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Diphtheria vaccine. Researcher pipettes pink liquid into phials during testing of a diphtheria vaccine. Corynebacterium diphtheriae, the bacterium which causes diphtheria, produces a toxin which kills muscle cells and causes life- threatening symptoms such as paralysis and heart failure. This toxin can be chemically modified by formalin treatment so that it retains its ability to generate an immune response, but loses its toxicity. Vaccination with the toxoid (modified toxin) sensitises the immune system to the toxin, so that susequent infection is met with an intense antibody response which neutralises the toxin. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROBERT LONGUEHAYE, NIBSC)