The factors that can weaken your immune system can be pretty diverse. They range from infections and medications to stress and unhealthy habits.

Chronic inflammation

One of the most common immune system enemies is chronic inflammation. It can hamper your immune system via complex mechanisms, making you more susceptible to infections. 

For example, scientists report that inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) play a vital role in initiating an acute immune response against pathogens.

Unfortunately, TNF-α can suppress your immune system when produced persistently due to chronic inflammation (1). On top of that, chronic inflammation increases your risk of many serious illnesses.

Medications

Many medications can also suppress your immune system. The list includes anti-inflammatory drugs for treating chronic inflammation, such as glucocorticosteroids and high doses of certain non-steroidal anti-inflammatory drugs (NSAIDs). 

One laboratory study found that Ibuprofen and other widely used NSAIDs reduce the ability of human cells to produce antibodies (2). 

Glucocorticosteroids are also widely used medications for suppressing the immune system in case of allergies, autoimmune conditions, and some malignancies.

Cancer treatment, including radiation and chemotherapy, is another factor with a potent suppressive effect on the immune system.

Infections

Acute inflammation due to specific infectious agents can exhaust your immune system and make you more susceptible to secondary infections. 

For example, pathogens such as the influenza virus can wreak havoc on your immunity and make you more susceptible to bacterial infections. 

Other infections that weaken your immune system include measles, mononucleosis, and AIDS.

Stress and insomnia

Both acute and chronic stress can have an impact on your immune system. That’s because immune cells have receptors for stress hormones such as cortisol, with well-known immunosuppressive effects (3).

Yet, chronic stress appears to be associated with stronger immune suppression than acute stress. Studies report that severe chronic stress that lasts for one month or longer is associated with the highest increase in the risk of infectious diseases (4). 

Other trials also confirm that psychological stress significantly increases the risk of infections (5). Scientists report increased susceptibility to different viruses associated with the common cold (6).

Inadequate sleep has been shown to cause a similar effect on the immune system as chronic stress (7).

Sleep is essential for your body’s ability to produce inflammatory molecules that help fight infectious agents such as antibodies. Therefore it may also take longer to recover from infection if you have trouble sleeping.

Unhealthy habits

Unhealthy habits such as smoking and alcohol use also hinder immune function. 

These habits lead to chronic inflammation, exhausting your immune system and making you more susceptible to viruses and bacteria. Smoking also increases your risk of bronchitis and pneumonia, perpetuating the cycle and overloading your defenses.

Overeating and a sedentary lifestyle can lead to weight gain and obesity, contributing to chronic inflammation and persistent elevation of TNF-α.

There are several things you can focus on to help the function of your immune system and bolster your body’s defenses:

• deal with chronic stress
• exercise in moderation
• maintain optimal body weight
• improve your sleep
• quit smoking
• reduce your alcohol intake

Reduce your stress levels

Dealing with chronic stress can have enormous benefits for your overall well-being, and your immune system health can really benefit from it. 

• Studies show that learning relaxation techniques and practicing them during stressful periods can significantly improve the function of the immune system and its ability to fight germs (8).

• You can combat stress via learning relaxation techniques, spending time outdoors, meeting with friends, owning pets, meditation, therapy, etc. 

• Spending more time outdoors in direct sunlight will also boost your vitamin D production, which is vital for your immune system. Keep in mind to expose yourself to direct sunlight in moderation to avoid burning and skin damage.

Exercise regularly

Physical exercise is an acute form of good stress which stimulates the production of both pro-inflammatory and anti-inflammatory molecules in your body. 

Regular moderate or high-intensity exercise leads to the production of more anti-inflammatory mediators that tend to prevail, which improves the regulation of your immune function.

Research shows a clear inverse relationship between moderate exercise training and the risk of infectious illnesses (9).

Yet, extreme intensities seen in athletes during competition training can have the opposite effects and increase the body's susceptibility to infections. Therefore, it is essential to exercise in moderation and focus on long-term consistency.

Maintain a healthy body weight

Obesity, more specifically visceral obesity, leads to chronic low-grade inflammation, which negatively affects your immune function and overall health.

Excess fatty tissue around the waist produces excessive amounts of inflammatory molecules such as TNF-α in your body, contributing to chronic disease and increasing susceptibility to infections (10).

• Researchers also warn that obesity increases the severity of certain viral infections in those patients, such as the flu and COVID-19 (11, 12). 

On the other hand, being underweight and malnourished also increases your susceptibility to infections because your immune system lacks the resources to fight pathogens. 

For example, inadequate protein status will hamper your immune response to produce antibodies which play a significant role in your defense mechanisms against viruses and bacteria.

Therefore it is vital to maintain optimal body weight and avoid obesity or malnourishment. The optimal body mass index (BMI) for adults is between 18.5 and 25 kg/m2. You can calculate your own BMI by dividing your weight (in kg) by the square of your height (in cm).

Get adequate sleep

Improving your sleep may help improve your tolerance to stress, body composition, weight loss efforts, and immune function. 

A recent study reported that quality sleep improves the function of your T-cells, which are a vital part of your immunity (13). 

T-cells can directly kill pathogens, activate other immune cells, produce inflammatory molecules and regulate your immune tolerance.

Foods can affect your immune function beyond their effect on your body weight. Your diet is also a source of essential nutrients and antioxidants, which are vital for a healthy immune function.

The wide range of vitamins and minerals includes vitamin A, vitamin C, vitamin D, vitamin E, zinc, selenium, and chromium. Vitamin A, C, E, selenium, polyphenols, and carotenoids also act as antioxidants that help regulate your immune function and reduce chronic inflammation (14).

Your body can produce antioxidants on its own and use those in your diet to neutralize the oxidative stress of active radicals, such as reactive oxygen species (free radicals) (15). These radicals are a natural byproduct of your metabolism but also a tool used by your immunity to damage germs, pathogens, and cancer cells (16, 17).

Since the oxidative stress produced by your immune system can harm healthy tissues as well, your antioxidant needs increase. Thus, food sources of antioxidants can help minimize the collateral damage caused by the immune response.

Yet, keep in mind that consuming too many antioxidants may disturb the balance of antioxidants and radicals. Studies report that excessive antioxidant consumption can lead to an "antioxidant paradox" and do more harm than good (18, 19). 

• Avoid overconsumption of certain antioxidants that can accumulate in your body, such as vitamin A, selenium, and vitamin E. Zinc is vital for your immunity, but too much of it can also have the opposite effect.

The balance between the essential fats in your diet can also either “make it or break it” regarding healthy immune function. These are the omega 3 and omega 6 fatty acids in your diet. 

All of them are important for producing inflammatory molecules and mediators by your immune system. Omega 3 is used primarily to synthesize anti-inflammatory metabolites, while omega 6 is converted into pro-inflammatory ones.

Thus, a diet that contains an optimal ratio between omega 3 and omega 6 fatty acids is essential for normal immune function. This ratio should be no higher than 4:1 in favor of omega 6.

Diets high in saturated fats may also lower your B-cell activity, immune cells that produce antibodies to fight germs and pathogens (20).