Inside the membrane, the bacteria produce an exotoxin, which is a poisonous secretion that causes the life-threatening symptoms of diphtheria.
The exotoxin is carried throughout the body in the bloodstream, destroying healthy tissue in other parts of the body.
The most serious complications caused by the exotoxin are inflammations of the heart muscle (myocarditis) and damage to the nervous system.
It is important not to try to remove the membrane because the trauma may increase the body's absorption of the exotoxin.
People who are sensitive (about 10%) must be desensitized with diluted antitoxin, since as of 2004 the antitoxin is the only specific substance that counteracts diphtheria exotoxin.