Clemens Peter Freiherr[1] von Pirquet (12 May 1874 – 28 February 1929) was an Austrian scientist and pediatrician best known for his contributions to the fields of bacteriology and immunology.

[2] Soon after, the observation with smallpox led Pirquet to realize that tuberculin, which Robert Koch isolated from the bacteria that cause tuberculosis in 1890, might lead to a similar type of reaction.

In 1909 he declined proposals to take a position at the Pasteur Institute in Paris and to become a professor at the Johns Hopkins University.

In 1910 he returned to Europe taking positions in Breslau (now Wrocław) and then Vienna.

On 28 February 1929 Clemens von Pirquet and his wife committed suicide with potassium cyanide.