Key Concepts The innate immune response to infection is mounted by leukocytes that respond in a nonspecific way to pathogens In contrast the adaptive immune response is mounted by lymphocytes that respond to specific antigens often a protein from a pathogen The adaptive immune response begins when certain lymphocytes are activated in a regulated step by step process triggered by interaction with a specific antigen Activated lymphocytes either destroy infected cells or produce antibodies that tag pathogens for destruction 2011 Pearson Education Inc Introduction Humans are susceptible to a multitude of disease causing bacteria viruses parasitic worms fungi and protists To understand how animals stay healthy for most of their lives biologists have investigated three observations 1 Wounds usually heal even if they become infected 2 Most people who contract a bacterial or viral illness eventually recover without medication 3 People who acquire bacterial or viral infections and recover frequently do not contract the same disease again 2011 Pearson Education Inc Introduction Immunity is a resistance to or protection against disease causing pathogens that prevents individuals from contracting a disease more than once Immunization is the conferring of immunity to a particular disease Vaccination is the introduction of a weakened or altered pathogen to prime the body s immune system so it fights later infections effectively 2011 Pearson Education Inc Introduction In the late 1700s Edward Jenner had a key insight about why milkmaids often avoided smallpox infections Because cows suffered from a smallpox like disease called cowpox Jenner reasoned that milkmaids were immune to smallpox because they had been exposed to cowpox To test his hypothesis he inoculated a boy with cowpox pathogens and then with smallpox pathogens As predicted the boy did not contract smallpox 2011 Pearson Education Inc Innate Immunity Innate immunity refers to immune system cells that are ready to respond to foreign invaders at all times The innate immune system is nonspecific and responds in the same way to all antigens An antigen is any foreign molecule that can initiate an immune response Cells that require activation confer adaptive immunity These cells respond in an extremely specific way to the particular strains of bacteria virus or fungus that enters the body 2011 Pearson Education Inc Barriers to Entry In humans and other animals the most important barrier to pathogen entry is the skin which presents both physical and chemical barriers Gaps in the skin barrier occur where the digestive tract reproductive tract gas exchange surfaces and sensory organs make contact with the environment 2011 Pearson Education Inc How Are Openings in the Body Protected Gaps have a protective physical barrier in the form of mucus a proteoglycan rich solution secreted by epithelial cells or other features that discourage pathogen entry Gaps in the body that are not covered with mucous layers are often protected by other types of secretions such as wax in the ears and lysozyme an antibiotic that occurs in tears Despite preventative measures however pathogens are still able to gain entry to tissues beneath the skin 2011 Pearson Education Inc 2011 Pearson Education Inc How Do Pathogens Gain Entry Flu viruses have an enzyme on their surface that disrupts the mucous lining of the respiratory tract When the outer surface of the virus makes contact with a host cell beneath the mucous layer the virus is able to enter the cell and begin an infection Falls wounds and other types of physical trauma provide another important mode of entry When the skin is broken bacteria and other pathogens gain direct access to the tissues inside 2011 Pearson Education Inc The Innate Immune Response The innate immune response the body s nonspecific response to pathogens involves leukocytes white blood cells such as mast cells macrophages and neutrophils Innate immune response leukocytes are alerted to the presence of foreign invaders by antigens that are found on the surfaces of pathogens but not on host cells Some innate system cells have membrane proteins known as pattern recognition receptors that bind to and are activated by pathogen specific compounds These proteins serve as on switches by initiating a cell response 2011 Pearson Education Inc 2011 Pearson Education Inc The Inflammatory Response in Humans The inflammatory response a multistep innate immune response in mammals occurs at the site of an injury The steps of this process are as follows 1 When skin breaks pathogens enter a wound 2 Platelets release blood clotting proteins at the wound site 3 Wounded tissues and macrophages at the wound site secrete chemokines signaling molecules that recruit immune cells by forming a gradient to mark the path to the site 2011 Pearson Education Inc The Inflammatory Response in Humans 4 Mast cells release chemical messengers that constrict blood vessels near the wound reducing blood flow and thus blood loss Mast cells also secrete histamine and other signaling molecules which dilate blood vessels slightly farther away from the wound making them more permeable 5 Neutrophils and macrophages remove pathogens by phagocytosis the engulfing and digesting of foreign particles 2011 Pearson Education Inc The Innate Immune Response 6 Macrophages secrete cytokines chemicals that attract other immune system cells to the site and activate cells involved in tissue repair They also induce fever elevated body temperature that aids in healing The inflammatory response continues until the foreign material is eliminated and the wound is repaired 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc Innate Immunity in Invertebrates Innate responses make up the entire immune system observed in millions of species of invertebrates For example If a pathogen succeeds in entering the main body cavity of an insect cells respond by synthesizing and secreting peptides with potent antibacterial or antifungal properties Sea stars have specialized cells that are similar in function to neutrophils and macrophages they secrete cytokines or engulf and destroy pathogens by phagocytosis 2011 Pearson Education Inc The Adaptive Immune Response Recognition The adaptive immune response also known as the acquired immune response is based on interactions between specific immune system cells and a specific antigen Antibodies are proteins that are produced and secreted by