The primary defensive cells in innate immune response are the Professional phagocytes. Macrophages, neutrophils and dendritic cells are called professional phagocytes.

The innate immune system relies on a large family of pattern recognition receptors (PRRs), which detect distinct evolutionarily conserved structures on pathogens, termed pathogen-associated molecular patterns (PAMPs).

PAMPs vs PRRs Difference
Upon PAMP recognition, PRRs activation transmits intracellular signal activating a multitude of intracellular signaling pathways, ultimately result in the activation of gene expression and synthesis of a broad range of molecules, including cytokines, chemokines, cell adhesion molecules, and immunoreceptors which together clear out the pathogen from the system or can activate adaptive immune response.



Present on the innate immune cells as cell surface receptors or cytosolic PRRs

Are evolutionary conserved structures on pathogens

PRRs can distinguish self cells and non self cells by recognizing  PAMPs

PAMPs are ‘signatures’ present only on pathogen

PRRs are generally glycoproteins

PAMPs can be carbohydrate, protein or even nucleic acids of bacteria and virus

PRRs include Toll like receptors (TLRs) TLRs are membrane-bound receptors localized at the cellular or endosomal membranes

PAMPs include flagellin protein that makes bacterial flagella, lipopolysaccharide layer of gram negative bacteria, peptidoglycan of bacterial cell wall, zymozan of yeast cell wall, nucleic acids of both bacteria and viruses.

Cytosolic PRRs, include retinoid acid-inducible gene I (RIG-I)-like receptors (RLRs) (393) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs)

Nucleic acids of virus and bacteria generally interacts with cytosolic PRRs

Some examples of PRRs and PAMPs interaction

  • Gram-negative bacteria are recognized by TLR4 via the lipid A portion of LPS
  • Lipoteichoic acid, lipoproteins, and peptidoglycan of gram-positive bacteria are detected by TLR2
  • Flagellin, the major constituent of the motility apparatus of flagellated bacteria, is recognized by TLR5
  • TLR3 recognizes intracellular dsRNA produced during viral replication
  • TLR7 and TLR8 are activated by single-stranded RNA (ssRNA)
Mogensen T. H. (2009). Pathogen recognition and inflammatory signaling in innate immune defenses. Clinical microbiology reviews, 22(2), 240–273.

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