How does Innate immune response recognize pathogens? PRRs and PAMPs

 Second Line of Defence is innate form of non specific defence that removes pathogen entered into the body after bypassing first line of defense by means of defensive cells, proteins, fever and inflammation

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

How macrophages and other cells of innate immune response recognize pathogens?

Pathogen-Associated Molecular Patterns (PAMPs)

PAMPs include carbohydrate, polypeptide, and nucleic acid “signatures” that are expressed by viruses, bacteria, and parasites. This helps immune cells for self-non self recognition. These signatures are present only on pathogen so that self non self cells recognition is possible. Microbes of different biochemical composition and with entirely different life cycles are recognized by relatively similar mechanisms by host PRRs.

Examples of PAMPs: 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 etc.

Pattern Recognition Receptors (PRRs)

The immune cells  has specific cell surface receptors called pattern recognition receptors(PRRs) that recognize pathogen-associated molecular patterns (PAMPs). Once recognized or binding of PAMPs to PRR transmits intracellular signals leading to the transcription and translation of chemical messengers like cytokines or anti viral proteins like interferon. This activation triggers phagocytosis of pathogen by professional phagocytes.

Cytosolic PRRs, that recognize PAMPs inside cell include retinoid acid-inducible gene I (RIG-I)-like receptors (RLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs).

Examples of PRR include Toll like receptors (TLRs), scavenging receptors etc.

TLR2 binds to LPS, TLR 5 recognizes flagellin

More: Second Line of defense of Immune system Summary

           First Line of Defense summary

Reference:

Mogensen T. H. (2009). Pathogen recognition and inflammatory signaling in innate immune defenses. Clinical microbiology reviews, 22(2), 240–273. https://doi.org/10.1128/CMR.00046-08