The diphtheria bacillus multiplies in the mucosal surface tissues of the upper respiratory tract (usually the pharynx) or in the skin of the body surface of susceptible persons, secretes exotoxin, which penetrates into the local and peripheral tissues and causes tissue necrosis and acute pseudomembranous inflammation. The fluid exuding from the blood vessels contains easily coagulated fibrin, which coagulates inflammatory cells, mucosal necrotic tissue and Corynebacterium diphtheriae together to form a pseudomembrane, which is grayish-white in color, with neat edges, and which is tightly adherent to submucosal tissues, and is not easy to swab away. #In a few patients, the lesions may invade the deeper tissues and form ulcerated surfaces. The epithelium of the laryngeal, tracheal and bronchial mucosa is ciliated, and the pseudomembrane formed is not tightly adherent to the mucosa and is easily ejected from the tracheal incision.

Diphtheria exotoxin is absorbed locally and then dispersed through lymph and blood to all tissues of the body, combining with cells to cause lesions, of which the myocardium and peripheral nerves are the most sensitive, and the lesions in the kidneys and adrenal cortex are also more significant. The amount of exotoxin absorbed is related to the location and extensiveness of the pseudomembrane, with the pharynx being the most susceptible to absorption, the tonsils the second, and the larynx and trachea the least. The more extensive the pseudomembrane, the greater the amount of toxin absorbed. The combination of toxin and tissue starts with a loose fashion, and the longer the time, the stronger the combination, which is not easy to be neutralized by the antitoxin. Corynebacterium diphtheriae generally stays in localized foci, does not enter the bloodstream, and occasionally reaches local lymph nodes.