What Is the Immune System?

The immune system is the body's defense against infections. The immune (ih-MYOON) system attacks germs and helps keep us healthy.

What Are the Parts of the Immune System?

Many cells and organs work together to protect the body. White blood cells, also called leukocytes (LOO-kuh-sytes), play an important role in the immune system.

Some types of white blood cells, called phagocytes (FAH-guh-sytes), chew up invading organisms. Others, called lymphocytes (LIM-fuh-sytes), help the body remember the invaders and destroy them.

One type of phagocyte is the neutrophil (NOO-truh-fil), which fights bacteria. When someone might have bacterial infection, doctors can order a blood test to see if it caused the body to have lots of neutrophils. Other types of phagocytes do their own jobs to make sure that the body responds to invaders.

The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start out in the bone marrow and either stay there and mature into B cells, or go to the thymus gland to mature into T cells. B lymphocytes are like the body's military intelligence system — they find their targets and send defenses to lock onto them. T cells are like the soldiers — they destroy the invaders that the intelligence system finds.

How Does the Immune System Work?

When the body senses foreign substances (called antigens), the immune system works to recognize the antigens and get rid of them.

B lymphocytes are triggered to make antibodies (also called immunoglobulins). These proteins lock onto specific antigens. After they're made, antibodies usually stay in our bodies in case we have to fight the same germ again. That's why someone who gets sick with a disease, like chickenpox, usually won't get sick from it again.

This is also how immunizations (vaccines) prevent some diseases. An immunization introduces the body to an antigen in a way that doesn't make someone sick. But it does let the body make antibodies that will protect the person from future attack by the germ.

Although antibodies can recognize an antigen and lock onto it, they can't destroy it without help. That's the job of the T cells. They destroy antigens tagged by antibodies or cells that are infected or somehow changed. (Some T cells are actually called "killer cells.") T cells also help signal other cells (like phagocytes) to do their jobs.

Antibodies also can:

  • neutralize toxins (poisonous or damaging substances) produced by different organisms
  • activate a group of proteins called complement that are part of the immune system. Complement helps kill bacteria, viruses, or infected cells.

These specialized cells and parts of the immune system offer the body protection against disease. This protection is called immunity.

Humans have three types of immunity — innate, adaptive, and passive:

  • Innate immunity: Everyone is born with innate (or natural) immunity, a type of general protection. For example, the skin acts as a barrier to block germs from entering the body. And the immune system recognizes when certain invaders are foreign and could be dangerous.
  • Adaptive immunity: Adaptive (or active) immunity develops throughout our lives. We develop adaptive immunity when we're exposed to diseases or when we're immunized against them with vaccines.
  • Passive immunity: Passive immunity is "borrowed" from another source and it lasts for a short time. For example, antibodies in a mother's breast milk give a baby temporary immunity to diseases the mother has been exposed to.

The immune system takes a while to develop and needs help from vaccines. By getting all your child's recommended vaccines on time, you can help keep your child as healthy as possible.

Back to Articles


Related Articles

Activity: Immune System

Do you know your immune system? Label the parts of the system that keeps you well.

Read More

Definition: Autoimmunity

Sometimes the immune system makes a mistake and attacks the body's own tissues or organs. This is called autoimmunity.

Read More

Word! Autoimmunity

Your immune system fights infections and illnesses. But sometimes the immune system makes a mistake and attacks part of the body. This is called autoimmunity.

Read More

Definition: Autoimmunity

Sometimes the immune system makes a mistake and attacks the body's own tissues or organs. This is called autoimmunity.

Read More

HIV and AIDS

Get the facts on HIV and AIDS in this article just for kids.

Read More

Arthritis

Kids can get a kind of arthritis that causes joint pain. Find out more in this article for kids.

Read More

Immunotherapy to Treat Cancer

This promising new type of cancer treatment stimulates a person's immune system so it is better able to fight disease.

Read More

Severe Combined Immunodeficiency

Severe combined immunodeficiency (SCID) is an immune deficiency that can be successfully treated if it's found early.

Read More

Lupus

Lupus is known as an autoimmune disease in which a person's immune system mistakenly works against the body's own tissues.

Read More

Lupus

Lupus is a disease that affects the immune system. Learn how lupus is treated, signs and symptoms, how to support a friend who has it, and more.

Read More

Blood Test: Immunoglobulin A (IgA)

Checking IgA levels can help doctors diagnose problems with the immune system, intestines, and kidneys. It's also used to evaluate autoimmune conditions, such as rheumatoid arthritis, lupus, and celiac disease.

Read More

Neutropenia

Certain cancers, or cancer treatment, can weaken the immune system, requiring a child to stay home to avoid exposure to germs. Here are ways to help your child make the best of it.

Read More

Blood Test: Thyroid Peroxidase Antibodies

The thyroid peroxidase antibodies test is primarily used to help diagnose and monitor autoimmune conditions involving the thyroid gland, including Hashimoto's thyroiditis and Graves disease.

Read More

Movie: Immune System

Watch this movie about your immune system - the system that keeps you healthy.

Read More

Immune System

The immune system is made up of special cells, proteins, tissues, and organs that defend people against germs and microorganisms.

Read More

Juvenile Idiopathic Arthritis (JIA)

Learn about juvenile idiopathic arthritis, a specific kind of arthritis that usually occurs in kids and teens younger than 17.

Read More

Juvenile Idiopathic Arthritis

In juvenile idiopathic arthritis (also called juvenile rheumatoid arthritis)), a person can develop swollen, warm, and painful joints. Learn more.

Read More

Food Allergies

Struggling with strawberries? Petrified of peanuts? Sorry you ate shellfish? Maybe you have a food allergy. Find out more in this article for kids.

Read More

The Danger of Antibiotic Overuse

Taking antibiotics too often or for the wrong reason has led to a dangerous rise in bacteria that no longer respond to medicine. Find out what you can do to prevent antibiotic overuse.

Read More

HIV and AIDS

Parents can help prevent HIV/AIDS by learning the facts and talking with their kids regularly about healthy behaviors, feelings, and sexuality.

Read More

HIV and AIDS

There is no cure for AIDS, which is why prevention is so important. Get the facts on HIV/AIDS, as well as how it affects the body and is treated, in this article.

Read More

Food Allergies

Food allergies can cause serious and even deadly reactions in kids, so it's important to know how to feed a child with food allergies and to prevent reactions.

Read More

What Are Glands?

You've heard of glands, but what are they? Find out in this article for kids.

Read More

Your Immune System

The immune system keeps you healthy. How does it work? Find out in this article for kids.

Read More

Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2020 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.

Search our entire site.