Pertussis toxin, produced and secreted by the whooping cough agent Bordetella pertussis, is one of the most complex soluble bacterial proteins. It is actively secreted through the B. pertussis cell envelope by the Ptl secretion system, a member of the widespread type IV secretion systems. The toxin is composed of five subunits (named S1 to S5 according to their decreasing molecular weights) arranged in an A–B structure. The A protomer is composed of the enzymatically active S1 subunit, which catalyzes ADP‐ribosylation of the α subunit of trimeric G proteins, thereby disturbing the metabolic functions of the target cells, leading to a variety of biological activities. The B oligomer is composed of 1S2:1S3:2S4:1S5 and is responsible for binding of the toxin to the target cell receptors and for intracellular trafficking via receptor‐mediated endocytosis and retrograde transport. The toxin is one of the most important virulence factors of B. pertussis and is a component of all current vaccines against whooping cough. Pertussis toxin is a major virulence factor of Bordetella pertussis, the etiological agent of whooping cough, and also one of the main antigens in all current pertussis vaccines. It is one of the most complex bacterial toxins, composed of five different subunits and organized in an A‐B structure, in which the A protomer expresses ADPribosyltransferase activity and the B oligomer is responsible for toxin binding to the target cell receptors. After binding the toxin traffics via a retrograde transport to the target substrate. As a fully assembled protein it is secreted through the bacterial cell wall by a type IV secretion system