The Immune System: What It Is and What It Does

Medically Reviewed By Adam Bernstein, MD, ScD

Your immune system is a complex network of organs, cells, and tissues that defend your body against foreign substances, such as bacteria, viruses, and toxins. The immune response is recorded in your body so that if a foreign substance enters your body again, the immune system can often quickly remove it. The immune system contains white blood cells, antibodies, the complement system, the lymphatic system, the spleen, the thymus, and bone marrow.

Read on to learn more about the different parts of the immune system, some common immune system disorders, and how to boost your immune system.

What are some of the different parts of the immune system?

Virus particles surround a person washing their hands with soap
Marc Tran/Stocksy United

Below are some of the main parts of the immune system that fight infection.

White blood cells

White blood cells, or leukocytes, travel through the body in the blood and lymphatic vessels, looking for antigens. An antigen, which is short for “antibody generator,” is something that sparks an immune response. When white blood cells find an antigen, they multiply and attempt to remove it while instructing other cells to do the same.


Antibodies are antigen-neutralizing proteins secreted by B lymphocytes, which are a type of white blood cell. Different types of B lymphocytes secrete different antibodies, depending on the bacteria or virus they encounter.

Complement system

The complement system contains a large number of plasma proteins. These proteins tag antigens to complement or complete the work that antibodies do. The complement system works through cleavage, or division. A cascade of proteins, enzymes, and receptors activates to remove microbes and damaged cells and regulate inflammation.

Lymphatic system

The lymphatic system is a network of lymph nodes, organs, and vessels. Lymph is a colorless fluid that contains different types of lymphocytes. It travels through your body, searching for invading microorganisms. Your lymph nodes and other lymphatic organs filter the lymph to remove the microorganisms.


The spleen is the largest organ in the lymphatic system. It contains two different types of tissue: the red pulp tissue filters the blood, and the white pulp tissue helps the immune system remove infections.

Bone marrow

Your bone marrow produces platelets, red blood cells, and white blood cells. The white blood cells produced in the bone marrow include T lymphocytes and B lymphocytes. T lymphocytes help protect your body from infections. B lymphocytes help make antibodies.


The thymus is a gland between the lungs and just below the neck. As part of the lymphatic system, it filters your blood and is responsible for the processing and maturation of white blood cells called T lymphocytes.  

What does the immune system do?

Your immune system is your body’s defense mechanism against antigens. The many parts of the immune system work together to identify and remove antigens through its complex network of organs, tissues, and cells.

Your immune system can remember an antigen. If it encounters the antigen again, it can quickly respond by producing the proper antibodies. In many cases, you may not get sick from an antigen after your immune system has already encountered it. This resistance to infection is called immunity.

Sometimes, you can develop an immune response even though there is no real threat. When your immune system attacks your healthy cells, you can develop an autoimmune disorder.

What are the different types of immunity?

There are three types of immunity. The sections below cover these in more detail.

Innate immunity

Innate immunity is protection you are born with. It includes barriers that stop antigens from entering the body, such as the skin, mucous membranes, and tonsils. The cells and other substances in your body that remove antigens are also part of your innate immunity.

Active immunity

Active immunity, also called adaptive immunity, occurs when a specific antigen enters your body or when you are vaccinated. Some cells store information about the antigen and will quickly respond if that antigen enters your body again. Active immunity against many diseases can last a lifetime.

Passive immunity

Passive immunity occurs when you receive antibodies instead of producing them in your own body. Newborn babies receive antibodies from their birthing parents and have passive immunity to some antigens. Although this kind of immunity is effective immediately, it is temporary.

What are some common immune system disorders?

According to the National Stem Cell Foundation, globally, almost 4% of people live with an autoimmune disorder.

Some common autoimmune disorders include type 1 diabetes, multiple sclerosis (MS), rheumatoid arthritis, lupus, and Crohn’s disease. The following sections discuss these conditions in more detail.

Type 1 diabetes

Type 1 diabetes occurs when the body’s immune system mistakenly interferes with the insulin-producing cells of the pancreas. As the immune system progressively removes these cells, daily insulin injections are necessary to move glucose into the body’s cells.

Symptoms may include:

Multiple sclerosis

MS is a condition wherein the body’s immune cells mistakenly damage the myelin sheath. This is the fatty covering that surrounds and protects nerve cells in the brain and spinal cord. This interferes with the transmission of nerve signals.

Symptoms may include:

  • muscle weakness
  • numbness or prickling sensations
  • coordination and balance difficulties
  • vision issues
  • problems with memory and thinking

Rheumatoid arthritis

Rheumatoid arthritis occurs when the body mistakenly damages healthy tissue and organs. The synovial membrane that lines the joints becomes inflamed. Although rheumatoid arthritis primarily affects the joints, it can also affect bones, cartilage, and other tissues.

Symptoms may include:

  • joints that are swollen, tender, and warm
  • limited movement due to stiffness


Lupus is a condition wherein the body mistakenly damages healthy tissue and organs. As more parts of the immune system become involved, it can cause severe inflammation.

Symptoms may include:

Crohn’s disease

Crohn’s disease is a kind of inflammatory bowel disease. It primarily affects the digestive tract. Chronic inflammation of the gastrointestinal tract may cause some areas of the intestinal wall to thicken.

Symptoms may include:

How can you boost your immune system?

There are many ways to boost your immune system. The sections below cover some of these in more detail.

Get adequate sleep

Getting enough quality sleep can boost Trusted Source Centers for Disease Control and Prevention (CDC) Governmental authority Go to source your immunity. Good sleep habits include sleeping in a completely dark room and going to bed at the same time every night.

Follow a healthy diet

Whole plant foods — such as fruits, vegetables, nuts, seeds, and legumes — are rich in nutrients and antioxidants that help decrease inflammation. Limiting saturated fats, cholesterol, salt, and added sugars can improve your overall health.  

Get moderate exercise

Moderate exercise can boost your immune system by reducing stress and anxiety and contributing to better sleep. It can also help keep your brain and heart healthy.

Avoid smoking

Avoiding smoking is an effective way to boost your immune system. Smoking increases your risk of developing certain immune system disorders.


Here are some other commonly asked questions about the immune system.

Which vitamins can strengthen the immune system?

According to the Centers for Disease Control and Prevention (CDC) Trusted Source Centers for Disease Control and Prevention (CDC) Governmental authority Go to source , vitamin A, vitamin D, and zinc can help boost your immune system and resistance to infections. Eating a varied and healthy diet can help you get the vitamins and minerals you need to keep your immune system strong.

What are the signs and symptoms of a weak immune system?

A weakened immune system can lead to frequent illnesses and infections. Someone with a weak immune system can experience fatigue, digestive issues, wounds that are slow to heal, and other symptoms consistent with illness.


Your immune system protects you from potentially harmful substances, such as bacteria, viruses, and parasites. It can remember antigens it has encountered before, increasing immunity and decreasing your risk of getting sick.

Common immune system disorders include type 1 diabetes, MS, rheumatoid arthritis, lupus, and Crohn’s disease. You can help boost your immune system by getting enough sleep, eating a healthy diet, and getting regular exercise.

If you think you are experiencing issues with your immune system, talk with your doctor to learn more about possible causes and how to help strengthen your immune system.

Was this helpful?

Medical Reviewer: Adam Bernstein, MD, ScD
Last Review Date: 2022 May 10
View All Allergies Articles
THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.