Physiologic Response Initiate Symptoms of Infectious Mononucleosis When EBV invades the human body, immune system releases cytokines initiating fever. The cytokines release in response to invasion of B lymphocytes. Since the immune system is fighting against an intruder therefore the number of white blood cells increase, a condition called lymphocytosis. This leads to pharyngitis. Fever and pharyngitis are the most common and initial symptoms of EBV.