The use of differentiation marker CD89 for the detection of cellular reservoirs of a mammalian immunodeficiency virus. Also the use of the differentiation marker CD89 for making a prognosis, diagnosing a remission, and evaluating the efficacy of treatment of the mammalian immunodeficiency. A multi-specific antibody that recognizes both at least one epitope of CD89 and at least one characteristic of the lymphocyte cells, a composition including the antibody, and the use of the antibody for treatment.