Checks & balances
The immune system exists in a constant state of balance between protective immunity and self-tolerance, i.e not attacking ‘self’. Multiple checks ensure this balance is not lost; however, overactivity of the immune system can lead to autoimmunity; to chronic inflammatory conditions, and, in the setting of organ transplantation, to organ rejection. On the other hand, the underactivity of the immune system is a result of global immunosuppression, which leads to a decreased ‘immune surveillance’ which often results in serious infections and cancer.
Different classes of T cells are the front line of control in the immune system with effector T cells (Teff) being responsible for killing non-self-cells, and T regulatory cells (Tregs) being responsible for dampening down the immune response preventing an over stimulation of the immune system. Put simply, Tregs are capable of down regulating the immune system by ‘putting on a brake’ to prevent over activation and resultant pathological immune responses.