VETS2006 Lecture Notes - Lecture 6: Fragment Antigen-Binding, Immunoglobulin M, Fragment Crystallizable Region
Antibodies
Antibody
- Each antibody (immunoglobulin) molecule consists of 2 pairs of proteins: 2 heavy chains and
2 light chains linked together by disulphide bonds to form Y-shape.
- There are two different types of light chain: lamda and kappa (diff animals have dif ratios f
oath but don't really know why) and five different types of heavy chain.
- Can be divided into 2 sections (Fab and Fc)
- Antibodies bind to antigens at the ends (top there) of the Fab region and bind to receptors
on other cells (e.g. Macrophages) via the Fc region.
- Because both chains are identical, the conformation of binding site on both arms are
identical, thus both sets bind to the same antigen.
Variable and Constant Regions of Antibody
- Light and heavy chains each contain a variable and a constant region.
- The amino acid sequence of the constant region of heavy and light chains is very similar
among different antibody molecules.
- The amino acid sequence of the variable region differs between different antibody
molecules.
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- The antigen binding site is formed from the variable regions of H and L chains.
General functions of antibody
- Antibody binds to its specific antigen. This can cause:
1. A direct effect: Blocking some activity of the antigen. (E.g. Neutralising a bacterial toxin,
neutralising a virus)
2. Initiate classical pathway of complement > production of membrane attack complexes and
destruction of bacterial cell.
3. Opsonization: Taggig atiges for destrutio. Atiody ids to antigen via Fab portion
and binds to Phagocytic cell via Fc portion (attaches to receptor on cell)
4. Antibody-dependent cell mediated cytotoxicity (ADCC): Later lectures.
Antibody Isotypes
- There are five classes of antibody, each with a slightly different structure and function.
- Each Isotypes has a different type of heavy chain.
- The type of heavy chain determines the nature of the Fc region, which in turn influences the
function of the antibody molecule.
1. Immunoglobulin G (IgG)
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- Gamma (y) heavy chains.
- Structure is basic antibody molecule.
- Most abundant antibody class in serum. (Up to 20-25 mg/ml serum in most animal species)
- Smallest antibody molecule > passes through the blood vessel walls into tissues more easily
than other isotypes.
- Major antibody in the second and subsequent times an antigen is encountered.
- IgG functions include complement activation, opsonisation, neutralisation of toxins.
- Chicken and other birds have IgY as their major antibody (similar structure to IgG) Equivalent
in terms of being the major antibody in second and subsequent encounters.
2. Immunoglobulin M (IgM)
- M (mu) heavy chains
- Consists of 5 subunits (10 antigen binding sites), each composed of two m heavy chains and
2 light chains linked by disulphide bonds,, which J chains joining 2 subunits.
- Second most abundant antibody class in serum.
- Largest antibody molecule, so does not readily move into tissues, but is important in the
response to infections in blood.
- IgM is an efficient antibody Isotype for:
A) Activation of the complement cascade (classical pathway)
B) Opsonization (in phagocytosis)
C) Neutralization of viruses.
D) Agglutination reactions (used in diagnostic tests)
- IgM is the major antibody in the first encounter with antigen. Some confused B cells will still
produce IgM during second reaction.
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Document Summary
Each antibody (immunoglobulin) molecule consists of 2 pairs of proteins: 2 heavy chains and. 2 light chains linked together by disulphide bonds to form y-shape. There are two different types of light chain: lamda and kappa (diff animals have dif ratios f oath but don"t really know why) and five different types of heavy chain. Can be divided into 2 sections (fab and fc) Antibodies bind to antigens at the ends (top there) of the fab region and bind to receptors on other cells (e. g. macrophages) via the fc region. Because both chains are identical, the conformation of binding site on both arms are identical, thus both sets bind to the same antigen. Light and heavy chains each contain a variable and a constant region. The amino acid sequence of the constant region of heavy and light chains is very similar among different antibody molecules. The amino acid sequence of the variable region differs between different antibody molecules.
