As you get older, your immune system ages with you. There's even a medical term for it – immunosenescence – the gradual decrease in immune function that comes with age. Similar to your walking or running speed, your body's ability to fight off infection inevitably slows.

However, you can do a lot to bolster your immune system and keep it healthy as possible. Here are some signs of a weakening immune system and proactive ways to support it.

Aging Effect Is Real

Your immune system keeps your body healthy by warding off foreign substances. Harmful invaders include bacteria, viruses, fungi and cancer cells. The immune system battles back through a complex network of blood cells and bodily organs. Lymph nodes are glands that harbor, then release, specialized white blood cells called lymphocytes. Lymph and blood vessels transport the infection-fighting lymphocytes throughout the body.

"Your age is the primary determinant of what's going to happen to your immune system," says Philippa Marrack, a professor and chair of biomedical research at National Jewish Health in Denver.

The bottom line is that your immune system is just not as robust as it used to be, Marrack explains. As a consequence, it takes longer for your body to figure out when you have an infection. Once detected, it takes longer for the immune system to deal with it – your body starts losing the race between bacteria or viruses. You get sick more often. Infections are more severe and more of a threat than when you were younger, and you recover from them more slowly.

Vaccinations are key for protecting you from infection. However, vaccines may not work as well as they used to. As the immune system changes, autoimmune diseases like rheumatoid arthritis are more likely to occur.

Experience does confer some benefit. "When you get older, your immune system is [still] pretty good at dealing with things you've already experienced," Marrack says. "But it's the new infections that you've never experienced before that are a real problem."

One example is the West Nile virus, Marrack says. West Nile virus disease, which affects older adults more severely, was far more lethal among those over 65 than younger adults when it first came through North America at the turn of the millennium.

What You Can Do

Vaccinate, vaccinate, vaccinate. That's the first line of defense against common infections such as flu and pneumonia. Getting adult vaccinations according to recommendations is the best way to protect yourself from infectious diseases, Marrack emphasizes. One example is shingles – a painful, persistent infection that tends to prey on seniors.

"Shingles is caused by the chickenpox virus hiding away in [our] nerve cells ever since we got chickenpox when we were young," Marrack says. An intact immune system keeps the virus under control, sometimes for decades. However, she says, "As your immune system gets less effective, it can come popping out."

Vaccination against the varicella zoster virus, which causes shingles, is recommended for adults over 50, according to the Centers for Disease Control and Prevention. Pneumococcal vaccines are advised for adults over 65 to prevent pneumonia and related conditions. Most adults should receive yearly flu shots. In addition, you may need periodic booster shots for tetanus or other conditions, depending on your health history. Work with your health provider to stay on top of immunization schedules.

A healthy immune system is closely tied to your overall health. Avoiding obesity and keeping chronic conditions like diabetes under control reduces immune-system stress.

Exercise boosts the immune system, evidence shows. Last week, a new study highlighted the importance of exercise to counter aging. A team made up of 125 male and female cyclists, ages 55 to 79, was compared to a control group of older adults who did not exercise regularly. The cyclists not only had intact strength and muscle mass, but also possessed immune systems equal to those of much younger adults.

Good nutrition from eating a balanced diet also keeps your immune system strong. On the other hand, smoking is as bad for your immune system as it is for your lungs. Heavy alcohol use or binges can compromise immunity as well. Sleep disorders like sleep apnea can also lower immunity, so seek treatment if needed.

Hope for a Stronger Immune System

Inflammation takes a toll on the immune system. "Unfortunately, what happens in many of the conditions of old age – and obesity in particular – is they come with low-grade inflammation, all the time," says Dr. Janko Nikolich-Žugich, a professor and chair of immunobiology at the University of Arizona College of Medicine–Tucson.

The good news is the growing body of research looking at inflammation-busting interventions. "A lot of new, fun things are happening," says Nikolich-Žugich, who is also co-director of the Arizona Center on Aging. Potential avenues include addressing imbalances of gut bacteria, also called the microbiome.

Calorie restriction is another possibility, he says, if done carefully to provide enough energy to keep the immune system properly fueled. Another potential track is medications such as rapamycin and metformin, which have shown age-delaying and anti-inflammatory effects in animal studies.

The ketogenic diet, or keto diet, is known to be anti-inflammatory, Nikolich-Žugich says. One unanswered question is whether exogenous ketones, like those contained in ketone drinks, can stave off the inflammatory process.

In his own laboratory, Nikolich-Žugich's team is working to understand how to reinvigorate white blood cells that are critical to immune function. Another challenge is restoring coordination throughout the immune system so that infection-fighting cells can meet microbial challenges in time.