The mechanism of pathogenesis of Bacillus thuringiensis (Bt) against insects is a complex process in which toxin interaction with larval midgut receptors is a key step. In Tribolium castaneum (Tc) larvae we have demonstrated by RNAi gene silencing that the Bt Cry3Ba toxin receptors Cadherin-like and Sodium solute symporter proteins act also as functional receptors of the less active Cry3Aa toxin. However, the toxicity outcome not only depends on the toxin interaction with specific receptors on larval midgut microvilli but also on how insects respond to toxin damage. Using qRT-PCR, we evaluated the systemic immune response of Tc larve after exposure to three Bt spore-crystal mixtures exhibiting differential activity against Tc (a mixture containing Cry3Ba toxin more active than that containing Cry3Aa toxin and the lepidopteran specific Cry1Ac spore-toxin mixture, non-active against Tc larvae) by comparing the expression of defense related genes encoding Apolipoprotein-III and two antimicrobial peptides Defensin3 and Defensin2. The Cry1Ac spore-toxin mixture showed no induction of any of the genes analyzed relative to non-intoxicated larvae. In contrast, all three genes were up-regulated following Cry3Ba spore-crystal intoxication whereas only Defensins gene expression was induced upon Cry3Aa spore-crystal treatment, evidencing a possible association between host immune response and larval susceptibility to Bt. Up-regulation of the antimicrobial peptide Defensin 3 was the highest following Cry3Ba spore-toxin intoxication of the larvae. Finally, we analyzed the antimicrobial spectra of two T. castaneum Defensin3 and Defensin2 peptide fragments, evaluating their activity against the three human microbial pathogens, Escherichia coli, Staphylococus aureus and Candida albicans. Results support T. castaneum antimicrobial peptides might have a potential therapeutical use tackling antibiotic-resistant infections.