Inx

■ Disulfide bond

B chain attaches to receptor

Binding site

A portion exerts toxic effects on cell

B portion is removed by exocytosis.

A separates from B component.

A portion exerts toxic effects on cell

B portion is removed by exocytosis.

A separates from B component.

Figure 19.12 The Action of A-B Exotoxins The A portion is toxic and the B portion binds to specific receptors on cells. After binding, the A-B molecule is taken up by endocytosis. After endocytosis, the contents of the vacuole become more acidic, causing the A and B portions to separate.The B portion remains in the vacuole and is removed by exoccytosis, while the A portion enters the cytoplasm of the cell and exerts its toxic effect.

The a-toxin of Clostridium perfringens, which causes gas gangrene, is a phospholipase. ■ phospholipid, p. 35 ■ gas gangrene, p. 701

Superantigens

Superantigens override the specificity of the T-cell response, causing toxic effects due to the massive release of cytokines by an

Helper T cell wm mm mmm

T-cell receptor

Groove for antigen -fragment

MHC II protein

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