An exotoxin is a diffusible protein that is secreted by a pathogen into the external medium. They are essentially a poison to animal or plant cells, and have the potential to cause significant illness and even death. For the most part, these proteins are produced to facilitate adhesion to, or invasion of the host. However, there are many others that are secreted to cause damage to the host's cells. This damage can be either physiological (manipulates normal cell function) or pathological (causes cell death).

Exotoxins vary in molecular structure, biological function, mechanism of secretion and immunological properties; the list is endless, and, thanks to the rapid mutation of microbiological species, growing by the day. They are, however, classifiable by their mode of action on animal cells:

• type I toxins – bind to cell surface receptors and stimulate transmembrane signals
• type II toxins – act directly on cell membranes, forming pores or disrupting lipid bilayers
• type III toxins – translocates an active enzymatic compound into the cell, which then interacts with an intracellular target molecule.

Examples of exotoxins include Tetanus toxin, Clostridium botulinum toxin (e.g., Botox), Shigella toxin, Diphtheria toxin.... It really is an endless list.

References

• Greenwood D, Slack R C B, Peuther J F, 2003, Medical Microbiology, 16th edition, Churchill Livingstone, 90