University of Michigan - Ann Arbor

Exotoxins can be grouped into three categories based on their target:
- Intracellular targeting
- Membrane disrupting
- Superantigens

The Three Classes of Exotoxins

Intracellular targeting toxins consist of two parts: A for activity and B for binding, and are known as A-B exotoxins. The B component determines the toxin's cellular specificity and facilitates its attachment to specific cell surface receptors. After the A-B toxin binds to the host cell, it is taken inside by endocytosis and trapped in a vacuole. As the vacuole acidifies, the A and B subunits separate. The A subunit then enters the cell cytoplasm and disrupts a specific internal cellular function.

Intracellular Targeting Exotoxins

Two membrane-disrupting exotoxins are **hemolysins** and **leukocidins**, which form pores in cell membranes, causing leakage of the cell contents and cell lysis.

Membrane Disrupting  Exotoxins

Which of the following correctly describes the three classes of exotoxins?

Which class of exotoxins can cause an excessive immune response by activating a large number of T-cells?

Which class of exotoxins targets the inside of host cells to interfere with cellular functions?

Superantigens are exotoxins that trigger a nonspecific stimulation of immune cells to secrete cytokines. The overproduction of cytokines, known as a cytokine storm, triggers a strong immune and inflammatory response that can lead to life-threatening conditions such as high fevers, low blood pressure, multi-organ failure, shock, and death.

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