What type of exotoxin includes an active portion and a binding portion?

AB toxins are characterized by their structure, which comprises two distinct components: an active portion (A) that has enzymatic activity, and a binding portion (B) that is responsible for attaching to the target cell. This arrangement allows the toxin to enter the host cell effectively—first, the binding portion facilitates the entry of the active portion, which then exerts its toxic effect inside the cell.

For example, many well-known AB toxins, such as diphtheria toxin and cholera toxin, demonstrate this mechanism. The B part specifically binds to receptors on the host cells, ensuring that the A part can deliver its toxic effect, such as inhibiting protein synthesis or disrupting cell signaling pathways.

The other types of toxins listed—Type I, Type II, and Type III toxins—do not share this specific structure. Type I toxins are typically surface-active and do not enter cells because they act on cell membranes, while Type II toxins primarily damage cell membranes directly. Type III toxins generally refer to a broader category of virulence factors but lack the distinct A and B subtypes associated with AB toxins.