Immunity Chapter 35 The Immune System An animal mustdefend itself from themany dangerouspathogens it mayencounter in theenvironment. The Immune System A host is susceptible to a parasite if it cant eliminatea parasite before it becomes established. The parasite is infective. The host is resistant if it is able to preventestablishment of the parasite. The parasite is noninfective. The Immune System Immune cells,red blood cells,and otherwhite bloodcells arederived frommultipotentialstem cells inthe bonemarrow. The Immune System Lymphocytes arisefrom stem cells inthe bone marrow. Newly formedlymphocytes are allalike, but they laterdevelop into B cellsor T cells,depending onwhere theycontinue theirmaturation. The Immune System As B and T cells are maturing in the bone marrow andthymus, their antigen receptors are tested for possibleself-reactivity. Lymphocytes bearing receptors for antigens alreadypresent in the body are destroyed by apoptosis orrendered nonfunctional. The Immune System Two major kinds of immunity have evolved thatcounter these invaders: Innate immunity Acquired immunity Innate Immunity Innate immunity ispresent before anyexposure topathogens and iseffective from thetime of birth. Involvesnonspecificresponses topathogens. Acquired Immunity Acquired immunity develops only after exposure toinducing agents such as microbes, toxins, or otherforeign substances. Involves a very specific response to pathogens. A Summary of Innate and Acquired Immunity External Defenses Intact skin and mucous membranes form physicalbarriers that block the entry of microorganisms andviruses. Certain cells of the mucous membranes producemucus - a viscous fluid that traps microbes and otherparticles. External Defenses In the trachea,ciliated epithelialcells sweepmucus and anyentrappedmicrobesupward,preventing themicrobes fromentering thelungs. External Defenses Secretions from the skin give the skin a pH between 3and 5, which is acidic enough to prevent colonization ofmany microbes. Also include proteins such as lysozyme, an enzyme thatdigests the cell walls of many bacteria. Internal Cellular and Chemical Defenses
Internal cellular defenses depend mainly onphagocytosis. Phagocytes are types of white blood cells that: Ingest invading microorganisms. Initiate the inflammatory response. Phagocytic Cells Phagocytes attach to their prey via surface receptors and engulf them, forming a vacuole that fuses with a lysosome. Phagocytic Cells Macrophages, a specific type of phagocyte, can befound migrating through the body. Also found in various organs of the lymphatic system. The Lymphatic System The lymphaticsystem plays anactive role indefending the bodyfrom pathogens. Antimicrobial Proteins
Numerous proteins function in innate defense byattacking microbes directly or by impeding theirreproduction. Antimicrobial Proteins
About 30 proteins make up the complement system,which can cause lysis of invading cells and help triggerinflammation. Interferons provide innate defense against viruses andhelp activate macrophages. Inflammatory Response
In local inflammation, histamine and otherchemicals released from injured cells promotechanges in blood vessels that allow more fluid,more phagocytes, and antimicrobial proteins toenter the tissues. Natural Killer Cells Natural killer (NK) cells patrol the body and attackvirus-infected body cells and cancer cells. Trigger apoptosis (programmed cell death) in the cellsthey attack. Acquired Immunity Acquired immunity is the bodys second major kindof defense. Involves the activity of lymphocytes. Acquired Immunity An antigen is any foreign molecule that is specifically recognized by lymphocytes and elicits a response from them. A lymphocyte actually recognizes and binds to just a small, accessible portion of the antigen called an epitope. Antigen Recognition by Lymphocytes
The vertebrate body is populated by two main types oflymphocytes which circulate through the blood: B lymphocytes (B cells) T lymphocytes (T cells) B Cell Receptors for Antigens
B cell receptorsbind to specific,intact antigens. Y-shaped: twoidentical heavychains & twoidentical lightchains. Variable regionsat the tip providediversity. T Cell Receptors for Antigens and the Role of the MHC
Each T cell receptorconsists of twodifferent polypeptidechains. The variable regionsform the antigenbinding site andprovide a diversity ofT cells. V V C C T Cell Receptors for Antigens and the Role of the MHC
T cells bind to small fragments of antigens that arebound to normal cell-surface proteins called MHCmolecules. MHC molecules are encoded by a family of genescalled the major histocompatibility complex. T Cell Receptors for Antigens and the Role of the MHC
Infected cells produce MHC molecules which bind toantigen fragments and then are transported to the cellsurface in a process called antigen presentation. A nearby T cell can then detect the antigen fragmentdisplayed on the cells surface. T Cell Receptors for Antigens and the Role of the MHC
Depending on their source, peptide antigens arehandled by different classes of MHC molecules. T Cell Receptors for Antigens and the Role of the MHC
Class I MHCmolecules, found onalmost all nucleatedcells of the body,display peptideantigens to cytotoxicT cells. T Cell Receptors for Antigens and the Role of the MHC
Class II MHCmolecules, locatedmainly on dendriticcells, macrophages,and B cells, displayantigens to helper Tcells. Clonal Selection of Lymphocytes
In a primary immune response, binding of anantigen to a mature lymphocyte induces thelymphocytes proliferation and differentiation, aprocess called clonal selection. Clonal Selection of Lymphocytes
Clonal selectionof B cellsgenerates a cloneof short-livedactivated effectorcells and a cloneof long-livedmemory cells. Effector cellsproduceantibodies for aspecific antigen. Clonal Selection of Lymphocytes
In the secondary immune response, memorycells facilitate a faster, more efficient response. Humoral vs. Cell-Mediated Response
Acquired immunity includes two branches: The humoral immune response involves the activationand clonal selection of B cells, resulting in the productionof secreted antibodies. The cell-mediated immune response involves theactivation and clonal selection of cytotoxic T cells. The Role of Helper T Cells in Acquired Immunity
Activated helper T cells secrete several differentcytokines (protein hormones) that stimulate otherlymphocytes. The Role of Helper T Cells in Acquired Immunity
The activated cytotoxic T cell secretesproteins that destroy the infected target cell. B Cells: A Response to Extracellular Pathogens
Activation of B cells is aided by cytokines and antigen binding to helper T cells. The clonal selection of B cells generates antibody-secreting plasma cells, the effector cells of humoral immunity. Antibody Classes A secretedantibody has thesame Y-shapedstructure as a B cellreceptor, but isntanchored in the cellmembrane. The five majorclasses ofantibodies differ intheir distributionsand functions withinthe body. Antibody-Mediated Disposal of Antigens
The binding ofantibodies toantigens is alsothe basis ofseveral antigendisposalmechanisms. Leads toelimination ofmicrobes byphagocytosisandcomplement- mediated lysis. Active and Passive Immunization
Active immunity develops naturally in response to aninfection. Can also develop following immunization, also calledvaccination. Active and Passive Immunization
In immunization, anonpathogenicform of a microbeor part of a microbeelicits an immuneresponse to animmunologicalmemory for thatmicrobe. Active and Passive Immunization
Passive immunity, which provides immediate, short- term protection, is conferred naturally when IgGcrosses the placenta from mother to fetus or when IgApasses from mother to infant in breast milk. Can be conferred artificially by injecting antibodies into anon-immune person. Distinguishing Self from Nonself
The immune system can wage war against cells fromother individuals. Transplanted tissues are usually destroyed by therecipients immune system. Blood Groups and Transfusions
Certain antigens on red blood cells determine whethera person has type A, B, AB, or O blood. Antibodies to nonself blood types already exist in thebody. Transfusion with incompatible blood leads todestruction of the transfused cells. Blood Groups and Transfusions
Another red blood cell antigen, the Rh factor createsdifficulties when an Rh-negative mother carriessuccessive Rh-positive fetuses. Disrupting the Balance
If the delicate balance of the immune system isdisrupted, the effects on the individual can range fromminor to often fatal consequences. Allergies Allergies are exaggerated (hypersensitive) responsesto certain antigens called allergens. In localized allergies such as hay fever, IgE antibodiesproduced after first exposure to an allergen attach toreceptors on mast cells. Allergies The next time the allergen enters the body, it binds to mast cellassociated IgE molecules. The mast cells then release histamine and other mediators that cause vascular changes and typical symptoms. Allergies An acute allergic response sometimes leads toanaphylactic shock a whole-body, life-threateningreaction that can occur within seconds of exposure toan allergen. Autoimmune Diseases In individuals with autoimmune diseases, the immunesystem loses tolerance for self and turns againstcertain molecules of the body. Autoimmune Diseases Rheumatoid arthritisis an autoimmunedisease that leads todamage and painfulinflammation of thecartilage and bone ofjoints. Autoimmune Diseases Other examples of autoimmune diseases include:
Systemic lupus erythematosus Multiple sclerosis Insulin-dependent diabetes Immunodeficiency Diseases
An inborn or primary immunodeficiency results fromhereditary or congenital defects that prevent properfunctioning of innate, humoral, and/or cell-mediateddefenses. Immunodeficiency Diseases
An acquired or secondary immunodeficiency resultsfrom exposure to various chemical and biologicalagents. Stress and the Immune System
Growing evidence shows that physical and emotionalstress can harm immunity. Acquired Immunodeficiency Syndrome (AIDS)
People with AIDS are highly susceptible toopportunistic infections and cancers that takeadvantage of an immune system in collapse. Acquired Immunodeficiency Syndrome (AIDS)
Because AIDS arises from the loss of helper T cells, both humoral and cell-mediated immune responses are impaired. The loss of helper T cells results from infection by the human immunodeficiency virus (HIV).