The Frei test was developed in 1925 by Wilhelm Siegmund Frei

Wilhelm Siegmund Frei

Wilhelm Siegmund Frei was a German dermatologist best known for his contributions to Durand-Nicholas-Favre disease, a sexually transmitted disease found mainly in tropical and subtropical climates...

, a German dermatologist, to identify lymphogranuloma inguinale. Antigen made from sterile pus

Pus

Pus is a viscous exudate, typically whitish-yellow, yellow, or yellow-brown, formed at the site of inflammatory during infection. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule or...

 aspirated from previously unruptured abscess

Abscess

An abscess is a collection of pus that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process or other foreign materials...

es, produced a reaction in patients with lymphogranuloma inguinale when injected intradermally. Other sources of antigen have been explored, most deriving from various tissues of mice infected with Chlamydia. The test is no longer used but stands as a milestone in our understanding of immunology.