Lymphocytes are a subset of white blood cells (WBC) that forms a framework of the immune system. They facilitate humoral and cellular immunity of the body against foreign proteins and pathogens. An increase in circulating lymphocytes (lymphocytosis) can be seen following infections such as infectious mononucleosis and pertussis, or in lymphoproliferative disorders. Lymphoproliferative disorders (LPD) are known complication of primary immunodeficiency (PID).

Viruses can cause number of primary infections that leads to different complications and induces malignancies. Primary infections in infants and children are common and usually asymptomatic. Outcome of LPD is very poor. Mortality rate is approximately 75%.