Chapter 43

The Immune System

Concept 43.1: Innate Concept 43.1: Innate immunity provides broad immunity provides broad defenses against infectiondefenses against infection• A pathogen that breaks through external

defenses encounters innate cellular and chemical mechanisms that impede its attack

External Defenses

• Skin and mucous membranes are physical barriers to entry of microorganisms and viruses

• Secretions of the skin and mucous membranes provide an environment hostile to microbes

External Defenses

• Secretions give the skin a pH between 3 and 5, acidic enough to prevent colonization of many microbes

• Skin secretions include proteins such as lysozyme, which digests bacterial cell walls

External Defenses

• Mucous membrane cells produce mucus, a viscous fluid that traps microbes and other particles

• In the trachea, ciliated epithelial cells sweep mucus and any entrapped microbes upward, preventing microbes from entering the lungs

Internal Cellular and Chemical Defenses

• Internal cellular defenses depend mainly on phagocytosis

• White blood cells called phagocytes ingest microorganisms and initiate inflammation

Phagocytic Cells

• Phagocytes attach to prey via surface receptors and engulf them, forming a vacuole that fuses with a lysosome

PseudopodiaMicrobes

MACROPHAGE

Lysosomecontainingenzymes

Vacuole

• Macrophages, a type of phagocyte, migrate through the body and are found in organs of the lymphatic system

• The lymphatic system defends against pathogens

Adenoid

Tonsil

Lymphnodes

Peyer’s patches(small intestine)

Spleen

Appendix

Lymphaticvessels Lymph

nodeMasses oflymphocytes andmacrophages

Lymphaticvessel

Bloodcapillary

Tissuecells

Lymphaticcapillary

Interstitialfluid

The Lymphatic System

• Drains excess fluid– Into veins near the heart

• Temporary reservoir for fluids• Carry hormones throughout the body• Carries fat molecules to the regular

vessels• Part of the immune system

Diseases of the Lymphatic System

Antimicrobial Proteins

• Proteins function in innate defense by attacking microbes directly or impeding their reproduction

• About 30 proteins make up the complement system, which causes lysis of invading cells and helps trigger inflammation

• Interferons provide innate defense against viruses and help activate macrophages

Inflammatory Response

• In local inflammation, histamine and other chemicals released from injured cells promote changes in blood vessels

• These changes allow more fluid, phagocytes, and antimicrobial proteins to enter tissues

Pathogen Pin

Chemical signals

CapillaryPhagocytic cells

Macrophage

Red blood cell

Bloodclottingelements

Blood clot

Phagocytosis

Natural Killer Cells“better safe than sorry”

• Natural killer (NK) cells attack virus-infected body cells and cancer cells– Recognizes cells that contain “non-self” proteins

• They trigger apoptosis in the cells they attack