Immunology Antibodies Flashcards
What is the structure of an antibody?
Structure
• symmetrical
• Two light (25kDa) chains, two heavy (50kDa) chains
• Each chain has amino and carboxyl terminal
• Chains held together by disulphide bridges
• Electrophoresis of globulins found in serum:
- Relative amounts (decreasing): A, γ, α, β
- Electrophoretic mobility- towards +ve electrode: A, α,
β, γ
• Different antibodies therefore have different charges
Describe more about the flexibility of the antibody.
- There is a hinge in the antibody which allows flexibility between the two Fab
- This allows the angle between the two antigen binding sites to change depending on the proximity of cell surface determinants, i.e. how close together antigens are
Discovery of Antibodies
• Rodney Porter
• Limited the digestion of gamma-globulin with purified papain, which produced 3 fragments in equal amounts
• 2 fragments had antigen binding activity (Fab)
• The third did not, but formed protein crystals (Fc)
Further elaboration on structure of antibodies?
• Both light and heavy chains can be divided into variable (where the sequences are different) and constant (same sequence) regions
• Each IG (immunoglobulin/antibody) domain, e.g. variable light, has
INTRAMOLECULAR DISULPHIDE BONDS to maintain their specific 3D
structure required for antigen binding
• Many cell surface proteins also have IG-like domains, and are said to belong to the IG super family
• The constant region binds to Fc receptors, which can lead to cell activation, e.g. NK cells (secondary effector
functions in immune response)
What is the antigen-binding site?
• Antigen binding occurs at 3 HYPERVARIABLE regions, known as COMPLEMENTARITY DETERMINING REGIONS (CDR’s)
• These have specific residue positron numbers
• The region of binding is a large undulating 3D structure (~750A = 10-10m), so is highly specific and there are a
significant number of interactions between the antibody and antigen surface
What are the forces involved in an antibody? (4)
Forces involved
• Hydrogen bonds
• Ionic bonds
• Hydrophobic interactions
• Van der Waals interactions
Forces involved non-covalent, therefore are relatively weak. This means that in order to have a HIGH AFFINITY, there can only be a short distance between the antigen and antibody, highly complementary nature, and a significant number of interactions.
What is Antibody Affinity?
The strength of the total non-covalent interactions between a single antigen binding site and a single epitope on the
antigen.
The affinity association constant K can be calculated: K varies from 104 to 1011 L/mol
What is an antibody?
What is an antibody?
• A protein that is produced in response to an antigen
• Binds specifically to the antigen
• Form the class known as IMMUNOGLOBULINS
• Large family of soluble GLYCOPROTEINS
• Produced by B lymphocytes
• Found in serum
• >107 different types
• Deficiency is life threatening
• After binding antigen, initiate secondary effector functions
- Complement activation
- Opsonisation
- Cell activation via specific antibody-binding receptors (Fc receptors)
What is antibody avidity?
The overall strength of multiple interactions between an antibody with multiple binding sites and a complex antigen with multiple epitopes
• This is a better measure of binding capacity in biological systems
• Monovalent interactions have a low affinity
• Bivalent interactions have a high affinity
• Polyvalent interactions have a very high affinity
What is cross-affinity in antibodies?
Antibodies elicited in response to one antigen can also recognise a different antigen, for example:
- Vaccination with cowpox induces antibodies which are able to recognise smallpox
- ABO blood group antigens are glycoproteins on red blood cells. Antibodies made against microbial agents on common intestinal bacteria may cross-react with the glycoproteins, which poses a problem for blood transfusions.
What are isotypes and allotypes?
• Isotypes are antibodies who are present in everybody, with a constant region.
• Allotypes are antibodies that contain single amino acid mutations, giving allelic polymorphisms which vary in the population
