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20.2: Introduction to the Immune System

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  • Worm Attack!

    Does this organism look like a space alien? A scary creature from a nightmare? In fact, it’s a 1-cm long worm in the genus Schistosoma. It may invade and take up residence in the human body, causing a very serious illness known as schistosomiasis. The worm gains access to the human body while it is in a microscopic life stage. It enters through a hair follicle when the skin comes into contact with contaminated water. The worm then grows and matures inside the human organism, causing disease.

    Screen Shot 2019-03-31 at 4.27.25 PM.png
    Figure \(\PageIndex{1}\): (Public Domain; Bruce Wetzel and Harry Schaefer, NCI NIH).

    Host vs. Pathogen

    The Schistosoma worm has a parasitic relationship with humans. In this type of relationship, one organism, called the parasite, lives on or in another organism, called the host. The parasite always benefits from the relationship and the host is always harmed. The human host of the Schistosoma worm is clearly harmed by the parasite when it invades the host’s tissues. The urinary tract or intestines may be infected, and signs and symptoms may include abdominal pain, diarrhea, bloody stool, or blood in the urine. Those who have been infected a long time may experience liver damage, kidney failure, infertility, or bladder cancer. In children, Schistosoma infection may cause poor growth and difficulty learning.

    Like the Schistosoma worm, many other organisms can make us sick if they manage to enter our body. Any such agent that can cause disease is called a pathogen. Most pathogens are microorganisms, although some, such as the Schistosoma worm, are much larger. In addition to worms, common types of pathogens of human hosts include bacteria, viruses, fungi, and single-celled organisms called protists. You can see examples of each of these types of pathogens in the table below. Fortunately for us, our immune system is able to keep most potential pathogens out of the body or to quickly destroy them if they do manage to get in. When you read this chapter, you’ll learn how your immune system usually keeps you safe from harm — including from scary creatures like the Schistosoma worm!

    Screen Shot 2019-03-31 at 4.26.04 PM.png
    Figure \(\PageIndex{2}\): Common pathogens that cause human disease. (E coli: (Rocky Mountain Lab, NIAID, NIH; public domain; via; EscherichiaColi) Herpes simplex: (CDC/ Dr. Erskine Palmer; public domain; via Death cap: (GLJIVARSKO DRUSTVO; CC BY 2.0;; Giardia lamblia: (Public Domain; CDC / Janice Haney Carr; via; Giardia lamblia) .

    What is the Immune System?

    The immune system is a host defense system. It comprises many biological structures —ranging from individual white blood cells to entire organs — as well as many complex biological processes. The function of the immune system is to protect the host from pathogens and other causes of disease such as tumor cells. To function properly, the immune system must be able to detect a wide variety of pathogens. It also must be able to distinguish the cells of pathogens from the host’s own cells and also to distinguish cancerous or damaged host cells from healthy cells. In humans and most other vertebrates, the immune system consists of layered defenses that have increased specificity for particular pathogens or tumor cells. The layered defenses of the human immune system are usually classified into two subsystems called the innate immune system and the adaptive immune system.

    Innate Immune System

    Phagocytosis ZP
    Figure \(\PageIndex{3}\): A leukocyte called a macrophage phagocytizes bacteria in the series of steps shown here. First, the bacterium is engulfed. Then it is digested. And, finally, the components of the bacterium are expelled. (OpenStax; CC BY 4.0; via

    The innate immune system provides a very quick but non-specific response to pathogens. It responds the same way regardless of the type of pathogen that is attacking the host. It includes barriers — such as the skin and mucous membranes — that normally keep pathogens out of the body. It also includes general responses to pathogens that manage to breach these barriers, including chemicals and cells that attack the pathogens inside the human host. For example, certain leukocytes (white blood cells) engulf and destroy pathogens they encounter in the process called phagocytosis, which is illustrated below. Exposure to pathogens leads to an immediate maximal response from the innate immune system.

    Adaptive Immune System

    The adaptive immune system is activated if pathogens successfully enter the body and manage to evade the general defenses of the innate immune system. An adaptive response is specific to the particular type of pathogen that has invaded the body or to cancerous cells. It takes longer to launch a specific attack, but once it is underway, its specificity makes it very effective. An adaptive response also usually leads to immunity. This is a state of resistance to a specific pathogen due to the ability of the adaptive immune system to “remember” the pathogen and immediately mount a strong attack tailored to that particular pathogen if it invades again in the future.

    Self vs. Non-Self

    Both innate and adaptive immune responses depend on the ability of the immune system to distinguish between self and non-self molecules. Self molecules are those components of an organism’s body that can be distinguished from foreign substances by the immune system. Virtually all body cells have surface proteins that are part of a complex called major histocompatibility complex (MHC). These proteins are one way the immune system recognizes body cells as self. Non-self proteins, in contrast, are recognized as foreign because they are different from self-proteins.

    Antigens and Antibodies

    Many non-self molecules comprise a class of compounds called antigens. Antigens, which are usually proteins, bind to specific receptors on immune system cells and elicit an adaptive immune response. Some adaptive immune system cells (B cells) respond to foreign antigens by producing antibodies. An antibody is a molecule that precisely matches and binds to a specific antigen. This may target the antigen (and the pathogen displaying it) for destruction by other immune cells.

    Antigens on the surface of pathogens are how the adaptive immune system recognizes specific pathogens. Antigen specificity allows for the generation of responses tailored to the specific pathogen. It is also how the adaptive immune system ”remembers” the same pathogen in the future.

    Immune Surveillance

    Another important role of the immune system is to identify and eliminate tumor cells. This is called immune surveillance. The transformed cells of tumors express antigens that are not found on normal body cells. The main response of the immune system to tumor cells is to destroy them. This is carried out primarily by aptly named killer T cells of the adaptive immune system.

    Lymphatic System

    The lymphatic system is a human organ system that is a vital part of the adaptive immune system. It is also part of the cardiovascular system and plays a major role in the digestive system (see the concept Lymphatic System). The major structures of the lymphatic system are shown in Figure \(\PageIndex{4}\).

    Figure \(\PageIndex{4}\): The lymphatic system includes the organs and vessels illustrated here. (by Illu lymphatic system.jpg [CC BY 3.0 ], via Wikimedia Commons; TE-Lymphatic system diagram.svg)

    The lymphatic system consists of several lymphatic organs and a body-wide network of lymphatic vessels that transport the fluid called lymph. Lymph is essentially blood plasma that has leaked from capillaries into tissue spaces. It includes many leukocytes, especially lymphocytes, which are the major cells of the lymphatic system. Like other leukocytes, lymphocytes defend the body. There are several different types of lymphocytes that fight pathogens or cancer cells as part of the adaptive immune system.

    Major lymphatic organs include the thymus and bone marrow. Their function is to form and/or mature lymphocytes. Other lymphatic organs include the spleen, tonsils, and lymph nodes, which are small clumps of lymphoid tissue clustered along lymphatic vessels. These other lymphatic organs harbor mature lymphocytes and filter lymph. They are sites where pathogens collect and adaptive immune responses generally begin.

    Neuroimmune System vs. Peripheral Immune System

    The brain and spinal cord are normally protected from pathogens in the blood by the selectively permeable blood-brain and blood-spinal cord barriers. These barriers are part of the neuroimmune system. The neuroimmune system has traditionally been considered to be distinct from the rest of the immune system, which is called the peripheral immune system, although that view may be changing (see the following Feature: Human Biology in the News). Unlike the peripheral system, in which leukocytes are the main cells, the main cells of the neuroimmune system are thought to be nervous system cells called glial cells. These cells are able to recognize and respond to pathogens, debris, and other potential dangers. Types of glial cells involved in neuroimmune responses include microglial cells and astrocytes.

    • Microglial cells are among the most prominent types of glial cells in the brain. One of their main functions is to phagocytize cellular debris that remains when neurons die. Microglial cells also “prune” obsolete synapses between neurons.
    • Astrocytes are glial cells that have a different immune function. They allow certain immune cells from the peripheral immune system to cross the blood-brain barrier into the brain to target both pathogens and damaged nervous tissue.

    Feature: Human Biology in the News

    “They’ll have to rewrite the textbooks!”

    That sort of response to a scientific discovery is sure to attract media attention, and it did. It’s what Kevin Lee, a neuroscientist at the University of Virginia, said in 2016 when his colleagues told him they had discovered human anatomical structures that had never before been detected. The structures were tiny lymphatic vessels in the meningeal layers surrounding the brain.

    How these lymphatic vessels could have gone unnoticed when all human body systems have been studied so completely is amazing in its own right. The suggested implications of the discovery are equally amazing:

    • The presence of these lymphatic vessels means that the brain is directly connected to the peripheral immune system, presumably allowing a close association between the human brain and human pathogens. This suggests an entirely new avenue by which humans and their pathogens may have influenced each other’s evolution. The researchers speculate that our pathogens may have even influenced the evolution of our social behaviors.
    • The researchers think there will also be many medical applications of their discovery. For example, the newly discovered lymphatic vessels may play a major role in neurological diseases that have an immune component, such as multiple sclerosis. The discovery might also affect how conditions such as autism spectrum disorders and schizophrenia are treated.


    • Any agent that can cause disease is called a pathogen. Most human pathogens are microorganisms such as bacteria and viruses. The immune system is the body system that defends the human host from pathogens and cancerous cells.
    • The innate immune system is a subset of the immune system that provides very quick but non-specific responses to pathogens. It includes multiple types of barriers to pathogens, leukocytes that phagocytize pathogens, and several other general responses.
    • The adaptive immune system is a subset of the immune system that provides specific responses tailored to particular pathogens. It takes longer to put into effect, but it may lead to immunity to the pathogens.
    • Both innate and adaptive immune responses depend on the ability of the immune system to distinguish between self and non-self molecules. Most body cells have major histocompatibility complex (MHC) proteins that identify them as self. Pathogens and tumor cells have non-self antigens that the immune system recognizes as foreign.
    • Antigens are proteins that bind to specific receptors on immune system cells and elicit an adaptive immune response. Generally, they are non-self molecules on pathogens or infected cells. Some immune cells (B cells) respond to foreign antigens by producing antibodies that bind with antigens and target pathogens for destruction.
    • An important role of the immune system is tumor surveillance. Killer T cells of the adaptive immune system find and destroy tumor cells, which they can identify from their abnormal antigens.
    • The lymphatic system is a human organ system that is a vital part of the adaptive immune system. It consists of several organs and a system of vessels that transport lymph. The main immune function of the lymphatic system is to produce, mature, and circulate lymphocytes, which are the main cells in the adaptive immune system.
    • The neuroimmune system that protects the central nervous system is thought to be distinct from the peripheral immune system that protects the rest of the human body. The blood-brain and blood-spinal cord barriers are one type of protection of the neuroimmune system. Glial cells also play a role in this system, for example, by carrying out phagocytosis.


    1. What is a pathogen?
    2. State the purpose of the immune system.
    3. Compare and contrast the innate and adaptive immune systems.
    4. Explain how the immune system distinguishes self molecules from non-self molecules.
    5. What are antigens?
    6. Define tumor surveillance.
    7. Briefly describe the lymphatic system and its role in immune function.
    8. Identify the neuroimmune system.
    9. Which of the following is NOT a function of the immune system?

      A. Protecting the body against fungi

      B. Protecting the body against bacteria

      C. Protecting the body against cancerous cells

      D. None of the above

    10. What does it mean that the immune system is not just composed of organs?

    11. What are the general relationships between the terms lymphocytes, leukocytes, and white blood cells?

    12. True or False. Phagocytosis occurs in the innate immune system.

    13. True or False. Major histocompatibility complex proteins are antibodies.

    14. True or False. Only the adaptive immune response requires the ability to distinguish between self and non-self.

    15. Why is the immune system considered to be “layered?”

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