Immune System

Your immune system is your body’s first-line defense against invaders like germs. It helps protect you from getting sick and promotes healing when you’re unwell or injured. You can strengthen your immune system by eating nutritious foods, exercising and getting enough sleep.


What is the immune system?

Your immune system is a large network of organs, white blood cells, proteins and chemicals. These parts all work together to protect you from germs and other invaders. Your immune system also helps your body heal from infections and injuries.


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What does the immune system do

Your immune system works hard to keep you healthy. It does this by:

  • Keeping invaders (like germs) out of your body.
  • Destroying invaders.
  • Limiting how much harm the invaders can do if they’re inside your body.
  • Healing damage to your body.
  • Adapting to new challenges and threats.

Invaders your immune system protects you against include:

How does the immune system work?

When your immune system is working properly, it:

  • Tells the difference between cells that are yours and those that don’t belong in your body.
  • Activates and mobilizes to kill germs that may harm you.
  • Ends an attack once the threat is gone.
  • Learns about germs after you’ve had contact with them and develops antibodies against them.
  • Sends out antibodies to destroy germs that try to enter your body in the future.

But things don’t always go this smoothly. Sometimes, your immune system doesn’t work properly. For example, it may be too weak to fight off invaders, or it may launch too strong of a response.

Weak immune system

Many different conditions can weaken your immune system and make you more susceptible to infection. Conditions at birth are less common than those that develop later in life, like Type 2 diabetes and cancer.

Overactive immune system

At the other end of the spectrum, your immune system may react too strongly to invaders (real or perceived). It may mount an attack when there’s no invader. Or it may keep attacking after getting rid of an invader. An overactive immune system can lead to problems like autoimmune diseases or allergic reactions.



Illustration showing different parts of your immune system and where they're located in your body.
The various organs, tissues and cells of your immune system are distributed throughout your body. They all work together to help keep you healthy.

What parts of your body make up the immune system?

Many parts of your body, including immune system organs and cells, work together to keep you healthy. The main components of your immune system are:

  • White blood cells. These immune system cells attack and eliminate harmful germs to keep you healthy. There are many types of white blood cells, and each type has a specific mission in your body’s defense system. Each type also has a different way of recognizing a problem, communicating with other cells and getting their job done.
  • Antibodies. These proteins protect you from invaders by binding to them and initiating their destruction.
  • Cytokines. These proteins serve as chemical messengers that tell your immune cells where to go and what to do. Different types of cytokines do different specific tasks, like regulating inflammation. Inflammation happens when your immune cells are warding off invaders or healing damage to your tissues.
  • Complement system. This is a group of proteins that teams up with other cells in your body to defend against invaders and promote healing from an injury or infection.
  • Lymph nodes. These small, bean-shaped organs are like colanders you use to drain pasta. They filter waste products from the fluid that drains from your tissues and cells (lymph) while keeping the good components, like nutrients. You have hundreds of lymph nodes throughout your body, and they’re a vital part of your lymphatic system
  • Spleen. This organ stores white blood cells that defend your body from invaders. It also filters your blood, recycling old and damaged cells to make new ones.
  • Tonsils and adenoids. Located in your throat and nasal passage, tonsils and adenoids can trap invaders (like bacteria or viruses) as soon as they enter your body.
  • Thymus. This small organ helps T-cells (a specific type of white blood cell) mature before they travel elsewhere in your body to protect you.
  • Bone marrow. This soft, fatty tissue inside your bones is like a factory for your blood cells. It makes the blood cells your body needs to survive, including white blood cells that support your immune system.
  • Skin. Your skin is a protective barrier that helps stop germs from entering your body. It produces oils and releases other protective immune system cells.
  • Mucosa. This three-layered membrane lines cavities and organs throughout your body. It secretes mucus that captures invaders, like germs, for your body to then clear out.

Innate immunity vs. acquired immunity

Innate immunity is protection that you’re born with. Your innate immune system is part of your body’s first-line defense. It responds to invaders right away by attacking any organism that shouldn’t be in your body. It doesn’t need prior training to tell the difference between cells that belong in your body and those that don’t.

The white blood cells involved in innate immunity don’t learn to recognize certain invaders. They also have no memory of attacking invaders and don’t offer protection against specific germs (or the infections they cause) in the future.

That’s where acquired immunity comes into play. Acquired immunity, also called adaptive or specific immunity, is protection your body gains (acquires) over time from exposure to germs. Certain white blood cells called lymphocytes remember specific invaders and can tell when they don’t belong in your body. So, if those invaders try to get in again, the lymphocytes can quickly spring into action and work with other cells to eliminate the threat.

Vaccines support your acquired immunity by training its cells to identify and destroy invaders before they make you sick.

Conditions and Disorders

What disorders and diseases can affect the immune system?

Conditions that can interfere with the normal workings of your immune system include:

  • Allergies. An allergy is your body’s reaction to a substance that’s normally harmless. Your immune system overreacts to the presence of that substance, leading to a range of symptoms from mild to severe.
  • Autoimmune diseases. These conditions occur when your immune system attacks its own healthy cells by mistake. Lupus and rheumatoid arthritis are examples of common autoimmune diseases.
  • Primary immunodeficiency diseases. These inherited conditions prevent your immune system from working properly. They make you more vulnerable to infections and certain diseases.
  • Infectious diseases. Infectious diseases happen when germs enter your body, replicate and cause damage. HIV and mononucleosis (mono) are examples of infectious diseases that weaken your immune system and can lead to serious illness.
  • Cancer. Certain types of cancer, like leukemia and lymphoma, can weaken your immune system. That’s because cancer cells may grow in your bone marrow or spread there from somewhere else. Cancer cells in your bone marrow interfere with the normal production of blood cells you need to fight infection.
  • Sepsis. Sepsis is an extreme immune response to infection. Your immune system starts damaging healthy tissues and organs. This causes potentially life-threatening inflammation throughout your body.

Signs and symptoms of immune system disorders

Signs and symptoms vary depending on the condition and may include:

  • Always feeling tired (fatigue).
  • Unexplained fever.
  • Unexplained weight loss.
  • Drenching night sweats.
  • Itchy skin.
  • Sore, aching muscles.
  • Fingers or toes that tingle or are numb.
  • Trouble concentrating or paying attention.
  • Hair loss.
  • Inflammation, rashes or redness anywhere on your body.
  • Swollen lymph nodes in your neck, armpits or groin.

Common tests that check the health of your immune system

Healthcare providers often use blood tests to check how well your immune system is working. Specific blood tests your provider may order include:


What medications can affect my immune system?

Some medications do important work in your body but in the process, can weaken your immune system. These include:

If you need any of these treatments, talk to your healthcare provider about how you can support your immune system.


How can I boost my immune system naturally?

No one likes getting sick, and it’s common to wonder how to improve or strengthen your immune system. Because your immune system is complex, there’s no fast and easy answer that works for everyone on how to build it up. That’s why it’s important to talk to a healthcare provider. They can give you individualized advice based on your medical history. They’ll also talk to you about your lifestyle and daily habits to see what changes you can make.

Here are some general tips to keep your immune system running smoothly:

  • Fill your plate with healthy foods. Fruits, veggies, lean sources of protein and whole grains are just some examples of foods that bolster immune function. Talk to your provider about how different ways of eating, like the Mediterranean diet, can help give you the vitamins you need for a healthy immune system.
  • Build exercise into your daily routine. Exercise helps many aspects of your health, including your immune function. Your provider can help you get started with an exercise plan that fits your medical needs and lifestyle.
  • Keep a weight that’s healthy for you. Researchers have linked a body mass index (BMI) greater than 30 (obesity) to poorer immune function. Ask your provider what your target weight range should be, and work together to reach it.
  • Catch enough ZZZs. Not getting enough sleep can prevent your immune system from working as it should.
  • Stay up-to-date on vaccines. Vaccines train your body to fight off germs that can make you sick. Talk to your provider about which vaccines you need and when.
  • Avoid smoking and all tobacco products. Tobacco use raises your risk for conditions that can harm your immune system, like rheumatoid arthritis. If you use tobacco, talk to your provider about effective ways to quit.

Additional Common Questions

Why is my immune system so weak?

Many different medical conditions, medications and lifestyle factors can weaken your immune system and prevent it from defending you as well as it should. If you feel like you’re always sick or have symptoms that never go away, make an appointment with a healthcare provider. They’ll determine if you have a weak immune system and what’s causing the issue.

A note from Cleveland Clinic

Like a home security system that guards against intruders and sounds the alarm when needed, your immune system is on-call and ready to signal for help when it perceives a threat. The cells and organs of your immune system work together to locate, identify and remove germs and other invaders to keep you safe and healthy. But guarding isn’t your immune system’s only duty. Its crew also heals the damage that intruders cause, just like you’d need someone to repair a broken window or door.

But even the best security system can malfunction sometimes. Autoimmune diseases or other conditions can disrupt your body’s ability to defend itself against intruders or repair damage. That’s why it’s important to see a healthcare provider regularly for checkups. They can find problems early and, if needed, provide treatment to keep your immune system working at its best.

Medically Reviewed

Last reviewed on 10/20/2023.

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