Chapter 43 The Immune System Pathogen Disease causing agent fungi bacterium virus Immune System Body s defenses which enable host to avoid infections Skin forms first line of defense against pathogens shortfall is openings that are required to allow gas exchange nutrition and reproduction Immune System forms second line of defense and detects foreign particles through receptor molecules which then allow for defense responses This occurs through the molecular recognition of foreign particles 2 types of molecular recognition are Innate and Adaptive immunity INNATE IMMUNITY Includes barrier defenses molecular recognition through receptor proteins Binding of innate immune receptor to foreign molecule activates internal defenses ADAPTIVE IMMUNITY More specific than innate immunity Has vast arsenal of receptors Activated after innate immune response and is also known as the ACQUIRED IMMUNE RESPONSE 43 1 IN INATE IMMUNITY RECOGNITION AND RESPONSE RELY ON TRAITS COMMON TO GROUPS OF PATHOGENS INVERETBERATES only have INNATE IMMUNITY INNATE IMMUNITY OF INVERTEBRATES Insects rely on their exoskeleton made up of Chitin as a first line of defense against most pathogens Chitin also lines the intestines of insects and it blocks the infection of many pathogens ingested with food LYSOZYME an enzyme that also breaks down bacterial cell walls also protects the insect s digestive system HEMOCYTES immune cells travel throughout the body in hemolymph to protect the insects by ingesting foreign substances PHAGOCYTOSIS Other hemocytes release chemicals and antimicrobial peptides Antimicrobial peptides help attack the cell wall of the invading bodies Macromolecules found on the outer membrane of bacteria and fungi sugars polysaccharides amino acids help serve as identity tags in the process of pathogen identification INNATE IMMUNITY OF VERTEBRATES Innate Immunity includes barrier defenses phagocytosis and antimicrobial peptides which are also found in invertebrates Vertebrates have unique aspects such as natural killer cells interferons and the inflammatory response Barrier Defenses Skin and the mucous membranes block many pathogens Mucous membranes produce mucous that trap pathogens and other particles Saliva tears and mucous provide a washing action that inhibits the colonization by fungi and bacteria These fluids contain lysozymes that destroy the cell walls of susceptible bacteria and fungi Sweat and Skin Oil also give our skin a high pH to prevent growth of bacteria Cellular Innate Defenses Mammals have Toll like receptors that bind to fragments of molecules normally absent from the vertebrate body but characteristic of a set of pathogens such as the nucleic acid of certain viruses This detection triggers phagocytosis and destruction The 2 Phagocytic cells are NEUTROPHILS circulate in blood and are attracted by signals from infected tissues to destroy pathogens or MACROPHAGES migrate across body or found in organs to perform phagocytosis 2 other types of phagocytic cells are DENDRTIIC Cells populate tissues in contact with environment and stimulate adaptive immunity against pathogens and kEOSINOPHILS discharge destructive enzymes against multicellular invaders NATURAL KILLER CELLS release chemicals to lead to cell death and inhibit the further spread of virus or cancer Antimicrobial Peptides and Proteins In mammals pathogen recognition triggers the production and release of a variety of peptides and proteins that attack pathogens or impede their reproduction INFTERFERONS and COMPLEMENT PROTEINS are unique to vertebrate immune systems Interferons are proteins that provide innate defense by interfering with viral infections Infected cells limit cell to cell spread of viruses by secreting Interferons which produce substance that inhibit viral replication The complement system has around 30 proteins in blood plasma and circulate in an inactive state and are activated by substances on the surface of many microbes This leads to biochemical reactions that lead to the lysis of invading cells Inflammatory Response Histamine is an important inflammatory signaling molecule that is stored in densely packed vesicles of mast cells which are found in connective tissue Histamine causes localized swelling by making blood vessels dilate and become more permeable Macrophages and neutrophils also participate by discharging cytokines signaling molecules that modulate immune responses This promotes blood flow and increased temperature The increased amount of histamine helps attract more phagocytic cells that carry out additional phagocytosis and more blood flow delivers antimicrobial peptides This leads to an accumulation of pus The inflammatory response could also be triggered by injury Fever also is another systemic inflammatory response Septic Shock could lead to an inflammatory response that is too strong elderly and infants Evasion of Innate Immunity by Pathogens Some bacteria have outer capsules that interfere with molecular recognition and phagocytosis while some can resist breakdown by lysozymes 43 2 IN ADAPTIVE IMMUNITY RECEPTORS PROVIDE PATHOGEN SPECIFIC RECOGNITION Vertebrates are unique in having both adaptive and innate immunity Adaptive response relies on T cells and B Cells which are types of white blood cells called lymphocytes Lymphocytes originate in the stem cells of bone marrow T CELLS are lymphocytes that migrate from the bone marrow to the thymus while B CELLS are lymphocytes that remain and mature in the bone marrow Any substance that elicits a B or T cell response is called an antigen In adaptive immunity recognition occurs when a B or T cell binds to an antigen via a protein called an ANTIGEN RECEPTOR The EPITOPE is the small region of the antigen that binds to the antigen receptor All of the receptors made by a single B or T cell are identical However when an infection occurs only the B or T cells with specific antigen receptors for the pathogen are activated which display specificity for a particular epitope ANTIGEN RECOGNITON BY B CELLS AND ANTIBODIES Each B cell antigen receptor is a Y shaped molecule consisting of 4 polypeptide chains 2 identical heavy chains and 2 identical light chains Disulfide bridges link the chains A trans membrane region near one end of each heavy chain anchors the receptor in the cell s plasma membrane A short tail region at the end of the heavy chain extends into the cytoplasm The light and heavy chains each have a constant C region and a variable V region which