Massive mortalities in hatchery-reared larvae of Argopecten purpuratus scallop are recurrent and have been associated regularly with Vibrio bacterial infections. Few studies have explored the immune system during the early development of scallops. To gain new insights on this subject, the expression patterns of four genes potentially involved in different phases of A. purpuratus immune response (sensing, signaling, and effectors) were assessed at different larval stages (trochophore, D-veliger, veliger, and pediveliger) and early juveniles from fall and spring spawning seasons. A Toll-like receptor (ApTLR), an inhibitor of nuclear factor kappa B (ApIκB), and two antimicrobial effectors (big defensin ApBD1 and ferritin Apfer1) were measured at each developmental stage. Gene expression was determined from control and challenged pools with V. splendidus VPAP18 (pathogenic strain) after 6, 15, and 24 h. Results revealed that expression patterns of all genes were consistent between the assessed spawning seasons. Regardless of the immune status, transcriptional levels of ApTLR, ApBD1, and Apfer1 gradually increased during ontogeny. ApIkB displayed a contrasted transcriptional pattern, with higher expression in the early ontogenic stages, suggesting its participation in other metabolic processes at earlier stages. Post-Vibrio challenge overexpression was observed mainly in the pediveligers and early juveniles for ApTLR and antimicrobial effectors, with ApBD1 showing the strongest and consistent response. Thus, ApBD1 protein availability was assessed by immunofluorescence. ApBD1 was present on every developmental stage (control and Vibrio challenged), but at a higher level in challenged pediveligers. Overall, results suggest a maturation of the immune system towards the late developmental stages (pediveligers and early juveniles) and a possible parental transfer of ApBD1. The present study contributes to scallop's early ontogeny immunobiology knowledge and for the establishment of the management disease process in hatcheries of A. purpuratus.