FUNDAMENTALS OF IMMUNOLOGY

• Immunology is the study of physiological mechanisms that humans and other animals use to defend their bodies from invading organisms like bacteria, fungi, viruses, parasites, and toxins
• Key terms in immunology:

  • Antigen: any molecule that binds to immunoglobulin or T cell receptor
  • Pathogen: microorganisms that can cause disease
  • Antibody (Ab): secreted immunoglobulin
  • Vaccination: deliberate induction of protective immunity to a pathogen
  • Immunization: the ability to resist infection
• Types of immunity:

  • Innate Immunity: host defense mechanisms that act from the start of an infection but do not adapt to a particular pathogen
  • Adaptive Immunity: response of antigen-specific B and T lymphocytes to an antigen, with immunological memory
• Cells of the immune system:

  • Lymphocytes: important in both humoral and cell-mediated immunity, with B-cells producing antibodies and T-cells having different functions
  • Plasma cells: fully differentiated B cells that secrete antibodies
  • Natural Killer Cells: kill cells infected with certain viruses, involved in both innate and adaptive immunity
• Components of blood:

  • Serum: cell-free liquid, minus the clotting factors
  • Plasma: cell-free liquid with clotting factors in solution, must use an anticoagulant
• Mechanism of Action of Antigen-Presenting Cells (APCs):

  • Professional APCs: efficient at phagocytosis and endocytosis, antigen presentation done by MHC class 2 molecules, express co-stimulatory molecules
  • Non-professional APCs: do not constitutively express MHC class 2 during presentation, responsible for little role of antigen presentation compared to professional APCs
• Antibodies are large proteins produced by plasma cells, used by the immune system to identify and neutralize pathogens like bacteria and viruses
• Basic structure of antibodies:

  • Antibodies are heavy globular plasma proteins or glycoproteins
  • Each antibody is a heterodimer with a molecular weight of approximately 150KD
  • Antibodies share a basic structure with heavy and light chains bonded via interchain disulphides linkages
• Approximate length of a light chain: 211-217 amino acids
• Different classes of antibodies:

  • IgG: makes up approximately 80% of serum antibodies, has a half-life of 7-23 days, is a monomer with 2-epitope binding sites, can cross the placenta, functions include immunity to newborns and neutralization of toxins
  • IgM: makes up about 13% of serum antibodies, has a half-life of about 5 days, most are pentamers with 10-epitope binding sites, functions include activation of classical pathway and defense against multivalent antigens
  • IgA: makes up around 6% of serum antibodies, is a dimer with 4-epitope binding sites, found mainly in body secretions like saliva and milk, functions as a secretory antibody and is effective against viruses like influenza
  • IgD: makes up about 0.2% of serum antibodies, is a monomer with 2-epitope binding sites, found on the surface of B-lymphocytes, functions include B cell activation and acting as a receptor for antigen binding
• • IgE: discovered by KandT Ishizaka, very low concentration in blood, binds to Fc receptor on basophils and mast cells, functions include immediate hypersensitivity and release of substances like histamine and vasoactive mediators
• Complement system:

  • Consists of a group of 14 proteins in the complement cascade, more than 10 regulatory proteins, at least 7 complement receptors
  • Nearly 5% of all serum proteins, can increase to 7% in inflammatory states
  • Serum complement is produced by hepatocytes, myeloid cells, and adipocytes
  • Biological functions include opsonization, initiation of an inflammatory response, direct lysis of gram-negative bacteria, B cell activation, T cell priming, and complement regulatory protein functions
• C1 Inhibitor:

  • A serine protease inhibitor
  • Binds to C1r and C1s and dissociates C1
  • Important roles inhibiting factor XII (Hageman factor) and prokalilrein of contact system of coagulation
• Factors involved in cleaving C3b, C4b:

  • Factor I, MCP (CD46)
  • Factor H, C4 binding protein
• Antigen-Antibody Interaction:

  • Antigens and antibodies combine specifically in an interaction called Antigen-Antibody reaction
  • Forms the basis for humoral immunity or antibody-mediated immunity
  • Used in detecting infectious disease-causing agents and specific antigens like enzymes
  • In vitro reactions are known as serological reactions
  • Three stages: formation of Ag-Ab complex, visible events like precipitation, agglutination, destruction of Ag or its neutralization
• Specificity of Antigen-Antibody Reaction:

  • Refers to the ability of an individual antibody combining site to react with only one antigenic determinant or the ability of a population of antibody molecules to react with only one antigen
• Binding Site of Antigen-Antibody Reaction:

  • Epitope is the part of an antigen recognized by the immune system
  • Paratope is the part of an antibody that recognizes the epitope
• Immune Complex:

  • Formed from the integral binding of antibody to a soluble antigen
  • Mechanisms of antigen-antibody interaction lead to inflammation
• Binding Force of Antigen-Antibody Reaction:

  • Due to closeness between antigen and antibody, non-covalent bonds, and antibody affinity
• Properties of Antigen-Antibody Reaction:

  • Explained by antibody affinity, antibody avidity, and cross-reaction
• Precipitation Reaction:

  • Occurs when a soluble Ag combines with its Ab in the presence of an electrolyte (NaCl) forming an insoluble precipitate of Ag-Ab complex
  • Function: occurs in liquid or gel media
• Cross Reaction:

  • Antiserum raised against an Ag can react with a similar Ag of another type
  • Weak bonds are involved in cross reactions
• Function of Precipitation Reaction:

  • Occurs in liquid or gel media
  • In liquid: plotting the amount of precipitate against increasing antigen concentration yields a precipitation curve
• Antigen-antibody reaction results in precipitation, with the amount of precipitate plotted against increasing antigen concentration yielding a precipitation curve
• The precipitation curve shows three zones:

  • Zone of Ab axis
  • Zone of equivalence
  • Zone of Ag axis
• In radial immunodiffusion (Mancini), agar gel or similar gels are used on plates or petriplates where both antigen and antibody diffuse freely in the gel system in all directions, forming a visible precipitation at the zone of equivalence
• Agglutination reaction occurs when a specific antigen is mixed with its antibodies in the presence of electrolytes at a suitable temperature and pH, leading to clumping or agglutination of particles
• Slide agglutination is a rapid method to determine the presence of agglutinating antibodies by adding antiserum to a uniform suspension of particulate antigen, observing for granulation as a positive test, commonly used for blood grouping and cross-matching
• Tube agglutination is a standard method for the quantitative estimation of antibodies, where the serum containing antibodies is serially diluted with saline in test tubes and mixed with a constant volume of antigen suspension, with the tube showing the highest agglutination referred to as the titre
• The Widal test is used for the estimation of typhoid fever, measuring the antibody content of the patient’s serum by adding a constant amount of Salmonella typhi antigen to the serially diluted serum
• Passive agglutination test alters the physical nature of the reaction by coating the antigen on the surface of a carrier particle, used for the diagnosis of Rheumatoid arthritis
• Agglutination inhibition provides a highly sensitive assay for small quantities of an antigen, exemplified by the first home pregnancy test
• Complement fixation tests utilize the ability of the antigen-antibody complex to fix complement, with the complement system comprising 11 proteins that bind to the Fc component of antibodies involved in the complex
• ELISA (Enzyme Linked ImmunoSorbent Assay) uses enzyme-labeled antigens and antibodies for the assay of antibodies and antigens, with three types: indirect, sandwich, and competitive ELISA
• Immunofluorescence involves the use of fluorescent dyes like fluorescein and phycoerythrin to detect the presence or absence of proteins in serum, determine blood groups, and develop immunoassays for various substances