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The immune system declines by about 2-3% a year from our 20s, which is why older people are more susceptible to infections, conditions like rheumatoid arthritis, and cancer. However, research from the United Kingdom has found that high levels of physical activity can stall the shrinking of the thymus gland. The thymus gland produces T cells, which, in combination with B cells, make up the innate immune system which keeps you safe from infection.