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Antibody Function & Structure, Schemes and Mind Maps of Immunology

Royal Holloway, University of London (RHUL)Immunology

Specifically bind to antigens in both the recognition phase (cellular receptors) and during the effector phase (synthesis and secretion) of humoral immunity.

Typology: Schemes and Mind Maps

2021/2022

Uploaded on 09/12/2022

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Antibody Function & Structure
•Specifically bind to antigens in both the
recognition phase (cellular receptors) and
during the effector phase (synthesis and
secretion) of humoral immunity
–Serology: the study of antibodies and their
reactions with antigens
•Antibodies present in serum, in mucosal secretions
and interstitial spaces of tissues
•Titer of antibodies relates to resistance to disease
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Antibody Function & Structure

  • Specifically bind to antigens in both the

recognition phase (cellular receptors) and

during the effector phase (synthesis and

secretion) of humoral immunity

  • Serology: the study of antibodies and their reactions with antigens - Antibodies present in serum, in mucosal secretions and interstitial spaces of tissues - Titer of antibodies relates to resistance to disease

Monoclonal Antibodies

  • Requires cultured myeloma cell lines that grow in normal culture medium but will not grow in a selective media because they lack genes (mutated) required for DNA synthesis. By fusing normal cells to these mutated tumor cells the necessary genes are supplied by the normal cell (B cells) and allow the “hybrid” to grow in the selective media.
  • Genes from myeloma cell make “normal B cell” immortal
    • Normal cells synthesize purine nucleotides and thymidylate by a de novo pathway requiring tetrahydrofolate
    • Aminopterin-treated cells (inhibitor of folate enzymes) use a salvage pathway to make purines and thymidine kinase (TK) makes thymidyalate
    • Thus aminopterin treated cells grow normally– Mutated tumor cells are unable to grow in HAT media
  • HAT media contains
    • Hypoxanthine: mutated myeloma cells cannot use hypoxanthine to make purines in the salvage pathway because they are mutated in the HGPRT gene (only mutated tumor cells)
    • Aminopterin: stops the action of tetrahydrofolate and cells cannot make purines (both myeloma and normal B cells)
    • Thymidine: supplies nucleotide base in media (for both myeloma and normal B cell)
  • Unfused myeloma cells will die because they do not have the gene products to replicate DNA– normal B cells will die becaue they are not immortal– only fused cells will live in HAT medium (immortal genes from myeloma and salvage pathway genes from normal B cell)
  • All Ig molecules share the same basic structural

characteristics but display great variability in the

regions that bind antigens

  • Ag binding occurs at the N-terminal end of the combination of the H and L chains (variable ends)
  • Constant regions have identical (nearly identical- there may be allotypic differences [i.e., differences that may be as small as 1 AA change) AA sequences - Variability may be as great as 10^7 -10^9 different Ab molecules in each individual
  • Different classes of Ig’s have different functions

(differences in ½ life and tissue distribution):

  • IgM: 1st^ Ig made in response to Ag, Activate C’ very well because of pentameric structure (J-chain)
  • IgG: 2nd^ Ig made in response to Ag, activates C’ well, crosses placenta, has greater avidity for Ag than IgM, binds to Fc receptor of Mè
  • IgA: dimer, crosses epithelial cell barriers (mucous membranes and gut), has J-chain and SC protein
  • IgE: has extra domain (C4) that is able to bind to Fc receptor on mast cell
  • Antibody molecules can be cleaved by papain or

pepsin to form fragments that maintain specific

functions—

  • Papain– cleaves to give 2 fragments of Fab and one Fc = Fab can bind, but not cross-link antigen; Fc cannot bind complement without Ag binding 1st
  • Pepsin– cleaves to give a F(ab’) 2 fragment= can bind and cross-link antigens and cell receptors
  • Antibodies can undergo
  • Affinity maturation : somatic mutations of variable region
  • Isotype switching - give different set of effector functions
  • Clinical uses of monoclonal Antibodies
    • Diagnostic/imaging & therapeutic
      • Detect pregnancy, microorganisms, blood levels of drugs, matching histocompatibility antigens, detecting antigens shed by tumors
      • detecting tumors by radiolabeling Ig to locate tumor and metastatic cells
      • Immunotoxins- lethal toxins bound to Ig to specifically target tumor (ricin, Shigella toxin, and diphtheria toxin: all inhibit protein synthesis) [each toxin has a binding portion and an effector portion. If binding portion left off effector portion cannot get into cells and is harmless. Binding portion substituted by Ig that gives specificity to toxin)
      • Catalytic Monoclonal Antibodies ( Abzymes )- use for dissolving blood clots, cleave viral proteins

Organization of Ig Genes

  • To explain diversity of Immune reactivity
    • Germ-Line Theory= germline contributed by germ cells (sperm and egg) contains a large repertoire of Ig genes. This theory assumes that 15% of the genome is dedicated to just the immune response.
    • Somatic Variation Theory= genome contains a small number of Ig genes from which a large number of Ig specificities are generated by somatic cells by mutation or recombination