Table 1.

Pattern recognition receptors of the innate immune response and their major known natural ligands

Receptor or related structure	Cell type or soluble factor	Examples of known natural microbial ligands
CD14	Myeloid cells and soluble forms	PAMPs from bacterial, fungal, and mycobacterial antigens
Mannose binding lectin	Soluble factor	Binds to mannosides found on bacteria and fungi; activates complement and opsonin for neutrophils
C-reactive protein	Soluble protein	Opsonin for Gram-positive bacteria
LPS-binding protein	Soluble protein	Binds to lipopolysaccharide in Gram-negative bacteria and lipoteichoic acid Gram-positive bacteria
C'3, alternative complement	Soluble proteins	Polysaccharide capsules of bacteria, fungi
MD1	B cells	Coreceptor for LPS on cell surface of B cells
MD2	Myeloid cells	Coreceptor for LPS on macrophages, neutrophils
TLR1	Myeloid cells	Lipopeptide, lipoteichoic acid, LPS of leptospirosis
TLR2	Myeloid cells	Peptidoglycan, lipopeptide, lipoarabinomannan, fungal cell wall components, LPS of leptospirosis
TLR3	Myeloid cells	Double-stranded viral RNA
TLR4	Myeloid cells	LPS, respiratory syncytial virus proteins
TLR5	Myeloid cells	Flagellin from Gram-positive or Gram-negative bacteria
TLR6	Myeloid cells	Zymosan (fungal constituents) along with TLR2
TLR9	Dendritric cells, B cells,	Unmethylated CpG motifs in prokaryotic DNA epithelial cells

LPS, lipopolysaccharide; PAMP, pathogen associated molecular pattern; TLR, Toll-like receptor.