This uncommon skin condition usually presents as red patches on the genitals. It is called plasma cell balanitis in men and plasma cell vulvitis in women because of the numerous plasma cells seen on skin biopsy. A plasma cell is a type of white blood cell found in some inflammatory conditions. Plasma cell balanitis affects the head of the penis (the glans) and is also known as balanitis circumscripta plasmacellularis or Zoon balanitis. Plasma cell vulvitis is most commonly found on the inside of the vulva at the entrance to the vagina (vestibule). It is also known as vulvitis circumscripta plasmacellularis.