The immune system is the organism’s natural weapon against disease. The first line of defense is formed by innate immunity cells (such as monocytes and NK cells), which provide non-specific and incomplete, but rapid and indispensable protection while waiting for the so-called acquired immunity (in which are involved T, B lymphocytes and NK cells). Acquired immunity takes longer to respond to health threats, but it is highly specific and permanent.
Acquired immunity cells respond to the inflammatory environment generated by innate immunity. Once their work has been completed, most of these cells die, leaving behind them the so-called memory cells, able to protect the organism quickly and specifically when it is attacked again by the same threat.
Memory lymphocytes can survive for decades. However, the immune system tends to keep the total number of T lymphocytes in the blood almost constant. Consequently, continuous stimulations of the immune system can significantly reduce the space for new lymphocytes and, therefore, the ability to react to new threats. This is what happens, for example, in the presence of chronic viral infections, such as that caused by Cytomegalovirus (CMV).