Chapter 18

• Adaptive Immune Response

  • Has specificity
  • Is specifically directed against a foreign invader
  • Highly specific for individual microbial pathogens
  • Adaptive immunity is learned (acquired)
  • Has "memory"
  • Allows rapid response to pathogens due to prior exposure to the antigens
  • Achieved by programming cells to respond
• 2 Basic strategies of the Adaptive Response

  • Humoral immunity
  • Cellular immunity (cell-mediated)
• Humoral immunity


  • Eliminates extracellular antigens
  • Bacteria
  • Toxins
  • Viruses in bloodstream
  • Mediated by B-cells
  • B-cells differentiate into plasma cells and produce antibody
  • Memory B-cells are long-lived
• Cell-mediated immunity


  • Eliminates antigens residing inside host cells
  • Viruses infecting cells
  • Aberrant cells (cells that have 'gone bad)
  • Mediated by T-cells
  • Effector Cytotoxic T-cells destroy infected body cells
  • Helper T-cells assist with adaptive immune responses
• Lymphocytes in specific immunity

  • T- cells - cell mediated response
  • B- cells - produce antibodies
• Overview of humoral and cell-mediated responses

  • Humoral immunity
  • B-cells
  • T-cells
  • T helper cells
  • T cytotoxic cells
• Antigens


  • Antigen = antibody generator
  • Any substance recognized by the body as foreign
  • Bacteria
  • Viruses
  • Fungi
  • Protozoa
  • Chemicals
  • Drugs
  • Food, etc.
• Types of Antigens

  • T-dependent antigens
  • T-independent antigens
• Characteristics of good antigens

  • High molecular weight
  • Small molecules are not usually good antigens
  • Haptens – too small to be antigenic on their own; typically complex with a larger molecule
  • More complex molecules are better antigens
  • All macromolecule classes can be antigens; proteins are the best antigens; Carbohydrates less antigenic; lipids and nucleic acids not very antigenic
• Bacterial cell

  • Molecule on surface is the epitope
  • Epitopes are discreet regions of the antigen molecule
• Antibodies

  • Are glycoproteins (protein + carbohydrate)
  • Also called immunogobulins
  • Are specific for the antigen that induced their production
  • Increase phagocytosis, neutralize toxins or viruses, activate complement
  • There are five classes of antibodies; IgG, IgM, IgA, IgD, IgE
• Structure of the antibody molecule

  • Y-shaped molecule. The two halves are identical
  • Held together by disulfide bonds
  • Variable and constant regions
  • The Antigen binds at the variable region. (Fab region at the ends of the Y)
  • Fab = antigen binding fragment
  • Stem is the constant region (Fc)
  • Amino acid sequence in the variable region determines the three-dimensional structure, and thus the specific three-dimensional epitope to which the Fab region is capable of binding
• Antibody-antigen interactions

  • Neutralization
  • Cross-linking, Agglutination or Precipitation
  • Opsonization
  • Complement activation
  • Antibody-Dependent Cellular Cytotoxicity
• Neutralization


  Abs can coat the surface of viral particles preventing them from attaching to their cell receptors
• Opsonin


  • Any molecule that forms a physical connection between a phagocyte and an antigen
  • Phagocytes do not require opsonins to phagocytose foreign material, opsonins make the job easier and increase efficiency
• Natural killer cells - effector lymphocytes that can destroy pathogens or several types of tumors. They work without having had prior exposure to a particular pathogen, and thus are considered part of the innate immune system. Natural killer cells recognize general signals of immune stress such as inflammation
• Functions of Antibodies

  • Neutralization
  • Cross-linking, Agglutination or Precipitation
  • Opsonization
  • Complement activation
  • Antibody-Dependent Cellular Cytotoxicity
• Classes of Antibodies

  • IgM
  • IgG
  • IgA
  • IgD
  • IgE
• IgM


  • First class produced during response
  • 5 - 13% of circulating antibodies
  • Pentamer (has 10 antigen-binding sites)
  • Agglutination, precipitation
  • Initiates complement cascade
• IgG

  • 80 - 85% of total antibody in serum
  • Exits bloodstream into tissues
  • 21 day half-life
  • Most versatile
  • Can cross the placenta
  • Only class that can do this
• IgA

  • 10 - 13% of antibody in blood
  • Most IgA is secreted form
  • Breast milk, tears, saliva
  • IgA very abundant in body
  • Important in mucosal immunity
  • Primarily acts as neutralizer
  • Traps pathogens in mucus for later removal
• IgE

  • Exists bound to basophils and mast cells
  • Bound via Fc region
  • Antigen binds to IgE, causes those cells to release chemical contents
  • Histamine, cytokines
  • Inflammation, allergic reactions
  • Anti-parasitic
• IgD

  • Membrane-bound monomer on the surface of B cells
  • Antigen-binding receptor
  • Not secrete by B cells; Trace amounts in serum (probably from dead/degraded B cells)
• Major Histocompatibility complex (MHC)


  • MHC molecules found on the surface of all nucleated cells of the body
  • Mature RBC, which lack a nucleus, have no MHC
  • Two classes of MHC involved in adaptive immunity
  • MHC class I – found on all nucleated cells; they present normal self-antigens as well as abnormal or nonself pathogens to the effector T cells involved in cellular immunity
  • MHC class II – molecules are only found on macrophages, dendritic cells, and B cells; they present abnormal or nonself pathogen antigens for the initial activation of T cells
• Antigen Presentation

  • All nucleated cells can present antigen with MHC I
  • Indicated normal vs. infected cells
  • Macrophages, dendritic cells, and B cells present antigens with MHC II
  • Macrophages and Dendritic cells recognize pathogens nonspecifically and phagocytose pathogens
  • B cells recognize pathogens specifically and are endocytosed; epitopes bind BCR
  • APC's process and present antigen with MHC II to T cells
• Dendritic cells as APC's


  • Dendritic cells recognizes and binds to pathogen
  • Pathogens that invade skin and mucous membrane (first line defenses)
  • Pathogen is recognized non-specifically
  • The pathogen is internalized by phagocytosis and processed (broken down by antimicrobial enzymes and proteases) in the phagolysosome of the cell
  • The most antigenic protein epitopes are selected for presentation
  • These epitopes associate with the MHC class II molecules
  • This complex is sent to the cell surface for presentation
• 1. cells and the antibody response

  • B-cell
  • Antigen-specific receptor: This receptor is the same as the antibody this cell will make after the antigen is encountered
  • B cells mature in the bone marrow
  • Naïve mature B cells have not been activated
  • Need helper T cells for activation
  • Self reactive B cells are eliminated
  • Have antigen specific receptors (~100k identical BCR on its surface)
  • BCR are membrane bound IgD or IgM monomers
• Clonal expansion and selection

  • Genetic rearrangement of hundreds of V, D, J gene segments create the diversity of receptor specificities
  • Large populations of B recognize a functionally limitless variety of antigens
  • Each cell only recognizes and responds to a single epitope
  • Clonal selection – selection of antigen specific lymphocytes
  • Clonal expansion – proliferation of a cell to generate many copies (clones of cells)
• A person has an estimated 1 billion different B-cells. Each individual B-cell reacts with a different epitope. Genetic rearrangements of VJD segments results in the millions of unique antigen binding sites for the BCR. BCR can bind to free antigen. Naïve lymphocyte – has not yet encountered its specific antigen.
• Animation about the generation of antibody diversity
• Billions of different B and T cells; each interacts with only a single epitope. Lymphocytes that recognize epitope respond.
• Activation of B cells

  • T-dependent activation of B cells
  • Naïve mature B cell with BCR interacts specifically with protein antigen
  • Protein antigen is internalized in B cell
  • Antigen is processed and presented with MHC II on surface of B cell
  • Helper T cell with recognizes antigen/MHC II
  • TCR recognizes antigen
  • CD4 molecule recognizes MHC II
  • Linked recognition – B cell and T helper cell recognize the same antigen
  • T helper cell is activated and secre
• Clonal expansion


  Proliferation of a cell to generate many copies (clones of cells)
• A person has an estimated 1 billion different B-cells
• Each individual B-cell reacts with a different epitope
• Genetic rearrangements of VJD segments


  Results in the millions of unique antigen binding sites for the BCR
• BCR


  Can bind to free antigen
• Naïve lymphocyte


  Has not yet encountered its specific antigen
• Billions of different B and T cells; each interacts with only a single epitope
• Lymphocytes that recognize epitope respond