Recognition molecules of innate immunity

Pattern recognition receptors (PRRs) recognize pathogen-associated molecular patterns (PAMPs), common structures found on broad classes of pathogens that are not on host cells. These host sensors are germ-line encoded and do not undergo somatic recombination.

There are 4 main groups of PRRs:
 * Membrane-bound phagocytic receptors (e.g. mannose receptor)
 * Membrane-bound signalling receptors (e.g. TLRs)
 * Cytoplasmic signalling receptors (e.g. NLRs)
 * Soluble proteins in serum (e.g. MBL)

Plasma membrane-bound
TLR-2/6

TLR-1/2

TLR-4 : LPS

TLR-5

Endosomal membrane-bound
In general, TLRs localized on endosomes recognize foreign nucleic acids. TLRs 3, 7 and 8 recognize viral nucleic acids.

TLR-3 : dsRNA

TLR-7 : ssRNA

TLR-8 : ssRNA

TLR-9 : DNA

NOD-like receptors (NLRs)
NLRs primarily recognize peptidoglycan.

RIG-1-like receptors (RLRs)
RLRs recognize viral RNA. Signalling pathways lead to the expression of type 1 IFNs.

Inflammatory cytokines
See inflammation

Upon pathogen recognition, resident macrophages release inflammatory cytokines (IL-1, IL-6 and TNF-α).

Specifically, IL-6 induces the acute-phase response, trigger changes in blood plasma protein concentrations.

C-reactive protein (CRP) and mannose-binding lectin (MBL)
See Complement activation

These proteins function like early-evolutionary antibodies. They can activate complement and function as opsonins.