Mononucleosis pertains to the condition wherein there is an abnormally large number of monocytes circulating in the blood. The monocyte is a type of white blood cell produced from the bone marrow. It migrates to the peripheral blood through diapedesis. In humans, the monocytes constitute the 2% to 10% of the white blood cells in the body. Its function involves phagocytosis, antigen presentation, and immune functions. Some of the monocytes are stored in the spleen while others migrate from the bloodstream to the tissues and differentiate into a macrophage or a myeloid dendritic cell.
In mononucleosis, the circulating monocytes exceed the normal level. It may be due to the presence of the Epstein-Barr virus. When the body is infected with that virus, the condition may lead to a disease with few (sometimes, no perceptible) symptoms. The common symptoms include fever, sore throat, tiredness, and enlarged lymph nodes. In this case, the condition is referred to as infectious mononucleosis, which is also called kissing disease or glandular fever. The disease may persist in two to four weeks. However, it may affect the liver and the spleen, which may become swollen and in few cases may rupture.