Table 1.

Innate immune receptors on microglia.

Receptor	Functions/comments	References
TLR	Pattern-recognition receptors that respond to self (DAMPs) and nonself (PAMPs) activators. Microglia are known to express TLR1-9. TLRs are implicated in neuroinflammation in response to bacterial and viral infections, Alzheimer's disease, prion diseases, and amyotrophic lateral sclerosis.	[59, 69]
NLR	Cytoplasmic pattern-recognition receptors. Microglia are known to express NOD2 in response to CNS infection and NALP3 inflammasome in Alzheimer's disease.	[109, 110]
Scavenger	Another group of pattern-recognition receptors. The receptors expressed on microglia are Class A, CD36, and RAGE.	[111, 112]
RLR	RIG-I is a pattern-recognition receptor that is expressed by microglia in response to viral infections.	[110, 113]
Complement	Complement receptors expressed include CR1, CR3 and CR4. These receptors bind complement proteins and activate complement pathway which is considered to be both beneficial and detrimental depending on the level of activation.	[114]
Cytokines	Some of the cytokine receptors expressed in microglia are IL-1R, TNFR (responsible for proinflammatory actions of cytokines IL-1 and TNF-α resp.), IL-10R, TGFR (responsible for the anti-inflammatory cytokines IL-10 and TGF-β), and CCR1-5 responsible for actions of chemokines. These are expressed and produced in neuroinflammation.	[115, 116]

TLR: toll-like receptor; DAMP: damage-associated molecular pattern; PAMP: pattern-associated molecular pattern; NLR: NOD-like receptors; NOD: nucleotide-binding and oligomerisation domain; RLR: RIG-like receptors; RIG: retinoic acid-inducible gene; CR: complement receptor; IL: interleukin; TNF: tumour necrosis factor; TGF: transforming growth factor.