The Immune System Chapter 43 Animal Defenses • Two categories: • Innate Immunity • Does not distinguish one infectious agent from another • Acquired Immunity • Responds in specific ways to particular toxins, microorganisms, aberrant body cells, and other substances marked as foreign molecules Innate Immunity • Two types of Innate Immunity: • External Defenses • First Line of Defense • Physical barriers (Epithelium) • Skin • Mucus membranes • Internal Defenses • Second Line of Defense • Phagocytic cells • Antimicrobial proteins • Both come into play when external defenses have been compromised Acquired Immunity • “The Immune System” • The third line of defense • Comes into play if the first two lines of defense have been compromised • Specific defensive proteins called antibodies are produced by lymphocytes First Line of Defense • Physical Barriers • Skin • Cannot normally be penetrated by bacteria and viruses • But, even minute abrasions allow their passage • Secreations from sebaceous and sweat glands keep the skin in a pH range of 3 to 5 (acidic) • Kills most microbes • Microbial colonization is also inhibited by saliva, tears, and mucus secretions that continually bathe exposed epithelium • All of these secretions contain antimicrobial proteins • An example is lysozyme, an enzyme that digests the cell walls of many bacteria.• Mucus Membranes • Line digestive, respiratory, and genitourinary tracts • Also prevents entry of harmful microbes • Traps microbes and particles • In the trachea, ciliated epithelial cells sweep out mucus and trapped microbes • Prevents these from entering the lungs • Exposes them to the acidic environment of the stomach • Kills most microbes • Hepatitis A is an exception (can survive this) Second Line of Defense • Depends mainly on phagocytosis • The ingestion of invading organisms by certain types of white blood cells Phagocytic Cells • Macrophages • 5% of leukocytes • Specialized cells that develop from monocytes • Large, long-lived phagocytes • Cells extend long pseudopodia, engulf the microbe into a vacuole which fuses with a lysosome. • Lysosomes kill in two ways • By generating toxins such as nitric oxide • By digetsing microbes with lysozyme • Evading Macrophages • Some microbes have outer capsules to which macrophages cannot attach • Others, like Mycobacterium tuberculosis, are resistant to lysosomal destruction • Esinophils • Only 1.5% of leukocytes • Help fight large parasitic invaders o e.g. Schistosoma mansoni (blood fluke) o They position themselves alongside the parasite and discharge destructive enzymes from cytoplasmic granules • Neutrophils • 60-70% of all leukocytes • Usually the first to arrive • Attracted to chemical signals released by infected tissue • Self-destruct while destroying invaders • Life Span is shortNatural Killer Cells • Do not attack microbes directly • They destroy infected cells • Typically those infected with viruses • Also attack abnormal body cells that could become cancerous • They attack the cell’s membrane and cause the cell to lyse Antimicrobial Proteins • A variety of proteins • They function in nonspecific defense • Attack microbes directly • Or impede microbe reproduction • Lysozyme and other antimicrobial proteins are known collectively as the complement system • (including about 20 serum proteins) Interferons • Another set of proteins • Nonspecific defense • Secreted by virus-infected cells • Do not benefit the infected cell but induce neighboring cells to produce chemicals that inhibit viral reproduction Inflammatory Response • Tissue damages leads to a localized inflammatory response • Could be injury • Could be invasion by microbes • Capillaries Respond by • Increased dilation • Increased permeability • Leads to increased redness, heat, and swelling Histamine Release • Inflammation sets off histamine release • Histamine is released by special leukocytes called basophils and by mast cells in connective tissue • Histamine triggers both increased dilation and permeability of nearby capillaries Enhanced Blood Flow • In addition to histamine, damaged tissues also release prostaglandins and other substances the also promote blood flow to the injured site • Enhances delivery of clotting elements • Blocks spread of microbes • Enhances migration of phagocytic cellsRapid Response to Invasion • Neutrophils are the first phagocytes to arrive • Next are macrophages • Macrophages consume pathogens and their products • Macrophages also consume damaged tissue cells and the remains of neutrophils • The dead phagocytic cells and fluids leaked from capillaries is called pus Systemic Response • If damage or infection is severe, a widespread non-specific response may occur • This is called a systemic response • The number of leukocytes in the blood may increase several-fold within a few hours • Fever is a systemic response • Increased body temperature • May be triggered by toxins or by pyrogens released from leukocytes • Inhibits growth of some microbes • Facilitates phagocytosis • Speeds up repair of tissue Septic Shock • An overwhelming systemic inflammatory response • Characterized by • High fever • Low Blood pressure • The most common form of death in U.S. Critical Care Units • Local Inflammation is an essential step toward healing • Widespread inflammation can be devastating Acquired Immunity • “The third line of defense” • The key cells of the third line of defense are lymphocytes • Two Main Types • B Cells • Mature in Bone marrow • T Cells • Mature in the Thymus • Both cell types circulate through the blood • Both are concentrated in the • Spleen • Live • Other Lymphatic tissuesLymphocytes • Recognize and respond to particular microbes and foreign molecules (antigens) • This is called specificity Antigens • Viruses • Bacteria • Fungi • Protozoa • Parasitic worms • Non-pathogens • Pollen • Transplanted tissue Antibodies • Proteins produced by B cells • Each antigen has a unique molecular shape • Stimulates certain B cells to secrete antibodies to interact specifically with it • B and T cells recognize specific antigens