Coreceptor for IL1RL2 in the IL-36 signaling system (By similarity). Coreceptor with IL1R1 in the IL-1 signaling system. Associates with IL1R1 bound to IL1B to form the high affinity interleukin-1 receptor complex which mediates interleukin-1-dependent activation of NF-kappa-B and other pathways. Signaling involves the recruitment of adapter molecules such as TOLLIP, MYD88, and IRAK1 or IRAK2 via the respective TIR domains of the receptor/coreceptor subunits. Recruits TOLLIP to the signaling complex. Does not bind to interleukin-1 alone; binding of IL1RN to IL1R1, prevents its association with IL1R1 to form a signaling complex. The cellular response is modulated through a non-signaling association with the membrane IL1R2 decoy receptor. Coreceptor for IL1RL1 in the IL-33 signaling system. Can bidirectionally induce pre- and postsynaptic differentiation of neurons by trans-synaptically binding to PTPRD (By similarity). May play a role in IL1B-mediated costimulation of IFNG production from T-helper 1 (Th1) cells (Probable). {ECO:0000250|UniProtKB:Q61730, ECO:0000269|PubMed:10799889, ECO:0000269|PubMed:9371760, ECO:0000305|PubMed:10653850, ECO:0000305|PubMed:19836339}. [Isoform 2]: Associates with secreted ligand-bound IL1R2 and increases the affinity of secreted IL1R2 for IL1B; this complex formation may be the dominant mechanism for neutralization of IL1B by secreted/soluble receptors (PubMed:12530978). Enhances the ability of secreted IL1R1 to inhibit IL-33 signaling (By similarity). . [Isoform 4]: Unable to mediate canonical IL-1 signaling (PubMed:19481478). Required for Src phosphorylation by IL1B. May be involved in IL1B-potentiated NMDA-induced calcium influx in neurons (By similarity). {ECO:0000250|UniProtKB:Q61730, ECO:0000269|PubMed:19481478}. This is the function of IL1RAP (interleukin 1 receptor accessory protein, ENSG00000196083).