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• Antigen


  Any agent (molecule) that can bind to components of the immune response: antibodies or T-cell receptor

  
• Immunogen


  Any agent capable of inducing an immune response

  
• Hapten


  Small-molecular-weight compounds that evoke an immune response only when they are attached to carrier proteins

  
• Epitope


  The structure of the antigen that is recognized by B-cell or T-cell

  
• Factors that determine immunogenicity of antigen

  • Foreignness
  • Molecular size
  • Chemical composition and heterogeneity
  • Dose and mode of entry of antigen

  
• A bacterial cell has molecules on its surface that are capable of stimulating the production of antibodies

  
• Foreignness and genetic composition

  • Antigens must be recognized as non-self by the biological system
  • The greater the phylogenetic (evolutionary relationship) distances between two species, the greater the structural (and therefore the antigenic) disparity between them
• Molecular size


  • The greater the molecular weight, the more potent is the molecule as an immunogen
  • Molecular Mass ≥ 10,000 Da: Immunogenic
  • Molecular Mass <10,000 Da: Poorly immunogenic
  • Exceptions: Few substances with molecular mass less than 1000 Da have proven to be immunogenic
• Chemical Composition and Heterogeneity

  • The majority of organic compounds have antigenic properties, except pure lipids and nucleic acids
  • Proteins, which are larger and more complex, are most immunogenic
  • Being even more complex, glycoproteins are more antigenic than pure proteins
• Dose and Mode of entry of antigen

  • An optimum dose is necessary to elicit good immune response
  • Insufficient dose: will not stimulate an immune response
  • Excessively high dose: may induce tolerance
  • The intensity of immune responses is also governed by the route by which the immunogen enters the host body

  
• Mucosa-associated lymphoid tissues (MALT) are secondary lymphoid organs

  
• Cells of the immune system

  • T cells
  • Helper T cells (CD4+ T cells)
  • Cytotoxic T cells (CD8+ T cells)
  • Regulatory T cells (CD4+CD25+ T cells)

  
• Humoral immunity


  Where special glycoproteins called antibodies are produced by B cells to destroy specific microbes

  
• Cell-mediated immunity


  Involves a variety of cell types, with antibodies playing only a minor role, if any

  
• Major types of adaptive immunity

  • Humoral immunity
  • Cell-mediated immunity

  
• Active acquired immunity


  Antibodies are produced within the person, usually provides long-lasting protection

  
• Passive acquired immunity


  Antibodies are received that were produced by another person(s) or by an animal, usually provides only temporary protection

  
• Types of active acquired immunity

  • Natural active acquired immunity
  • Artificial active acquired immunity

  
• Vaccine


  Material that can artificially induce immunity to an infectious disease, usually following injection or ingestion of the vaccine

  
• Types of available vaccines

  • Attenuated vaccines
  • Inactivated vaccines
  • Subunit vaccines
  • Conjugate vaccines
  • Toxoid vaccines

  
• Attenuated vaccines


  • Use a weakened (or attenuated) form of the germ that causes a disease
  • Used to protect against: Measles, mumps, rubella (MMR combined vaccine); Smallpox; Chickenpox

  
• Inactivated vaccines


  • Use the killed version of the germ that causes a disease
  • Inactivated vaccines usually don't provide immunity (protection) that's as strong as live vaccines
  • May need several doses over time (booster shots) to get ongoing immunity against diseases
  • Used to protect against: Hepatitis A; Flu (shot only); Polio (shot only); Rabies

  
• Subunit, recombinant, polysaccharide, and conjugate vaccines

  • Use specific pieces of the germ — like its protein, sugar, or capsid (a casing around the germ)
  • Specificity elicits a very strong immune response
  • Used to protect against: Hib (Haemophilus influenzae type b) disease; Hepatitis B; HPV (Human papillomavirus); Whooping cough (part of the DTaP combined vaccine); Pneumococcal disease; Meningococcal disease; Shingles

  
• Toxoid vaccines


  • Use a toxin (harmful product) made by the germ that causes a disease
  • Create immunity to the parts of the germ that cause a disease instead of the germ itself
  • Used to protect against: Diphtheria; Tetanus
• Passive acquired immunity


  Antibodies produced in one person are transferred to another person to protect the latter from infection providing temporary protection

  
• Types of passive acquired immunity

  • Natural passive acquired immunity
  • Artificial passive-acquired immunity
• Vaccines protect against


  • Hib (Haemophilus influenzae type b) disease
  • Hepatitis B
  • HPV (Human papillomavirus)
  • Whooping cough (part of the DTaP combined vaccine)
  • Pneumococcal disease
  • Meningococcal disease
  • Shingles

  
• Toxoid vaccines


  • Use a toxin (harmful product) made by the germ that causes a disease
  • Create immunity to the parts of the germ that cause a disease instead of the germ itself

  
• Diseases toxoid vaccines protect against

  • Diphtheria
  • Tetanus
• Types of passive acquired immunity

  • Natural passive acquired immunity
  • Artificial passive-acquired immunity
• Natural passive acquired immunity

  Small antibodies, IgG, present in the mother's blood cross the placenta to reach the fetus

  
• Artificial passive-acquired immunity


  Antibodies from an immune person are transferred to a susceptible person (e.g., hepatitis B immunoglobulin)

  
• Antibodies


  • Proteins produced by lymphocytes in response to the presence of an antigen
  • Belong to a class of proteins called immunoglobulins—globular glycoproteins in the blood that participate in immune reactions

  
• Antibodies


  • The amount and type of antibodies produced by a given antigenic stimulation depend on the nature of the antigen, the site of antigenic stimulus, the amount of antigen, and the number of times the person is exposed to the antigen
  • The majority of antigens are referred to as T-dependent antigens because T cells are required in their processing
  • The processing of T-independent antigens requires only B cells

  
• Antibody production


  1. B cells developing into plasma cells, which are capable of secreting antibodies

  2. The initial immune response to an antigen is called the primary response; it takes 10 to 14 days for antibodies to be produced

  3. The increased production of antibodies following the second exposure to a particular antigen is called the secondary response

  
• Humoral Immunity


  • Immune responses to antigens in the blood are usually initiated in the spleen
  • Responses to microbes and other antigens in tissues are generated in lymph nodes located near the infected area

  
• Antibody structure and function

  • Antibodies are a class of glycoproteins called immunoglobulins; five types—IgA, IgD, IgE, IgG, and IgM
  • All antibodies are immunoglobulins, but not all immunoglobulins are antibodies

  
• Antigen-antibody complex


  • Formed when the antibody combines with an antigen
  • Can activate the complement cascade, causing activation of leukocytes, lysis of bacterial cells, and increased phagocytosis as a result of opsonization

  
• Complement cascade


  A collective term that describes a system of about 20 proteins, many of which are enzyme precursors
• Cell-mediated immunity (CMI)


  • A complex system of interactions among many types of cells and cellular secretions (cytokines)
  • Can control chronic infections by intracellular pathogens (e.g., certain bacteria, protozoa, fungi, and viruses)
  • Examples of cells that participate in CMI: macrophages, TH cells, TC cells, NK cells, and granulocytes