What is the immune system? A guide to your body's first-line of defense against infection

The immune system is a complex network of cells, tissues, and organs that fight off infection.
Your immune system can be affected by sleep, nutrition, hormones, and exercise.
You can help improve your immune system, but some people are chronically immunocompromised.
Visit Insider’s Health Reference library for more advice.

Your immune system protects your body against invaders like harmful germs, including viruses, bacteria, and fungi, and toxins from harmful chemicals.

This system comprises a large network of cells, molecules, tissues, and organs that work together via several biological processes to identify and neutralize threats. Some of the major components of your immune system include your skin, tonsils, spleen, bone marrow, white blood cells, and lymph nodes.

Several factors, including sleep, nutrition, stress, hormones, and exercise, can affect your immunity for better or for worse. Certain health conditions, medical treatments, and factors like age, pregnancy, and smoking can also affect your immunity.

Here’s what you need to know about your immune system and how it functions.

Your immune system, explained

The immune system is your body's internal defense system. According to the Cleveland Clinic, your immune system primarily does two things:

It protects you against germs and tries to keep you from getting sick.
If you do get sick, it fights the infection to help you recover.

These are some of the major components of your immune system:

Skin: Your skin is the first line of defense against germs or toxins. It acts like a barrier that prevents germs from entering your body. It also secretes fluids and other substances that destroy germs.
Mucous membranes: Your respiratory, reproductive, urinary, and digestive tracts are lined with mucus, which traps germs and expels them. For example, when you have a cold, your body produces mucus to flush germs out of your nose, sinuses, and airways.
White blood cells: These cells are a key part of your immune system. They locate, identify, and attack germs. There are several types of white blood cells; each one has a specific role to play in your immune response.
Lymph nodes: These are small glands located all over your body, including in your armpit, neck, and groin regions. They are part of your lymphatic system, which filters out germs via a clear, watery fluid known as lymph. This process can help prevent germs from reaching other parts of your body.
Spleen: Your spleen produces, stores, and dispatches white blood cells. For instance, if your body detects germs in your bloodstream, your spleen and lymph nodes manufacture and dispatch thousands of white blood cells to fight them.
Tonsils: Located in your throat, your tonsils trap harmful germs as soon as they enter your body, fighting them to prevent respiratory infections like influenza (flu) and pneumonia.
Bone marrow: Your bone marrow produces several different types of immune cells, including white blood cells. Millions of new blood cells are produced in your bone marrow and added to your bloodstream every day.
Thymus: Located behind your breastbone, this small organ produces white blood cells that catalogue germs and remember them in future, so that it's easier for your body to fight those germs if they attack you again.

How does your immune system fight infection?

If germs get past your defenses, your body launches an immune response to fight them. This involves several types of white blood cells:

B lymphocytes: These cells produce antibodies, which are special proteins designed to identify and weaken specific germs.
T lymphocytes: These cells attack the germs identified by the antibodies. They also release cytokines, which are small proteins that orchestrate your entire immune response by controlling the growth and activity of all your blood cells and immune cells.
Phagocytes: These cells engulf germs and destroy them.

Once your body has dealt with a certain invader, it remembers it. If the same germ attacks you again, your body recognizes it and sends out the appropriate antibodies to fight it. This can help prevent reinfection. This ability to protect against a certain illness is known as "immunity."

"The generation of long-lasting protective memory is one of the most unique and important characteristics of the immune system. Memory is essential in allowing individuals to defend themselves from infections to which they have previously been exposed," says Tulip Jhaveri, MD, who specializes in pathology and infectious diseases.

Vaccines help the body develop immunity to germs and infections. This is called immunization and it counts on your immune system's memory. Though different vaccines take different approaches to the immunization process, they all trigger your immune response to fight off a particular kind of illness (like measles, smallpox, etc.).

After you receive a vaccine, your body remembers how to fight that particular germ and generates a long-lasting immune response. Certain vaccines require a booster shot, which jogs your immune system's memory.

How do you check your immune system?

Blood tests that measure your levels of blood cells, immune system cells, and immunoglobulins (infection-fighting proteins) can help determine whether your immune system is healthy or not.

Jhaveri says that recurring illnesses could indicate that one's immune system isn't functioning as it should. Some signs of a weakened immune system include:

Frequent or prolonged colds
Recurring gastrointestinal problems like gas, constipation, or diarrhea
Slow healing of wounds
Frequent infections, like pneumonia, ear infections, or sinusitis
Persistent fatigue
High stress levels

Certain factors and health conditions can compromise your immunity and affect your body's ability to fight diseases. These include:

HIV and AIDS, which damage immune cells, making your body vulnerable to other infections
Asthma, which causes your immune system to severely overreact to harmless substances
Leukemia and lymphoma, which affect the normal functioning of your lymphocytes (white blood cells)
Cancer treatments, like radiation and chemotherapy, which can also damage and destroy immune cells in the process of destroying cancer cells
Bone marrow or organ transplants, which necessitate deliberately weakening the patient's immune system so it doesn't reject the donated cells
Smoking, which harms your immune system and raises your chances of developing an illness

Age also plays a role in immunity. As you get older, your immune system is more susceptible to infections, diseases, and cancers; this is referred to as "immune senescence." Infants and young children are also more prone to infections; their immunity develops over time as they encounter germs and their body learns how to fight them. "Even pregnancy is a state of compromised immunity to a certain extent," says Jhaveri.

Sometimes, the immune system is unable to differentiate between harmful substances and healthy tissues, causing it to attack and destroy healthy cells, which is known as autoimmune disorder. There are over 80 types of autoimmune disorders - some of the more common ones include Type 1 diabetes, multiple sclerosis, lupus, rheumatoid arthritis, and celiac disease.

Insider's takeaway

Your immune system consists of several components working together to fight off infection-causing germs. One of the most remarkable characteristics of the immune system is that it is able to remember how to defeat germs it has encountered before; a process by which you develop immunity to that illness. Vaccination, also known as immunization, relies on your immune system's memory to help build immunity against harmful germs that cause illnesses.

Certain health conditions and medical treatments, as well as factors like age, smoking, and pregnancy can compromise your immune system and its ability to function. Frequent illnesses and chronic fatigue are some of the signs of a weakened system. Blood tests can help determine whether your immune system is functioning as it should.

Jhaveri says your best chance at boosting your immune system is to eat a healthy diet, get plenty of sleep, exercise regularly, stop smoking, limit your alcohol consumption, and reduce stress.