Insect olfactory proteins: A comprehensive review with a special emphasis on the role of odorant‐binding proteins in insect immunity

The insect olfactory system is essential for survival, enabling the detection of chemical cues critical for feeding, reproduction, and avoiding threats. Semiochemicals, including pheromones and allelochemicals, are processed through specialized organs, primarily the antennae and maxillary palps, which contain sensilla housing olfactory receptor neurons (ORNs). Odorant-binding proteins (OBPs) transport volatile compounds to odorant receptors (ORs) on sensory neurons, initiating precise signal transduction. Rapid signal termination, vital for sensitivity, is achieved by odorant-degrading enzymes (ODEs) that prevent receptor saturation. Evolutionary adaptations optimize OBPs and ORs for species needs, such as the detection of foreign odors. Environmental factors, including temperature, nutritional state, and circadian rhythms, further modulate olfactory sensitivity. In this review article, we underline the interaction between olfactory proteins and insect immunity. Reports coming from different laboratories, point to the role of olfactory proteins in defense response, including its cellular, humoral, and behavioral aspects. Beyond chemosensitization, the olfactory system contributes to insect immunity by regulating pathogen recognition and immune signaling. OBPs interact with Toll-like receptors, regulating antimicrobial responses and gut microbiota stability. Symbiotic bacteria influence OBP expression, linking olfaction to systemic immunity. Finally, some odorant-binding proteins and chemosensory proteins possess direct antimicrobial activity. In conclusion, the insect olfactory system integrates sensory and immune functions through molecular and neuronal components, reflecting its evolutionary versatility. Blood-feeding insects, for example, Aedes aegypti or Rhodnius prolixus, exhibit heightened detection of host odors during reproductive cycles, while starved insects prioritize food-related cue.