In this chapter, you learned about the immune system. Specifically, you learned that:

• 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, infected cells, and tumor cells have non-self proteins called 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. 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 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. Neuroglia also play a role in this system, for example, by carrying out phagocytosis.
• 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 or filter the fluid called lymph. The main immune function of the lymphatic system is to produce, mature, harbor, and circulate white blood cells called lymphocytes, which are the main cells in the adaptive immune system, and are circulated in lymph.

• • The return of lymph to the bloodstream is one of the functions of the lymphatic system. Lymph flows from tissue spaces, where it leaks out of blood vessels, to major veins in the upper chest. It is then returned to the cardiovascular system. Lymph is similar in composition to blood plasma. Its main cellular components are lymphocytes.
  • Lymphatic vessels called lacteals are found in villi that line the small intestine. Lacteals absorb fatty acids from the digestion of lipids in the digestive system. The fatty acids are then transported through the network of lymphatic vessels to the bloodstream.
  • Lymphocytes, which include B cells and T cells, are the subset of leukocytes involved in adaptive immune responses. They may create a lasting memory of and immunity to specific pathogens.
  • All lymphocytes are produced in bone marrow and then go through a process of maturation, in which they “learn” to distinguish self from non-self. B cells mature in the bone marrow, and T cells mature in the thymus. Both the bone marrow and thymus are considered primary lymphatic organs.
  • Secondary lymphatic organs include the tonsils, spleen, and lymph nodes. There are four pairs of tonsils that encircle the throat. The spleen filters blood, as well as lymph. There are hundreds of lymph nodes located in clusters along the lymphatic vessels. All of these secondary organs filter lymph and store lymphocytes, so they are sites where pathogens encounter and activate lymphocytes and initiate adaptive immune responses.
• Unlike the adaptive immune system, the innate immune system does not confer immunity. The innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses.

• • The body’s first line of defense consists of three different types of barriers that keep most pathogens out of body tissues. The types of barriers are mechanical, chemical, and biological barriers.

• • • Mechanical barriers — which include the skin, mucous membranes, and fluids (such as tears and urine) — physically block pathogens from entering the body.
    • Chemical barriers — such as enzymes in sweat, saliva, and semen — kill pathogens on body surfaces.
    • Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.
  • If pathogens breach the protective barriers, inflammation occurs. This creates a physical barrier against the spread of infection and repairs tissue damage. Inflammation is triggered by chemicals (such as cytokines and histamines), and it causes swelling, redness, and warmth.
  • The complement system is a complex biochemical mechanism that helps antibodies kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the cell membrane of pathogens and bursting of the cells.
  • Cellular responses of the innate immune system involve various types of leukocytes (white blood cells). For example, neutrophils, macrophages, and dendritic cells phagocytize pathogens. Basophils and mast cells release chemicals that trigger inflammation. Natural killer cells destroy cancerous or virus-infected cells, and eosinophils kill parasites.
  • Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.
• The main cells of the adaptive immune system are lymphocytes. There are two major types of lymphocytes: T cells and B cells. Both types must be activated by foreign antigens to become functioning immune cells.

• • Most activated T cells become either killer T cells or helper T cells. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing cytokines that control other types of leukocytes.
  • Activated B cells form plasma cells that secrete antibodies, which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the complement system.
• After an adaptive immune response occurs, long-lasting memory B cells and memory T cells may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.
• Immunity may be active or passive.

• •  Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.
  • Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.
• Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus (HIV) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host’s cell membrane.
• An allergy is a disorder in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes allergies is called an allergen. Common allergens include pollen, dust mites, mold, specific foods (such as peanuts), insect stings, and certain medications (such as aspirin).

• • The prevalence of allergies has been increasing for decades, especially in developed countries, where they are much more common than in developing countries. The hygiene hypothesis posits that this has occurred because humans evolved to cope with more pathogens than we now typically face in our relatively sterile environments in developed countries. As a result, the immune system “keeps busy” by attacking harmless antigens.
  • Allergies occur when B cells are first activated to produce large amounts of antibodies to an otherwise harmless allergen, and the antibodies attach to mast cells. On subsequent exposures to the allergen, the mast cells immediately release cytokines and histamines that cause inflammation.
  • Mild allergy symptoms are frequently treated with antihistamines that counter histamines and reduce allergy symptoms. A severe systemic allergic reaction, called anaphylaxis, is a medical emergency that is usually treated with injections of epinephrine. Immunotherapy for allergies involves injecting increasing amounts of allergens to desensitize the immune system to them.
• Autoimmune diseases occur when the immune system fails to recognize the body’s own molecules as self and attacks them, causing damage to tissues and organs. A family history of autoimmunity and female gender are risk factors for autoimmune diseases.

• • In some autoimmune diseases, such as type I diabetes, the immune system attacks and damages specific body cells. In other autoimmune diseases, such as systemic lupus erythematosus, many different tissues and organs may be attacked and injured. Autoimmune diseases generally cannot be cured, but their symptoms can often be managed with drugs or other treatments.
• Immunodeficiency occurs when the immune system is not working properly, generally because one or more of its components are inactive. As a result, the immune system is unable to fight off pathogens or cancers that a normal immune system would be able to resist.

• • Primary immunodeficiency is present at birth and caused by rare genetic diseases. An example is severe combined immunodeficiency. Secondary immunodeficiency occurs because of some event or exposure experienced after birth. Possible causes include substance abuse, obesity, and malnutrition, among others.
  • The most common cause of immunodeficiency in the world today is human immunodeficiency virus (HIV), which infects and destroys helper T cells. HIV is transmitted through mucous membranes or body fluids. The virus may eventually lead to such low levels of helper T cells that opportunistic infections occur. When this happens, the patient is diagnosed with acquired immunodeficiency syndrome (AIDS). Medications can control the multiplication of HIV in the human body, but it can’t eliminate the virus completely.

Up to this point, this book has covered body systems that carry out processes within individuals, such as the digestive, muscular, and immune systems. Read the next chapter to learn about the body system that allows humans to produce new individuals — the reproductive system.