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13.1: The Immune System

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    Our immune system works all day every day to fight off infections and diseases. The immune system consists of bone marrow, spleen, tonsils, thymus, lymph nodes, and lymphatic vessels. Because the lymph node and lymphatic vessels are part of the immune system, the immune system is sometimes referred to as the lymphatic system.
    The immune system produces, stores, and carries white blood cells around the body. The job of white blood cells is to fight off infection. The enemies the white blood cells are fighting are called antigens. Antigens are the markers on foreign substances that cause an immune response. Every pathogen, or disease causing agent, that enters the body has a different antigen. Common pathogens include bacteria, virus’, parasitic worms, fungi, and prions.

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    • Lymph: a clear fluid that travels through the lymphatic system and carries cells that help fight infections and other diseases.
    • Lymph nodes: rounded masses of lymphatic tissue that is surrounded by a capsule of connective tissue. Lymph nodes filter lymph, and they store white blood cells. They are located along lymphatic vessels.
    • Lymph vessels: thin tubes that carry lymph and white blood cells through the lymphatic system. They branch, like blood vessels, into all the tissues of the body.
    • The thymus: an organ in the chest behind the breastbone. T lymphocytes grow and multiply in the thymus.
    • The spleen: an organ on the left side of the abdomen, near the stomach. It produces some white blood cells, filters the blood, stores blood cells, and destroys old blood cells.
    • White blood cells: cells are made by bone marrow and help the body fight infection and other diseases. There are lots of types of white blood cells.

    Because we have many different key players in our immune system and each has a specialized role, it can be helpful to learn about the immune system through a visualization. Imagine that our white blood cells are warriors fighting a battle or are superheroes saving our body. The CDC developed a program for 9-12 year olds called BAM! that describes the immune system as the Immune Platoon, a team of super-powered white blood cells that protect our bodies. Each member of the immune platoon has a different super power, much like each type of white blood cells. When a pathogen enters the body, the antigen is read by the cells. Some white blood cells absorb the pathogen, others release antibodies that knock out the antigen, and others destroy the damaged cells.
    Types of white blood cells include: Macrophages, eosinophils, basophils, mast cells, natural killer cells, dendritic cells, t-cells, and b-cells. For information on the role ach type of cell plays in the immune response please click here.

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    The CDC has also published the “Ask a Scientist” series, which provides two graphic comic strips to help explain how the body fights off disease and how it can become infected with germs.

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    Common Pathogen: Influenza

    What is Influenza (also called Flu)?

    The flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is by getting a flu vaccine each year.

    Signs and Symptoms of Flu

    People who have the flu often feel some or all of these signs and symptoms:

    • Fever* or feeling feverish/chills
    • Cough
    • Sore throat
    • Runny or stuffy nose
    • Muscle or body aches
    • Headaches
    • Fatigue (very tired)
    • Some people may have vomiting and diarrhea, though this is more common in children than adults.

    *It’s important to note that not everyone with flu will have a fever.

    How Flu Spreads

    Most experts believe that flu viruses spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth, eyes or possibly their nose.

    Period of Contagiousness

    You may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick. Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Some people, especially young children and people with weakened immune systems, might be able to infect others for an even longer time.

    Onset of Symptoms

    The time from when a person is exposed to flu virus to when symptoms begin is about 1 to 4 days, with an average of about 2 days.

    Complications of Flu

    Complications of flu can include bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.

    People at High Risk from Flu

    Anyone can get the flu (even healthy people), and serious problems related to the flu can happen at any age, but some people are at high risk of developing serious flu-related complications if they get sick. This includes people 65 years and older, people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women, and young children.

    Preventing Flu

    The first and most important step in preventing flu is to get a flu vaccination each year. CDC also recommends everyday preventive actions (like staying away from people who are sick, covering coughs and sneezes and frequent handwashing) to help slow the spread of germs that cause respiratory (nose, throat, and lungs) illnesses, like flu.

    Diagnosing Flu

    It is very difficult to distinguish the flu from other viral or bacterial causes of respiratory illnesses on the basis of symptoms alone. There are tests available to diagnose flu. For more information, see Diagnosing Flu.


    There are influenza antiviral drugs that can be used to treat flu illness.

    For more information, see “Seasonal Influenza, More Information.”

    Contributors and Attributions

    Public Domain Content

    This page titled 13.1: The Immune System is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Kelly Falcone.