 The host immune response involves cytotoxic (CD8-positive) T cells against infected B lymphocytes, resulting in enlarged, atypical lymphocytes (Downey cells).When the infection is acute (recent onset, instead of chronic), heterophile antibodies are produced.Cytomegalovirus, adenovirus and "Toxoplasma gondii" (toxoplasmosis) infections can cause symptoms similar to infectious mononucleosis, but a heterophile antibody test will test negative and differentiate those infections from infectious mononucleosis.