Okay, so, for exotoxins of microbes (the toxins the microbes create and shed, as compared to the toxins formed within the microbe that are released as the microbe is lysed - destroyed - which are endotoxins), there can be a two part system referred to as A-B. They're two different toxins (proteins), but here the B helps the A into the cell. The B has the receptors to attach to the cell wall, which then causes phagocytosis - the cell "eats" it. The A part attaches to the B part, so when the B gets eaten, so does the A. Now, in most cases, the B doesn't do much of anything else. It's the A protein that causes the problems (acts as the toxin).
With me so far?
Clostridium difficile (C. diff), has two enterotoxins (toxins that work in your GI tract) called A and B.
Sigh. Really? REALLY? We couldn't call them C and D (you know, for C. diff)? One of the reasons this shit is so hard. Don't even get me started on "di-" versus "bis-" prefixes.