Table 1.

Pattern recognition receptors in animals that have homologs in cnidarians and that could play a role in cnidarian-dinoflagellate recognition^a

PRRb	Descriptionc	MAMPs recognized
TLRs	Large group of transmembrane proteins (10 paralogs in humans); extracellular domain, set of LRRs; intracellular domain, TIR domain launches signal transduction pathway to activate NF-κB	LRRs bind a large variety of MAMPs, including glycans, LPS, PG, double-stranded RNA, flagellin, GPI anchors, unmethylated CpG DNA
Lectins	Secreted forms, e.g., MBL; membrane anchored forms, e.g., diverse C-type lectins	Glycans on glycolipids or glycoproteins
Scavenger receptors	Diverse, multidomain, cell surface glycoproteins; referred to as molecular flypaper due to their ability to bind a diversity of MAMPs; some mediate phagocytosis	Broad range, including LPS, double-stranded RNA, polyanionic ligands
Complement receptors	Bind complement in phagocytes, leading to phagocytosis activation	Complement and microbes opsonized with complement
NODs	Like TLRs, presence of numerous LRRs; presence of 1 or more CARD domains, thereby activating downstream pathways via CARD-CARD interactions; large family (as many as 30 in humans); evolutionarily ancient (plants to humans)	Recognize intracellular MAMPs, including LPS

^aNot all known PRRs are included. This information is compiled from several reviews (173, 233, 377).

^bTLR, Toll-like receptor; NOD, nucleotide-binding oligomerization domain protein.

^cLRR, leucine-rich repeat; MBL, mannose-binding lectin.