CHAPTER 40 – THE IMMUNE SYSTEM AND DISEASE

40-1 Infectious Disease

A disease is any change, other than an injury, that disrupts the normal functions of the body. Some diseases, such as hemophilia, are inherited. Others are caused by materials in the environment, such as cigarette smoke. Some others are produced by things such as bacteria, viruses, or pathogens.

The Germ Theory of Disease The germ theory of disease states that infectious diseases are caused by microorganisms, or germs. This theory came about through the works of a French chemist named Louis Pasteur and a German bacteriologist named Robert Koch. Prior to this theory, people thought diseases were brought on by curses or evil spirits.

Koch’s Postulates Koch developed a series of guidelines still used today to identify the microorganism that causes a specific disease. Koch’s postulates state the following: 1. The pathogen should always be found in the body of a sick organism and should not be found in a healthy one. 2. The pathogen must be isolated and grown in the laboratory in pure culture 3. When the purified pathogens are placed in a new host, they should cause the same disease that infected the original host. 4. The injected pathogen should be re-isolated from the second host. It should be identical to the original pathogen.

This made us realize that if a particular pathogen could be identified, maybe the disease it caused could be prevented or cured.

Toxins – poisons that produce illness by parasites that live and feed inside an infected organism. Some remove nutrients from the digestive system. Others destroy blood cells and neurons. A virus, which is a nonliving pathogen, can use the materials of a host cell to make copies of the virus until the cell bursts.

How Diseases are Spread The best method for fighting any disease is to avoid it. Infectious diseases are transmitted in a number of ways. Some infectious diseases are spread from one person to another through coughing, sneezing, or physical contact. Other infectious diseases are spread through contaminated water or food. Others are spread by infected animals.

Animals that carry disease-causing organisms from person to person are called vectors. (ex. Ticks or mosquitoes). It is important to try to avoid contact with vectors. Some ways are avoiding tall grass and wooded areas and spraying bug spray.

The common cold, mumps, measles, and the flu spread form one person to another through coughing, sneezing, or hand-to-hand contact. To avoid the spread, it is important to cover your mouth with a tissue when you cough or sneeze and washing your hands thoroughly.

Sexually transmitted diseases (STDs) are spread through sexual contact Sexually transmitted diseases (STDs) are spread through sexual contact. Syphilis and gonorrhea are caused by bacteria. Others, like hepatitis B, hepatitis C, genital herpes, and AIDS are caused by viruses. The consequences of having an STD can be severe. Gonorrhea and chlamydia can permanently damage the reproductive system. Syphilis and AIDS can be fatal.

Fighting Infectious Diseases If a disease was not able to be prevented, drugs have been developed for use against all sorts of pathogens. Antibiotics are compounds that kill bacteria without harming the cells of humans or animals. Antibiotics interfere with the cellular processes of microorganisms. Antibiotics have no effect on viruses, only bacteria.

40-2 The Immune System

Skin, mucus, sweat, and tears help to keep pathogens out Skin, mucus, sweat, and tears help to keep pathogens out. Your body’s most important nonspecific defense is the skin. Very few pathogens can get through the skin’s surface. Oil and sweat glands kill many types of bacteria. When skin is broken, pathogens can enter your body and multiply. This can cause an infection, which causes swelling, redness, and pain.

Pathogens can also enter your body through your mouth and nose Pathogens can also enter your body through your mouth and nose. Mucus in your nose and throat traps viruses and bacteria and cilia push them away from your lungs. Stomach acid and digestive enzymes destroy many pathogens that make their way to your stomach. Mucus, saliva, sweat, and tears contain lysozyme, which is an enzyme that breaks down the cell walls of many bacteria.

If pathogens do manage to enter your body, they may multiply quickly, releasing toxins into your tissues. When this happens, the inflammatory response, which is a second line of defense, is activated. The inflammatory response is a nonspecific defense reaction to tissue damage caused by injury or infection. Blood vessels near the wound expand and white blood cells leak from the vessels to enter the infected tissues. Many of these white blood cells are phagocytes, which engulf and destroy bacteria.

When pathogens are detected, the immune system produces millions of white blood cells, which fight the infection. The immune system also releases chemicals that increase the core body temperature. This is a fever.

Interferons are a group of proteins produced by virus-infected cells that help other cells resist viral infection.

Specific Defenses If a pathogen is able to get past the body’s non- specific defenses, the immune system reacts with a series of specific defenses that attack the disease-causing agent. These defenses are called immune response. Antigen- a substance that triggers this response. The body can produce two different immune responses: humoral immunity and cell-mediated immunity.

Humoral immunity – immunity against pathogens in the body fluids (blood and lymph). It is produced by lymphocytes (a type of white blood cell). An antibody molecule is the basic functional unit of the humoral immune response. Antibody – a protein that helps destroy pathogens

Cell-Mediated Immunity Cell-mediated immunity relies on lymphocytes, not antibodies. Helper T cells regulate the production of antibodies by B cells. Other T cells can attack antigen-carrying cells directly. The most effective attacking cells in the immune system are killer T cells. These transfer proteins into the cell membrane of a pathogen, causing fluid from inside the cell to leak out of the cell to rupture and die.

Killer T cells make it difficult to do organ transplants Killer T cells make it difficult to do organ transplants. The immune system recognizes cells. When an organ from one person is transplanted into another, the second person’s immune system recognizes the transplanted organ as foreign and attacks it. This is known as rejection. To prevent this, doctors look for a donor whose markings are nearly identical to the markers of the recipient. Recipients must take drugs that suppress the cell-mediated immune response.

People can acquire permanent immunity to certain diseases People can acquire permanent immunity to certain diseases. Once the body has been exposed to a pathogen, millions of memory B and T cells remain capable of producing specific antibodies to that pathogen. Memory B cells make sure the disease never gets a chance to develop a second time. Vaccination- the injection of a weakened or mild form of a pathogen to produce immunity.

Active immunity is produced because the body of the recipient has the ability to produce an immediate active immune response against the pathogen.

40-3 Immune System Disorders

There are two main types of disorders There are two main types of disorders. In the first, the immune system may overreact to an antigen, producing discomfort or even disease. In the second, the cellular nature of the immune response is a potential weak point.

Allergies – result when antigens from allergens bind to mast cells (a type of immune cell) When allergy-causing antigens attach themselves to mast cells, the activated mast cells release chemicals called histamines. Histamines produce sneezing, runny eyes and nose, and other irritations that make a person with allergies so uncomfortable. Some allergens include ragweed pollen, dust, and dust mites.

Allergic reactions can cause Asthma. Asthma is when the smooth muscle contractions reduce the size of air passageways in the lungs and make breathing very difficult. A particular antigen usually triggers an asthma attack.

Autoimmune Disease When the immune system makes a mistake and attacks the body’s own cells, it produces an autoimmune disease. Examples: juvenile-onset diabetes and multiple sclerosis

AIDS AIDS (Acquired Immune Deficiency Syndrome) is caused by a virus. The virus is known as HIV (human immunodeficiency virus). Once HIV enters the body, it attaches to receptors on the surfaces of the T cells that help other lymphocytes respond to infection. The body does produce antibodies against HIV, but HIV replicates within cells of the immune system, so it is less likely to bind to antibodies. Gradually, HIV kills off most of the helper T cells.

40-4 Cancer

Cancer is a life-threatening disease in which cells multiply uncontrollably and destroy healthy tissue.

Cancers begin when something goes wrong with the controls that normally regulate cell growth and division. Tumor – a mass of growing tissue, resulting from a single cell or a group of cells that begins to grow and divide uncontrollably. Cancerous tumors are malignant, which means they can invade and destroy surrounding healthy tissue. Metastasis- the spread of cancerous tumors beyond their original site.

Causes of Cancer Cancer may be inherited, may be caused by viruses (ex – HPV), or may result from mutations in DNA.

Fighting Cancer Treatments for cancer fall into three general categories: surgery, radiation therapy, and drug therapy (chemotherapy).