You know exercise builds muscles, strengthens bones, keeps your heart healthy and your mind sharp. But it also does something that you might not think much about: It helps keep your immune system — your internal defenses against infection — in tip-top shape.

If you take time for some physical exertion each day, it helps get your body ready to attack bacteria, viruses and toxins that can sneak in and make you sick.

But how much exercise is effective? Do too little or too much, and it won’t have the best effect on your immune system.

Clinical immunologist  Leonard Calabrese, DO, answers common questions about how exercise can impact your immunity and how to use your workouts to shut out a world of would-be invaders.

Q: How does breaking a sweat boost immunity?

A: If you exercise moderately on a regular basis, it tunes up the immune system in many ways. It enhances your broad-based defenses against viral infections, such as those causing upper respiratory infections.

Working out regularly also reduces the risk of many chronic diseases such as cardiovascular, respiratory illnesses and metabolic diseases, such as type 2 diabetes.

Q: How much exercise do you need?

A: Fortunately, you don’t have to push yourself to the limit to rev up your immune system. In fact, your immune system needs less of a workout than you get with your average cardio routine.

Focus on getting 20-30 minutes of moderate exercise, five days a week, and your immune system will thank you.

Q: What types of exercise are most helpful?

A: Moderate exercises, including biking or walking briskly in your neighborhood, are good ways to get your blood flowing. Swimming is also a good option for non-weight-bearing exercise for your joints.

Also try mind-body exercises such as Tai-Chi, Qi Gong and yoga, which are all options that help keep your joints flexible. These exercises also reduce chronic stress, which in itself is a powerful immune booster. These exercises can also help alleviate osteoarthritis and fibromyalgia symptoms.

Q: Can too much exercise set your immune system back?

A: Yes, though, the level to which it can slow your system down is still up for debate.

Research shows that exercising for more than 1.5 hours without refueling your body or giving it enough time to recover suppresses your immune responses for up to a few days. During that time, your cortisol levels rise, your white blood cell count drops and you’re more likely to develop a respiratory infection.

This problem usually affects elite athletes, such as marathon runners, most.

On the flip side, staying sedentary also increases your risk of infection, inflammation and chronic disease.

Q: Can exercise make an illness worse?

A: Yes, it can. This is a complex issue, but I like to tell my patients to do a “neck check.” If your symptoms are mostly of a mild cold without fever or lower respiratory symptoms, such as a productive cough, wheezing or shortness of breath, mild exercise can actually reduce congestion and may make you feel better.

If your symptoms are primarily in your lungs or you have a significant fever its better to rest until things settle down. Regardless, good hydration is important.