EXERCISE boosts your Immune system
The immune system needs physical activity to function properly.
Exercise raises our blood pressure and heart rate which causes the release of certain chemicals (cytokines). These cytokines mobilize immune cells, prompting them into action.
(Exercise—>Cytokines—>Increased Immune Cell Activity)
Some immune cells patrol for viruses/bacteria and attack them when they find them.
Other immune cells remember the bacteria/viruses they have seen in the past. When they see these bacteria/viruses again, they recognize them, and can quickly activate the immune system.
Both of these types of immune cells are sent out on patrol during exercise via the chemical signals of cytokines.
This immune cell recruitment lasts for several hours, which is why consistency in exercise is important. These immune cells patrol on the days that you exercise, so it is important to get some physical activity daily. (Being physically active twice during the day may also be helpful for this reason.)
Active muscles stimulate macrophages/ another type of immune cell.
Exercise is important to prevent illness. But once you are sick, your immune system is naturally activated, and these processes are already started, so it is best to rest and hydrate when sick.