Overview Barriers help an animal to defend itself from the many dangerous pathogens it may encounter The immune system recognizes foreign bodies and responds with the production of immune cells and proteins Two major kinds of defense have evolved innate immunity and acquired immunity Both invertebrates and vertebrates depend on innate immunity to fight infection Vertebrates have an additional immune defense known as acquired immunity Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings 1 Innate immunity is present before any exposure to pathogens and is effective from the time of birth Innate immunity consists of external barriers plus internal cellular and chemical defenses It involves nonspecific responses to pathogens and is a rapid response found in all classes of plant and animal life 2 Acquired immunity or adaptive immunity develops after exposure to agents such as microbes toxins or other foreign substances It involves a very specific response to pathogens and is a slow response found in Vertebrates Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Concept 43 1 In innate immunity recognition and response rely on shared traits of pathogens Innate Immunity of Invertebrates In insects an exoskeleton made of chitin forms the first barrier to pathogens Innate defenses of Invertebrates are low pH antimicrobial peptides and phagocytosis in which an enzyme lysozyme digests microbial cell walls The immune system recognizes bacteria and fungi by structures on their cell walls An immune response varies with the class of pathogen encountered Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Innate Immunity of Vertebrates The immune system of mammals is the best understood of the vertebrates Innate defenses include barrier defenses low pH phagocytosis and antimicrobial peptides similar to defense of Invertebrates Additional defenses unique to vertebrates are 1 The Innate defenses with WBC 2 Inflammatory response 3 NK cells Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Barrier Defenses of Vertebrates Barrier defenses include the skin and mucous membranes of the respiratory urinary and reproductive tracts Mucus traps and allows for the removal of microbes Many body fluids including saliva mucus and tears are hostile to microbes The low pH of skin and the digestive system prevents growth of microbes Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings 1 Cellular Innate Defenses of Vertebrates with WBC White blood cells leukocytes engulf pathogens in the body A white blood cell engulfs a microbe then fuses with a lysozyme to destroy the microbe Some other examples of Leukocytes that carry phagocytosis are Neutrophils Eosinophils Dendritic cells Macrophages produced by differentiation of Monocytes Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings 2 Inflammatory Responses of Vertebrates Following an injury mast cells release histamine which promotes changes in blood vessels this is part of the inflammatory response These changes increase local blood supply and allow more phagocytes and antimicrobial proteins to enter tissues Pus a fluid rich in white blood cells dead microbes and cell debris accumulates at the site of inflammation Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 43 8 3 Pathogen Mast cell Splinter Chemical Macrophage signals Capillary Red blood cells Phagocytic cell Fluid Phagocytosis 3 Natural Killer Cells All cells in the body except red blood cells have a class 1 MHC protein on their surface Cancerous or infected cells no longer express this protein natural killer NK cells attack these damaged cells Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Innate Immune System Evasion by Pathogens Inflammation can be either local or systemic throughout the body Fever is a systemic inflammatory response triggered by pyrogens released by toxins from pathogens Some pathogens avoid destruction by modifying their surface to prevent recognition or by resisting breakdown following phagocytosis Tuberculosis TB is one such disease and kills more than a million people a year Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Concept 43 2 In acquired immunity lymphocyte receptors provide pathogen specific recognition A type of white blood cells called lymphocytes recognize and respond to antigens foreign molecules Lymphocytes that mature in the thymus above the heart are called T cells and those that mature in bone marrow are called B cells Lymphocytes contribute to immunological memory an enhanced response to a foreign molecule encountered previously Cytokines signalling molecules are secreted by macrophages and dendritic cells to recruit and activate lymphocytes Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Acquired Immunity B cells and T cells have receptor proteins that can bind foreign molecules known as Antigen Each individual lymphocyte is specialized to recognize a specific type of molecule Acquired Immunity of 2 types Humoral immune Response In HIR B cells after recognizing an Antigen give rise to plasma cells which secrete proteins called antibodies or immunoglobulins Ig Cell mediated Immune response T cells bind to antigen fragments and then activate macrophages natural killer cells and antigen specific cytotoxic T cells Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings An antigen is any foreign molecule to which a lymphocyte responds Antigenbinding site Antigenbinding site C C V V V Disulfide bridge V Antigenbinding site C C Light chain Variable regions V V Constant regions C C Transmembrane region Plasma membrane Heavy chains chain chain Disulfide bridge B cell Cytoplasm of B cell a B cell receptor Cytoplasm of T cell T cell b T cell receptor A single B cell or T cell has about 100 000 identical antigen receptors Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 43 10 Antigenbinding sites Antigen binding sites V V Antibody A Antigen Antibody C C C V V C C Antibody B B cell receptor Epitopes antigenic determinants Lymphocyte Development The acquired immune system has three important properties Receptor diversity A lack of reactivity against host cells Immunological memory The first exposure to a specific antigen represents the primary immune response