Lesson Overview 35.1 Infectious Diseases 35.2 Defenses Against Infection 35.4 Immune System Disorders

Standards SC.912.L.14.6 Explain the significance of genetic factors, environmental factors, and pathogenic agents to health from the perspectives of both individual and public health. SC.912.L.14.52 Explain the basic functions of the human immune system, including specific and nonspecific immune response, vaccines, and antibiotics.

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THINK ABOUT IT For thousands of years, people believed that diseases were caused by curses, evil spirits, or vapors rising from foul marshes or dead plants and animals. In fact, malaria was named after the Italian words mal aria, meaning “bad air.” This isn’t all that surprising, because, until microscopes were invented, most causes of disease were invisible to the human eye!

Causes of Infectious Disease During the mid-nineteenth century, Louis Pasteur and Robert Koch established a scientific explanation for infectious disease. Pasteur’s and Koch’s observations and experiments led them to conclude that infectious diseases are caused by microorganisms. Microorganisms were commonly called “germs,” so this conclusion was called the germ theory of disease.

Agents of Disease Infectious diseases are caused by pathogens—organisms that invade the body and disrupt its normal functions. Examples of pathogens are viruses, bacteria, single-celled eukaryotes, fungi, and parasites.

Agents of Disease Viruses are NONLIVING particles that replicate by inserting their genetic material into a host cell and taking over many of the host cell’s functions. Viruses cause: common cold Influenza chicken pox warts

Agents of Disease Bacteria cause disease by breaking down the tissues of an infected organism for food, or by releasing toxins that interfere with normal activity in the host. Bacteria cause: streptococcus infections Diphtheria Botulism anthrax

Agents of Disease Different types of fungus may infect the surface of the skin, mouth, throat, fingernails and toenails. Dangerous infections may spread from the lungs to other organs. The fungus Trichophyton interdigitale causes athlete’s foot.

Agents of Disease The single-celled eukaryote Plasmodium causes malaria, a very damaging infectious disease. The single-celled eukaryote Trypanosoma brucei feeds off nutrients in its host’s blood and causes African sleeping sickness. Both Plasmodium and Trypanosoma brucei are spread to humans by insects. Giardia intestinalis causes infection of the digestive tract and is transmitted in infected water.

Symbionts vs. Pathogens What’s the difference between harmless microorganisms and pathogens that cause disease? SYMBIONTS = The “good guys” obtain nutrients, grow, and reproduce without disturbing normal body functions. PATHOGENS = The “bad guys” cause problems in various ways.

Symbionts vs. Pathogens Some viruses and bacteria directly destroy the cells of their host. Other bacteria and single-celled parasites release poisons that kill the host’s cells or interfere with their normal functions. Parasitic worms may block blood flow through blood vessels or organs, take up the host’s nutrients, or disrupt other body functions.

How Diseases Spread How are infectious diseases spread? Some diseases are spread through: Coughing Sneezing physical contact exchange of body fluids contaminated water or food Some spread to humans from infected animals

How Diseases Spread Pathogens are often spread by symptoms of disease, such as sneezing, coughing, or diarrhea. In many cases, these symptoms are changes in host behavior that help pathogens spread and infect new hosts. If a virus infects only one host, that virus will die when the host’s immune system kills it or when the host dies. For that reason, natural selection favors pathogens with adaptations that help them spread from host to host.

Zoonoses: The Animal Connection Sometimes an animal carries, or transfers, zoonotic diseases from an animal host to a human host. These carriers, called vectors, transport the pathogen but usually do not get sick themselves. Mosquitos can transfer West Nile virus between birds and humans.

Practice & Discuss in your Body Book p.9 What are ways pathogens can cause disease in their hosts? How do vectors contribute to the spread of disease? What are the ways we could prevent the spread of disease?

Nonspecific Defenses What are the body’s nonspecific defenses against pathogens? Nonspecific defenses include the skin, tears and other secretions, the inflammatory response, interferons, and fever.

Nonspecific Defenses The body’s first defense against pathogens is a combination of physical and chemical barriers. These barriers include the skin, tears and other secretions, the inflammatory response, interferons, and fever. These barriers are called nonspecific defenses because they act against a wide range of pathogens.

First Line of Defense The skin is the most widespread line of defense. Other nonspecific defenses protect parts of the body that are not covered by skin, such as the mouth, nose, and eyes. Saliva, mucus, and tears contain lysozyme, an enzyme that breaks down bacterial cell walls. Mucus in your nose and throat traps pathogens. Then, cilia push the mucous-trapped pathogens away from your lungs. Stomach secretions destroy many pathogens that are swallowed.

Second Line of Defense If pathogens make it into the body, through a cut in the skin, for example, the body’s second line of defense swings into action. These mechanisms include the inflammatory response, the actions of interferons, and fever.

Inflammatory Response The inflammatory response causes infected areas to become red and painful, or inflamed. The response begins when pathogens stimulate cells called mast cells to release chemicals known as histamines. Histamines increase the flow of blood and fluids to the affected area. Add this picture to page 9

Inflammatory Response Fluid leaking from expanded blood vessels causes the area to swell. White blood cells move from blood vessels into infected tissues.

Inflammatory Response Many of these white blood cells are phagocytes, which engulf and destroy bacteria. All this activity around a wound may cause a local rise in temperature. That’s why a wounded area sometimes feels warm. Add this picture to page 9

Interferons When viruses infect body cells, certain host cells produce proteins that inhibit synthesis of viral proteins and help block viral replication. Scientists named these proteins interferons because they “interfere” with viral growth. Interferons slow down the progress of infection and “buy time” for specific immune defenses to respond.

Fever The immune system also releases chemicals that increase body temperature, producing a fever. The increased body temperature may slow down or stop the growth of some pathogens. Higher body temperature also speeds up several parts of the immune response.

Specific Defenses: The Immune System What is the function of the immune system’s specific defenses? The immune system’s specific defenses distinguish between “self” and “other,” and they inactivate or kill any foreign substance or cell that enters the body. A healthy immune system recognizes all cells and proteins that belong in the body, and treats these cells and proteins as “self.” This ability to recognize “self” is essential, because the immune system controls powerful cellular and chemical weapons that could cause problems if turned against the body’s own cells.

Recognizing “Nonself” The immune system recognizes foreign organisms and molecules, as “other,” or “nonself.” Once the immune system recognizes invaders as “others,” it uses cellular and chemical weapons to attack them. After encountering a specific invader, the immune system “remembers” the invader, enabling a more rapid and effective response if that same pathogen or a similar one attacks again. This specific recognition, response, and memory are called the immune response.

Antigens Specific immune defenses are triggered by molecules called antigens. An antigen is any foreign substance that can stimulate an immune response. Typically, antigens are located on the outer surfaces of bacteria, viruses, or parasites. The immune system responds to antigens by increasing the number of cells that either attack the invaders directly or that produce proteins called antibodies.

Lymphocytes The main working cells of the immune response are B lymphocytes (B cells) and T lymphocytes (T cells). B cells are produced in, and mature in, red bone marrow. B cells have embedded antibodies and discover antigens in body fluids.

Lymphocytes T cells are produced in the bone marrow but mature in the thymus—an endocrine gland. T cells must be presented with an antigen by infected body cells or immune cells that have encountered antigens.

Lymphocytes Each B cell and T cell is capable of recognizing one specific antigen. A person’s genes determine the particular B and T cells that are produced. When mature, both types of cells travel to lymph nodes and the spleen, where they will encounter antigens.

Humoral Immunity The immune response that defends against antigens in body fluids, such as blood and lymph, is called humoral immunity.

Cell-Mediated Immunity Another part of the immune response, which depends on the action of macrophages and several types of T cells, is called cell-mediated immunity. This part of the immune system defends the body against viruses, fungi, and single-celled pathogens. T cells also protect the body from its own cells when they become cancerous.

Practice & Discuss in your Body Book p.9 Why is the structure of an antibody so important? Tell why the statement is false: The immune system cannot combat pathogens it has not encountered before. Illustrate an analogy that compares the body’s nonspecific defenses to a large building’s security system.