Editing Exotoxin (section)

=== Type I: cell surface-active ===

Type I toxins bind to a receptor on the cell surface and stimulate intracellular signaling pathways.  Two examples are described below.

==== Superantigens ====

[[Superantigens]] are produced by several bacteria.  The best-characterized superantigens are those produced by the strains of ''[[Staphylococcus aureus]]'' and ''[[Streptococcus pyogenes]]'' that cause [[toxic shock syndrome]].  Superantigens bridge the [[MHC class II]] protein on [[antigen-presenting cells]] with the [[T-cell receptor]] on the surface of [[T cells]] with a particular Vβ chain. As a consequence, up to 50% of all T cells are activated, leading to massive secretion of proinflammatory [[cytokines]], which produce the symptoms of toxic shock.

==== Heat-stable enterotoxins ====

Some strains of ''[[E. coli]]'' produce [[heat-stable enterotoxin]]s (ST), which are small peptides that are able to withstand heat treatment at 100&nbsp;°C.  Different STs recognize distinct receptors on the cell surface and thereby affect different intracellular signaling pathways.  For example, STa [[enterotoxins]] bind and activate membrane-bound guanylate cyclase, which leads to the intracellular accumulation of [[cyclic GMP]] and downstream effects on several signaling pathways.  These events lead to the loss of electrolytes and water from intestinal cells.