Quick Quiz Complete each of these statements with always , sometimes , or never .

1. Quick Quiz Complete each of these statements with always, sometimes, or never. I ____ wash my hands before meals. When preparing fruits and vegetables, I ____ wash them thoroughly. I am ____ careful to use only my own eating utensils, drinking cups, towels and grooming items. I ____ cover my mouth when I cough or sneeze. If I spend time in wooded areas, I ____ wear insect repellent. For each of your responses, explain how your behavior could affect your chances of getting or spreading an infectious disease.

2. Physical and Chemical Defenses Your body’s first line of defense against infectious disease includes both physical and chemical defenses that prevent pathogens from entering your body.

3. Skin • Your skin serves as both a physical and a chemical barrier against pathogens. • Sweat acts as a chemical barrier. • Old skin cells are shed constantly, and the pathogens on these cells are shed, too.

4. Mucous Membranes • The openings into your body, such as your mouth, eyes, and nose, are covered by protective linings called mucous membranes (MYOO kus). • Mucus traps many pathogens and washes them away. • Mucus contains chemicals and specialized cells that attack pathogens.

5. Cilia • Some of your body’s mucous membranes are lined with tiny hairlike structures called cilia (SIL ee uh). • Together, cilia and mucus help trap and remove pathogens.

6. Saliva and Tears • Your saliva and tears can trap pathogens and wash them away. • Saliva and tears also contain chemicals that attack pathogens.

7. Digestive System • Chemicals in your digestive system kill many pathogens. • The normal motions of the digestive system move pathogens out. • Bacteria that normally live in your digestive system produce substances that can harm or kill invading bacteria.

8. Stomach acidWhen you swallow certain pathogens with food, water, or mucus, acids in your stomach can kill those pathogens. Your Body’s Physical and Chemical Defenses Saliva and tearsSaliva and tears can wash away some pathogens. Mucous membranesMucus traps some pathogens, preventing them from entering your body. CiliaCilia help move mucus and pathogens out of your body when you cough or sneeze. SkinYour skin is an effective barrier against many pathogens.

9. Inflammation • Inflammation (in fluh MAY shun) is your body’s general response to all kinds of injury, from cuts and scrapes to internal damage. • Inflammation fights infection and promotes the healing process.

10. Phagocytes • Within seconds after your body is injured, the damaged cells release chemicals that cause blood vessels in the injured area to enlarge. • Blood, other fluids, and white blood cells called phagocytes (FAG uh syts) leak out of the enlarged vessels. • Phagocytes engulf and destroy pathogens.

11. Phagocytes This micrograph shows a phagocyte (blue) attacking bacteria (pink). Phagocytes kill pathogens by engulfing and then digesting them.

12. Healing • Phagocytes also give off substances that cause healing to begin. • The inflammation process heals the damage, and the inflammation subsides.

13. The Immune System The immune system (ih MYOON) fights disease by producing a separate set of weapons for each kind of pathogen it encounters.

14. The Immune Response • When a pathogen enters your body for the first time, it often causes disease. • White blood cells called lymphocytes (LIM fuh syts) carry out most of the immune system’s functions. • If a pathogen that has previously attacked your body enters your body again your immune system will quickly recognize the pathogen and launch an immediate attack. • Immunity (ih MYOON ih tee) is your body’s ability to destroy pathogens that it has previously encountered before the pathogens are able to cause disease.

15. T Cells T cells perform several functions. • Killer Tcells destroy any body cell that has been infected by a pathogen. • Helper Tcells produce chemicals that stimulate other T cells and B cells to fight off infection. • Suppressor Tcells produce chemicals that “turn off” other immune system cells when an infection has been brought under control.

16. B Cells • The B lymphocytes, or B cells, produce antibodies. • Antibodies (AN tih bahd eez) are proteins that attach to the surface of pathogens or to the toxins produced by pathogens. • This binding action keeps the pathogen or toxin from harming the body. • The memory capacity of B cells explains why you develop immunity to some diseases you’ve already had.

17. The Immune Response

18. The Lymphatic System • The lymphatic system (lim FAT ik) is a network of vessels that collects fluid from your tissues and returns it to the bloodstream. • The fluid flowing through the lymphatic system is called lymph (limf). • The lymphatic vessels have hundreds of small stations, called lymph nodes.

19. Lymph nodes Lymph vessel The Lymphatic System Your lymphatic system is a complex network of vessels and nodes.

20. Passive and Active Immunity Passive Immunity • Immunity acquired by receiving antibodies from a source other than one’s own immune system is called passive immunity. • This type of immunity is temporary, not lifelong. • It occurs naturally in babies, who receive antibodies from their mothers before birth. • Passive immunity can be artificially acquired.

21. Passive and Active Immunity Active Immunity • Active immunity results from either having a disease or from receiving a vaccine. • Injections, which cause you to become immune to a disease, are called immunizations(im yuh nih ZAY shunz), or vaccinations. • The substance that is injected is called a vaccine (vak SEEN). • Vaccines contain small amounts of dead or modified pathogens or their toxins.