AB toxins are proteins often used by pathogens as an attack molecule. However, some organisms use them as a defense molecule, when threatened by a predator. This PhD thesis provides insights on the understanding of the molecular mechanism of two AB toxins, representing the two distinct roles of attack and defense. Cholera toxin (CT) and its cousin LT from Escherichia coli are very similar proteins, both causing diarrheal diseases with different severity. In this work, we investigated the structural differences between the two toxins. We then provided a hypothesis to the difference in toxicity and disease severity, which may lead to the design of better therapeutics. The second target is a fungal toxin: Coprinopsis cinerae toxin 2 (CCTX2). CCTX2 is a defense protein used by the fungus against nematodes attacks. This work describes the molecular structure of the protein and explores its potential function. This work on CCTX2 paves the way for the design of safer nematicides.