ANTIBODIES: THE MEDIATORS OF HUMORAL IMMUNITY

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ANTIBODIES THE MEDIATORS OF HUMORAL IMMUNITY
Two Forms of Antibody or Immunoglobulin 1.
Membrane of B cells 2. Secreted by Plasma Cells
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IMMUNOGLOBULIN MOLECULE PEPTIDE COMPOSITION
Heavy chains (H) 50 kDa 5 main
isotypes-m,d,g,a,e
Light chains (L) 25 kDa 2 isotypes--k,l
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HEAVY AND LIGHT CHAINS HAVE CONSTANT AND VARIABLE
REGIONS
Variable (V) regions interact with antigen Heavy
chain constant (C) regions are responsible for Ig
function
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IMMUNOGLOBULIN MOLECULES ARE GLOBULAR
Hinge region yields flexibility
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STRUCTURE OF Ig DOMAINS
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CHARACTERISTICS OF Ig DOMAINS

• Each globular domain is approximately the same
  size (110 a.a)
• Each domain has two layers of polypeptide chain
  linked with a disulfide
• bond
• The characteristic b chain structure creates
  the Ig domains that typify the
• Ig gene superfamily

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WU AND KABAT PLOTS REVEAL REGIONS OF
HYPERVARIABILITY
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HYPERVARIABLE REGIONS ARE LOCATED IN DISCRETE
LOOPS
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STRUCTURAL ORGANIZATION OF MAIN HUMAN ISOTYPES
Note Differences in glycosylation sites, length,
and flexibility
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EACH ISOTYPE HAS SPECIFIC PROPERTIES AND FUNCTIONS
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IgM AND IgA CAN FORM MULTIMERS
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ANTIGENIC PROPERTIES OF IMMUNOGLOBULINS
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Isotypes have selective distribution
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POLYMERIC ISOTYPES CAN BE TRANSCYTOSED
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BACTERIA CAUSE DISEASE BY SECRETING TOXINS
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High Affinity IgG and IgA Can Neutralize Toxins
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High Affinity IgG and IgA Can Inhibit Viral
Infection
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Ig Can Block Adherence of Bacteria
IgA at mucosal surfaces IgG at other sites
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OPSONIZATION (IgG and IgA)
Opsonization requires matching between FcR and Ig
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ADCC(IgG)
ADCC also requires FcR and Ig isotype matching
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MAST CELL DEGRANULATION (IgE)
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FUNCTION AND DISTRIBUTION OF Ig ISOTYPES