Chapter 43 Immune System - TJHSST Splash Page by mifei


									                                    Biology: Seventh Edition
                               Unit 7: Animal Form and Function
                                Chapter 43: The Immune System
                               Adapted from Notes by: Jessica Liu

1)   Innate immunity provides broad defenses against infection
        a.      External Defenses
                 i.   Intact skin
                ii.   Mucous membranes. Mucus—viscous fluid that traps microbes and other
                iii.   Secretions- Acidic environment of skin and stomach destroys many microbes
                       and pathogens
                *.    Lysozyme—enzyme that digests the cell walls of many bacteria; present saliva,
                       tears, and mucous secretions; destroy bacteria as they enter upper respiratory
                       tract and openings around eyes
        b.     Internal Cellular and Chemical Defenses
              *.     Phagocytosis—the ingestion of invading microorganisms by certain types of white
                       blood cells [phagocytes] that produce antimicrobial proteins and help initiate
                       inflammation to limit spread of microbes
              ii. Phagocytic Cells
                             1.     Attach to microbes via surface receptors that bind to structures found
                                 on microorganisms but not in body cells
                             2.     Engulfs microbes, forming vacuole that fuses with lysosome; toxic
                                 forms of oxygen e.g. nitric oxide moison microbes; lysozyme and other
                                 enzymes degrade microbial components
              iii.    Antimicrobial Proteins
                             1.     Complement system—made up of ~30 serum proteins; in the absence
                                 of infection, are inactive; substances on surface of microbes trigger
                                 activation, leading to lysis of invading cells; help trigger inflammation,
                                 play role in acquired defense
                             2.     Interferon—α and β: secreted by virus-infected body cells; induce
                                 neighboring uninfected cells to produce substances to inhibit viral
                                 reproduction; interferon γ: helps activate macrophages
               iv.    Inflammatory Response
                             1.     Inflammatory response—triggered by injury or entry of pathogens
                             2.     Histamine—trigger dilation and increased permeability of nearby
                                 capillaries, promoting blood flow and inflammation and swelling [from
                                 leaking fluid]
                             3.     Mast cells—found in connective tissues; cells in which histamine is
                             4.     Inflammation helps deliver antimicrobial proteins and clotting
                                 elements to injured area, beginning repair process and blocking spread
                                 of microbes to other parts of body
                             5.     Chemokines—direct migration of phagocytes and signal them to
                                 increase production of microbe-killing compounds; secrete by blood
                                 vessel endothelial cells at or near site of injury or infection
                 v.    Natural Killer Cells
                             1.     Natural killer (NK) cells—attack virus-infected body cells and
                                 cancer cells; surface receptors recognize general features on surface of
                                 targets; releases chemicals the lead to death of cell by apoptosis
                             2.     Apoptosis—programmed cell death
2)   In acquired immunity, lymphocytes provide specific defenses against infection
        a.      Cytokines—proteins that help activate lymphocytes and other cells of immune system
        b.     Antigen—usually proteins or polysaccharides; recognized by lymphocytes as foreign
            and elicits response from lymphocytes; dissolved in extracellular fluid, protrude from
            surface of foreign cells
        c.      Epitope (antigenic determinant)—small, accessible portion of antigen to which
            lymphocyte binds
        d.     Antigen Recognition by Lymphocytes
                       i. Two main types of lymphocytes: B lymphocyte (B cell); T lymphocyte (T
                    ii.    Circulate through blood, concentrated in spleen, lymph nods, lymphoid
                    iii.    Antigen receptors—antigen-specific receptors in plasma membrane of
                   lymphocyte; all of ~100,000 on a cell are identical—specificity for epitope on an
                   antigen and defends against that antigen and closely related antigens
                    iv.     B Cell Receptors for Antigens
                         1.      B Cell receptor (membrane antibodies, membrane
                              immunoglobulins)—Y-shaped molecule consisting of four polypeptide
                              chains: two heavy chains [identical], two light chains linked by
                              disulfide bridge
                         2.      Transmembrane region anchors receptor in plasma membrane, short
                              region of tail extends into cytoplasm
                         3.      Variable (V) regions—regions at tips of Y light and heavy chains;
                              vary from one B cell to another; form antigen-binding site
                         4.      Constant (C) regions—everything not in V region; almost the same
                              in all cells
                         5.      Bonding between antigen binding site and antigen by noncovalent
                              bonds between chemical groups
                      v.    T Cell Receptors for Antigens and the Role of the MHC
                         1.      T cell receptor—consists of α-chain, β-chain linked by disulfide
                              bridge; transmembrane region anchors molecule in plasma membrane;
                              V regions for single antigen-binding site; recognize fragments of
                              antigens bound to normal MHC surface proteins
                         2.      Major histocompatibility complex (MHC)—family of genes that
                              codes for MHC molecule, which performs antigen presentation
                                   a.       Class I MHC molecules—found on almost all nucleated
                                         cells of body; bind peptides from foreign antigens that have
                                         been synthesized within cell e.g. in cancer cells; when bound
                                         to antigens, are recognized by cytotoxic T cells
                                   b.      Class II MHC molecules—made by dendritic cells,
                                         macrophages, B cells; bind peptides derived from foreign
                                         material that have been internalized or fragmented through
                                         pagocytosis or endocytosis; antigen-presenting cells—
                                         dendritic cells, macrophages, B cells; display internalized
                                         antigens to helper T cells
                         3.      Antigen Presentation—MHC molecule binds with fragment of
                              protein antigen within cell and brings it to cell surface as it moves
                              toward plasma membrane
                         4.      Because of many different MHC alleles, most people are
                              heterozygous for all MHC genes and can produce broad array of MHC
                              molecules; two people unlikely to have same set of MHC
                              molecules  chemical fingerprint of individual; marks cells as “self”
       e.    Lymphocyte Development
                   i. Thymus—gland in thoracic cavity above heart where lymphocytes develop
                   to T cells
                   ii.    B cells mature in bone marrow
                   iii.    Clonal Selection of Lymphocytes
                         1.      “Selection” of B cell or T cell by antigen activates lymphocyte and
                              causes it to divide, forming two clones of daughter cells
                         2.      Effector cells—combat antigen; short-lived
                         3.      Memory cells—bear receptors specific for same inducing antigen;
                         4.      Clonal selection—antigen-driven cloning of lymphocytes
                         5.      Primary immune response—proliferation of lymphocytes the first
                              time the body is exposed to a particular antigen; peak 10 to 17 days
                              after initial exposure
                         6.      Plasma cells—antibody-secreting effector B cells
                         7.      Secondary immune response—faster, more prolonged response the
                              second time the body is exposed to antigen
                         8.      Immunological memory—immune system’s ability to generate
                              secondary immune response; depends on clones of memory T and B
3)   Humoral and cell-mediated immunity defend against different types of threats
a.      Humoral immune response—activation and clonal selection of B cells
b.     Cell-mediated immune response—activation and clonal selection of cytotoxic T cells
c.      Helper T cell—responds to peptide antigens displayed on antigen-presenting cells and
     stimulates activation of B cells and cytotoxic T cells
d.     Helper T Cells: A Response to Nearly All Antigens
             i. Helper T cell recognizes class II MHC molecule-antigen complex on antigen
                presenting cell proliferates into active helper T cell clone and memory helper
                T cell
             ii.      CD4—binds class II MHC molecule to keep helper T cell and antigen-
                presenting cell joined while activation of helper T cell proceeds
            iii.      Active helper T cells secrete cytokines that stimulate other lymphocytes to
                promote humoral and cell-mediated response
            iv. Naïve helper T cells—helper T cells that have not yet detected antigen;
            v. Macrophages present antigens to memory T cells; B cells present antigens to
                helper T cells in humoral response
e.      Cytotoxic T Cells: A Response to Infected Cells and Cancer Cells
               i. Cytotoxic T cells eliminate infected body cells
               ii.     Nonself proteins synthesized in target cells associate with class I MHC
                molecule and are displayed on cell surface to be recognized by cytotoxic T cell
               iii.     CD8—binds to class I MHC molecule keeps target cell and cytotoxic T
                cell in contact during activation of cytotoxic T cell
               iv.     When cytotoxic T cell binds to MHC molecule-antigen complex it
                differentiates and becomes active killer; process promoted by cytokines from
                nearby helper T cells
                 v.     Activated cytotoxic T cell secretes proteins that lead to its destruction
                 vi.     Perforin—forms pores in target cell membrane
                 vii.     Granzyme—proteolytic enzyme that enters target cell by endocytosis;
                initiate apoptosis and lead to fragmentation of nucleus  release of apoptotic
f.      B Cells: A Response to Extracellular Pathogens
               i. B cell takes in antigen from surface of B cell by endocytosis and presents it
                to helper T cell; T cell bearing receptor for antigen bind to B cell, secrete
                cytokines to activate B cell
               ii.     B cell stimulated by antigen and cytokines proliferates and differentiates
                into clone of plasma cells and memory B cells
              iii.     Macrophage and dendritic cell can present peptide fragments from wide
                variety of antigens, but B cell binds and presents only antigen to which it
                specifically bonds
               iv. T-dependent antigens—antigens that induce antibody production only with
                assistance from helper T cells
               v.      T-independent antigens—can evoke B cell response without involvement of
                helper T cells; e.g. polysaccharides of many bacterial capsules, proteins from
                bacteria flagella; generally weaker response, no memory B cells
               vi.     Antibody Classes
                      1.      Immunoglobulins
                                a.     IgM (pentamer)—first Ig class produced after initial
                                    exposure to antigen, then concentration declines; promotes
                                    neutralization and agglutination of antigens; effective in
                                    complement activation
                                b.    IgG (monomer)—most abundant in blood, present in tissue
                                    fluid; only Ig class that crosses placenta, confers passive
                                    immunity on fetus; promotes opsonization, neutralization,
                                    agglutination of antigens, less effective in complement
                                c.     IgA (dimer)—present in secretions; provides localized
                                    defense of mucous membranes by agglutination and
                                    neutralization of antigens; J-chain + secretory component
                                d.    IgE (monomer)—triggers release of histamine from mast
                                    cells and basophils that cause allergic reactions
                                e.     IgD—present on surface of naïve B cells that have not been
                                    exposed to antigens; acts as antigen receptor in clonal
                                    selection of B cells
               vii.     Antibody-Mediated Disposal of Antigens
                         1.      Binding of antibodies to antigens inactivates antigens by:
                                    a.     Viral neutralization—block binding to host; opsonization—
                                        increases phagocytosis by increasing macrophage attachment
                                        to microbes
                                    b.    Agglutination—clumping of antigen bearing particles,
                                        possible because each antibody molecule has at least 2 antigen
                                        binding sites that can bind to identical epitopes on different
                                        particles to link them together
                                    c.    Precipitation—antibodies cross-link soluble antigen
                                        molecules; form immobile aggregate
                                    d.    Membrane attack complex (MAC)—forms pore in
                                        membrane, cause cell to swell and lyse when complement
                                        system activated
                          2.     Neutralization, opsonization, agglutination, precipitation enhance
                          3.     Activation of complement system causes cell lysis
       g.   Active and Passive Immunization
                     i. Active immunity—immunity conferred by natural exposure to infectious
                    agent; depends on activation of person’s own lymphocytes and resulting
                    memory cells
                     ii.    Immunization (vaccination)—develops active immunity; include
                    inactivated bacterial toxins, killed microbes, weakened microbes to induce
                    immediate immune response and immunological memory through memory
                    cells  quick secondary response
                     iii. Passive immunity—achieve immunity by transfer antibodies from
                    immune individual to non-immune individual; does not result from action of
                    recipient’s B and T cells; antibodies received immediately help destroy microbes;
                    last only as long as transferred antibodies live; occurs naturally in transfer of
                    antibodies from mother to child to protect infant against infection while their
                    immune system is maturing; occurs artificially in treatment of rabies
4)   The immune system’s ability to distinguish self from nonself limits tissue transplantation
       a.    Blood Groups and Transfusions
                     i. A type blood: Anti-B antibodies; B type: Anti-A; AB type: No anti-A or
                    anti-B; O type: Anti-A and Anti-B
                     ii.    O is universal donor; AB is universal recipient
                     iii.    Antigens against foreign blood type present even in absence of exposure
                    to foreign blood type because antibodies arise in response to normal bacterial
                    inhabitants of body
                     iv.     Anti blood group antibodies are always IgM; no memory cells generated
                     v.     Rh factor—induces immune response in which memory cells generated;
                    later exposure leads to anti-Rh IgG antibodies
5)   Exaggerated, self-directed, or diminished immune responses can cause disease
       a.    Allergies
                     i. Allergens—antigens that in exaggerated response cause allergies;
                    commonly involve IgE
                     ii.    Degranulation—mast cells release histamine and inflammatory agents
                    against pollen
                     iii.    Histamine increases permeability of small blood vessels
                     iv.     Antihistamines block histamine receptors
                     v.     Anaphylactic shock—whole-body, life-threatening reaction; occurs when
                    widespread degranulation by mast cells causes blood vessel dilation, sudden
                    drop in blood pressure; can be countered by epinephrine
       b.   Autoimmune Diseases
                     i. Autoimmune diseases—immune system turns against self
                     ii.    Autoimmune disease arise from failure in immune system regulation
       c.    Immunodeficiency Diseases
                    i. yeah like AIDS and stuff.

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