Posttraumatic stress disorder (PTSD) is one of the most prevalent psychiatric diseases, often associated with other co-morbid mental disorders and somatic health complaints. In this review, we attempt to present an up-to-date and comprehensive picture of the available data about immunological alterations in PTSD. A brief description of PTSD is followed by an introduction to the immune system. Then, a current review of available data on the immune system in PTSD is presented. The aim of this paper is to familiarize the reader with key issues of the psychoneuroimmunological interactions and difficulties related to studying a performance of the immune system in PTSD.