Stress is a function of our primal origins. When the body is under stress, it boosts production of cortisol to support the ‘fight or flight’ response we all have at the heart of our operating system. If the hormone remains elevated in the bloodstream for long periods of time, though, it wears down the immune system. Every cell contains a tiny clock called a telomere, which shortens each time the cell divides. Short telomeres are linked to a range of human diseases, including HIV, osteoporosis, heart disease and aging. Previous studies have shown that an enzyme within the cell, called telomerase, keeps immune cells young by preserving their telomere length and ability to continue dividing. UCLA scientists have found that the stress hormone cortisol suppresses immune cells’ ability to activate their telomerase. This may explain why the cells of persons under chronic stress have shorter telomeres.