Transcription factor that plays a key role in the humoral immune response as part of the peptidoglycan recognition protein (IMD) signaling pathway (PubMed:10619029, PubMed:8816802, PubMed:11269501, PubMed:11872802, PubMed:22022271, PubMed:29924997). Rel-p68 subunit translocates to the nucleus where it binds to the promoter of the Cecropin A1 gene and probably other antimicrobial peptide genes (PubMed:11269501, PubMed:29924997). I-kappa-B kinase complex (IKKbeta and key) and PGRP-LC are essential signaling components in transmitting the lipopolysaccharide (LPS) signal leading to cact degradation for NF-kappa-B (rel) activation (PubMed:11018014, PubMed:11872802). Part of a Toll-related receptor pathway that functions in the apoptosis of unfit cells during cell competition (PubMed:25477468). Also part of some antiviral immunity: activated downstream of Sting signaling, which detects double-stranded RNA (dsRNA) from viruses, and promotes expression of antiviral effector genes (PubMed:29934091, PubMed:30119996, PubMed:33262294, PubMed:34261127, PubMed:34261128). May be part of a NF-kappa-B and Tollo signaling cascade that regulates development of the peripheral nervous system (PubMed:18000549). Possibly post-transcriptionally regulates the neuron-specific genes sc and ase, by promoting the rapid turnover of their transcripts in the wing imaginal disk (PubMed:18000549). Rel-p68 subunit interacts with Dredd (PubMed:11269502). Interacts with DMAP1 (PubMed:24947515). Interacts with akirin; interaction is immune stimulation-dependent; activates selected rel target gene promoters (PubMed:25180232). Expressed both maternally and zygotically. By bacteria and fungi. Phosphorylated by lipopolysaccharide (LPS)-activated I-kappa-B kinase complex before being cleaved. Rel-p110 subunit is cleaved within seconds of an immune challenge into Rel-p49 subunit and Rel-p68 subunit. Rel-p110 subunit reappears after 45 minutes. Larvae infected with Gram-negative bacteria fail to activate tracheal expression of the antibacterial peptide gene Drs (PubMed:22022271). Ectopic bristles develop on the dorsocentral, scutellar and lateral regions of the noctum, with one or more ectopic bristles per hemi-notum (PubMed:18000549).