Immunology Chapter 7

==Antibody Structure==

• 2 heavy chains
• 2 light chains
• heavy chains linked to 2 disulfide bonds
• each chain composed of immunoglobulin domain

@@Immunoglobulin Domain@@

• each domain has 2 layers of β-pleated sheets with 3-4 antiparallel strands
• 2 layers held together by disulfide bonds

@@Variable & Constant Regions@@

• immunoglobulin domains named according to variable or constant region
• ends of heavy chain and light chain contains hypervariable regions and framework regions
• combination of hypervariable regions from heavy and light chain = antigen binding site
• hypervariable region AKA CDR because confirmation must be complementary to antigen for binding
• between CH1 & CH2 = hinge region
  • flexible for binding
• cleavage of antibody
  • 1.) papain = cleaves Ab above disulfide bridges linking heavy chains
  • result = 1 Fc region and 2 Fab fragments
  • 2.) pepsin = cleaves Ab below disulfide bonds holding 2 heavy chains together
  • result = 1 F(ab’)2

^^Antibody Isotope^^

• IgA = α
• IgG = γ
• IgD = δ
• IgE = ε
• IgM = μ
• no hinge region = ε and μ
• 1 disulfide bond = α and δ
• 2 disulfide bonds = γ
• 2 immunoglobulin regions = α, γ, and δ
• 3 immunoglobulin regions = ε and μ
• tail pieces = α, δ, and μ
  • α and μ tail pieces bind to each other and to J chain to form multimers
• no tail pieces = γ and ε

%%IgM%%

• produced first
• location = blood
• function = activates complement

%%IgG%%

• main isotope
• location = blood and extracellular fluid
• function = complement activation and opsonization

%%IgA%%

• primary component secreted onto mucus membranes (AKA secretory antibody)
• function = neutralization

%%IgE%%

• found in low levels in blood and extracellular fluid
• location = close to mast cells just beneath the skin and mucus membranes
• mast cells become activated when IgE binds antigen
• mast cells produce cytokines

%%IgD%%

• in small concentration in blood

^^Differences in Light Chain^^

• 2 types:
  • κ
  • λ
• more κ than λ in humans
• more λ than κ in mice
• no functional difference
• difference = framework regions of the variable and constant regions

==Polyclonal vs. Monoclonal Antibodies==

• antibody recognizes epitope
• antigen injected into animal → animal produces antibodies against every epitope of antigen
• monoclonal = specific (one kind)
• polyclonal = not specific (multiple kinds)
• @@Monoclonal steps:@@
  • monoclonal antibodies = inject antigen → polyclonal antibodies
  • spleen isolated and activated B cells from spleen are grown in tissue culture
  • spleen cells fused with myeloma cells → become immortalized
  • immortalized spleen cells → hybridomas = are diluted so that a single cell is present in each well of a tissue culture plate
  • one B cell can only produce antibody specific for one epitope, each well produces different antibodies for different epitopes

^^Antibody binding to Antigen^^

%%Epitope Structure%%

1.) Linear Epitope

• amino acid adjacent to each other in primary protein
• small (6 amino acids)
• to be recognized by antibody, it must be present on surface of protein or protein must be denatured

2.) Conformational Epitope (discontinuous epitope)

• recognizes shaped formed by folded protein
• actual amino acid in forming this shaped could be far apart in primary protein sequence

%%Affinity%%

• measurement of strength for antibody binding in ONE site
• affinity = equilibrium constant
• repeated antigen = increase in affinity → tighter antigen binding → affinity maturation

%%Avidity%%

• antibodies have multiple antigen binding sites
• each binding site can bind to an epitope
• avidity = measurement of strength for antibody binding in ALL sites
• example = IgM has low affinity than IgG but high avidity
• after affinity maturation, IgG increases to where it is equivalent to IgM

^^Antibody Functions^^

%%1.) Neutralize bacterial toxins%%

• exotoxins produced and secreted by bacteria
• toxin binds to receptor on target cell → internalized and active portion of toxin exerts its effect on cell
• antibodies can bind to receptor binding portion of toxin → blocks binding of toxin to its receptor → doesn’t get internalized and cannot carry out toxin function
• neutralizing antibodies = IgG in tissue or IgA on mucosal surface

%%2.) Prevent infection by virus%%

• viral attachment protein must bind to specific receptor to infect cell
• antibodies bind to viral attachment proteins = blocks virus from infecting cell
• virus neutralization = IgG and IgA

%%3.) Block attachment by bacteria%%

• bacteria attach to cell surface and colonize surface during infection
• antibodies bind to adhesins = block attachment

%%4.) Activation of complement vis classical pathway%%

%%5.) Antibody Dependent Cell Cytotoxicity (ADCC)%%

%%6.) Antibody Mediated Opsonization%%

• aggregation of immunoglobulin = cross-linking of Fc receptors → activation of macrophage, phagocytosis, and destruction of bacteria
• free immunoglobulin = no cross-linking of Fc receptors → no destruction of bacteria
• Opsonization by antibody and complement can occur at the same time = increase effectiveness
  • antibody binds to Fc receptor and C3b binds to Cr1 → macrophage membrane fuse → phagosome → lysosome in phagosome → enzyme degrades bacteria

%%7.) Degranulation of eosinophils and other phagocytes%%

• cross-linking Fc receptor on parasite too big for phagocytosis → lysosome from phagocyte fuses with cell membrane releasing contents on surface
• eosinophils = primary cells in response to parasite infection
• cross-linking receptors = IgE and FcεRI

%%8.) Activation and degranulation of mast cells%%

• mast cells have FcεRI on surface that binds to IgE
• cross-link IgE = mast cell activation
• activated mast cell = degranulation, releasing contents of granules that contains histamine
  • can produce prostaglandin D2, leukotriene C4, and TNF α = inflammation
• activated mast cell = allergic reaction

3 ways fighting infection:

• recruit leukocytes to infection area by increasing vascular permeability and inflammation
• increase flow of lymph to lymph nodes where antigen can be captured for processing and presentation
• trigger muscle contraction (coughing) or increase peristalsis in intestine (pooping)