Unlike Exotoxins, they are typically heat resistant, active only in sizable amounts, and remain bound to cells. Mechanism of action is very diverse. Most important clinical problems are fever and shock. Can be lethal. Siderophores Iron plays special role in control of infection.
Most bacteria require iron to synthesize cytochromes.
But most bacteria have to find way to get iron or cannot grow. Mutants that cannot synthesize enterochelin lose virulence. These mutants can regain virulence if pure enterochelin is injected along with mutant bacteria.