Exercise and physical activity are often encouraged as part of a healthy lifestyle. Being active can also impart immune protection against infection and illness. However, there are differences between adults and children in how exercise can affect components of the immune system. Because children are growing and the immune system is developing, it is important to understand these issues from a growth perspective. This entry discusses how exercise affects the immune system, how this is different between children and adults, and the implications for immune function and risk of infection.