Corynebacterium diphtheriae is a Gram-positive, non-motile bacterium that can cause serious illness in unvaccinated humans. C. diphtheriae infects the epithelial cells of the upper respiratory tract from where they secrete a potent toxin that is absorbed and disseminated through lymph channels and blood to the susceptible tissues of the body. The toxin is a single protein chain of 62 kDa which is "nicked" at proteolytic processing sites to produce an A chain of 21 kDa and a B chain of 40 kDa, held together by a disulfide bridge. The A-chain carries the ADP-ribosylation enzymatic activity responsible for the toxic action. This antibody reacts with diphtheria toxoid.