not specific to one exact strain, but conserved enough to reliably signal "this is a pathogen."
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Specific TLRs and what they detect
TLR4 detects LPS (lipopolysaccharide, found on Gram-negative bacteria). TLR2 detects peptidoglycan (found on Gram-positive bacteria). TLR3, 7, and 8 detect viral RNA. TLR9 detects bacterial or viral CpG DNA motifs.
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The signaling pathway
Once a TLR binds its matching PAMP, it triggers a signaling cascade through NF-κB, a transcription factor that turns on genes for pro-inflammatory cytokines (TNF-α, IL-1, IL-6, IL-12) and type I interferons — mounting a rapid inflammatory and antiviral response.
Applied Walkthrough
1
A Gram-negative bacterium invades the body. Its outer membrane contains LPS, a PAMP recognized by TLR4 on innate immune cells like macrophages.
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Binding of LPS to TLR4 triggers the NF-κB signaling pathway inside the macrophage.
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NF-κB activation switches on genes producing pro-inflammatory cytokines (TNF-α, IL-1, IL-6, IL-12), driving local inflammation and recruiting more immune cells to the site.
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This entire response happens within minutes to hours of exposure — far faster than an adaptive immune response could mount, since TLRs don't require prior sensitization to this specific pathogen.
Exam Application
Exams test whether you can match specific TLRs to the PAMPs they detect (TLR4-LPS, TLR2-peptidoglycan, TLR3/7/8-viral RNA, TLR9-CpG DNA), and whether you understand the general PAMP → TLR → NF-κB → cytokine signaling sequence.
⚠ Common Trap
The most common trap is confusing which TLR detects which pathogen type — particularly mixing up TLR4 (Gram-negative LPS) with TLR2 (Gram-positive peptidoglycan). Anchor these to "4" for the classically-studied Gram-negative LPS receptor and "2" for the simpler Gram-positive cell wall component.
✓ Quick Self-Check
1. What does TLR4 recognize, and on what type of bacteria?
LPS (lipopolysaccharide), found on Gram-negative bacteria.
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2. What does TLR2 recognize, and on what type of bacteria?
Peptidoglycan, found on Gram-positive bacteria.
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3. What do TLR3, 7, and 8 have in common?
They all detect viral RNA.
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4. What transcription factor do TLRs activate to trigger cytokine production?
NF-κB.
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5. What is a PAMP?
A Pathogen-Associated Molecular Pattern — a broad, conserved structural feature shared across classes of pathogens, allowing innate immune cells to recognize threats without prior exposure.
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