The ‘Graying’ of T Cells – Immune fitness is twofold: attacking illness-causing cells like bacteria, viruses and cancer and “remembering” past invaders

The elderly suffer more serious complications from infections and benefit less from vaccines.

In a healthy person, T cells circulate in the blood and quietly scan the body for threats using proteins on the cell’s surface. If a T cell encounters another cell it deems dangerous, the T cell undergoes activation, a molecular cascade in which it switches from surveillance mode to attack mode. The activated cells then rapidly replicate to build an army and destroy the enemy.

Source: The ‘Graying’ of T Cells | Harvard Medical School