Syphilis or Lue is a venereal infectious disease, whose etiological agent is Treponema Pallidum. The infection is spread through contact solutions on skin. Treponema antigenic structure is not completely known, but three different kinds of antigens can be distinguished: lipoid antigen, proteic antigen and polysaccharide antigen. The diagnosis can be directly done on Treponema Pallidum (TPHA method) or indirectly (serum diagnosis), highlighting specific antibodies anti-Treponema (reagines). TPHA is a specific treponemic test which uses the species-specific polysaccharide antigen. It is a passive hemoagglutination test, which uses chicken red blood cells sensitized with an extract of Treponema Pallidum. The red blood cells react and agglutinate with the specific antibodies eventually present in the serum.