Lecture 13 Antigens, Receptors and Immunoglobulins

1
Lecture 13Antigens, Receptors and Immunoglobulins
2
Antigenic Determinants (Epitopes)
3
Receptor-Epitope Concepts

• Most biological systems can be viewed in terms of
  interactions between receptors and epitopes
  (hand and glove).
• Specificity of the interaction between the
  receptor and epitope is determined by the amino
  acid sequence of the receptor (tailoring the
  glove).
• If the glove is too big, the glove falls off.
• If the glove is too small, you cant get your
  hand into it.
• If the glove is missing fingers

4
Examples of Receptor-Epitope Interactions

• Enzymatic reactions
• Antigen recognition
• Antigen-Antibody reactions
• Biological chaperones (stress proteins)
• Cellular communications
• Cell-mediated cytotoxicity
• Cellular homing (MALT, chemotaxis, etc.)

5
Basic Immunoglobulin Structure
6
The Prototype Immunoglobulin Molecule

• Heavy Chains (five types ??????????????
• Light Chains (two types ??and?????
• Fab fragment
• Fc fragment
• Constant and variable regions
• Carbohydrate
• Disulfide linkages

Fab
VH
VL
CH1
CL
CH2
Fc
CH3
7
Immunoglobulin G (IgG)

• 2 Heavy and 2 Light chains
• 2 Fab and 1 Fc fragment
• 4 Subclasses (IgG1, IgG2, IgG3, IgG4)
• Mol. Wt. 150,000
• 70-75 of serum immunoglobulin.
• The major antibody of the secondary immune
  response
• Change in affinity with time

8
Immunoglobulin M (IgM)

• 10 Heavy and 10 Light chains
• 10 Fab and 5 Fc fragments
• Mol. Wt. 900,000
• lt10 of serum immunoglobulin.
• Single J Chain (15 kDA)
• The predominant "early" antibody
• Most primitive immunoglobulin
• No change in affinity with time

9
Immunoglobulin A (IgA)

• Monomeric- Serum IgA
• 15-20 of serum immunoglobulins
• 2 Heavy and 2 Light
• 2 Subclasses (IgA1 and IgA2)
• Found in serum

10
Secretory Immunoglobulin A (sIgA)
J-Chain
Secretory Component

• 4 heavy chains and 4 light chains (dimeric)
• J-Chain and secretory component
• The major immunoglobulin of secretions
• Not found in serum

11
Immunoglobulin E (IgE)

• 2 Heavy and 2 Light chains
• Mol. Wt. 190,000
• Trace serum protein
• Note CH4 region on H chain
• Associated with atopic or anaphylactic
  hypersensitivity
• May play role in immunity to helminthic parasites

CH4
12
Immunoglobulin D (IgD)

• 2 heavy and 2 light chains
• lt1 of serum immunoglobulins
• Serves as a membrane receptor on B lymphocytes
• May play role in antigen-stimulated lymphocyte
  differentiation

13
Take Home Lessons

• What is the basic immunoglobulin structure?
• What are the characteristics or features of the
  various fragments of the immunoglobulin molecule?
• How is the specificity of an antibody defined
  by the amino acid sequence of the immunoglobulin
  molecule?
• What features distinguish each immunoglobulin
  class?