Antibodies Flashcards
What type of molecule is an antibody/immunoglobulin?
glycoprotein
What are 3 functions of antibodies?
- complement activation (IgM + IgG)
- Opsonisation
- Cell activation via specific antibody binding receptors - Fc receptors
What is the basic structure of immunoglobulins?
- 2 light (25kDa) chains, 2 heavy (50kDa) chains
- each chain has amino and carboxyl terminal
- chains held together by disulphide bridges
What type of bonds hold immunoglobulin chains together?
disulphide bonds
What is the order of amounts of the globulins found in serum in decreasing order?
A, γ, α, β
What allows flexibility of immunoglobulins?
there is a hinge in the antibody which allows flexibility between the two Fab
What is the benefit of immunoglobulins having flexibility?
- allows angle between the two antigen binding sites to change angle depending on the proximity of cell surface determinants (how cloes together the antigens are)
Label the antigen binding sites + variable + constant regions of the light and heavy chains on the diagram.
How are the variable and light chains divided in immunoglobulins?
both light and heavy chains can be divided into variable (where the sequences are different) and constant (same sequence) regions
Where are the disulphide bonds located within the immunoglobulins?
each domain e.g. variable light, has intramolecular disulphide bonds to maintain their specific 3D structure required for antigen binding
Which region of immunoglobulins binds to cells and what does this result in?
- constant region binds to Fc receptors
- can lead to cell activation e.g. NK cells (secondary effector functions in immune response)
How many hypervariable regions exist on each antibody binding site of each immunoglobulins?
3 hypervariable regions
What is another name for the 3 hypervariable regions?
complementarity determining regions (CDRs)
What is the role of the hypervariable regions at each antigen binding site?
highly specific region, significant number of interactinos between the antibody and antigen surface
What are 4 forces involved with antibody binding?
- hydrogen bonds
- ionic bonds
- hydrophobic interactions
- van der Waals interactions
What are 4 forces involved with antibody binding?
- hydrogen bonds
- ionic bonds
- hydrophobic interactions
- van der Waals interactions
What creates high affinity bonding between antigen and antibody?
non-covalent bonds are relatively weak - to have hihg affinity must only be short distance between antigen + antibody, highly complimentary, and significant number of interactinos
What creates high affinity bonding between antigen and antibody?
non-covalent bonds are relatively weak - to have hihg affinity must only be short distance between antigen + antibody, highly complimentary, and significant number of interactinos
What is the calaculation used to work out antibody affinity for a single antigen binding site and a single epitope on the antigen?
What is antibody affinity avidity?
- Affinity = The strength of the total non-covalent interactions between a single antigen binding site and a single epitope on the antigen.
- Avidity = The overall strength of multiple interactions between an antibody with multiple binding sites and a complex antigen with multiple epitopes
What are 2 examples of cross-reactivity with antibodies?
- vaccination with cowpox induced abs to smallpox
- antibodies against microbial agents can cross react with ABO blood group antigens
What are antibody isotypes vs allotypes?
- isotypes = antibodies present in everybody with a constant region
- allotypes = antibodies that contain single amino acid mutations, giving allelic polymorphisms which vary in the population
How many heavy chain domains are there in each of the antibody classes?
- IgG: 3
- IgA: 3
- IgM: 4
- IgD: 3
- IgE: 4
What is the heavy chain globulin class for each of the antibody classes?
- IgG: γ
- IgA: α
- IgM: µ
- IgD: δ
- IgE: ε
What are the light chain globulins in each of all 5 classes of immunoglobulin?
κ/λ
Which immunoglobulins have subclasses and what are they?
- IgG: IgG1-4
- IgA: IgA1 + IgA2
In what form does IgG exist?
monomer
Where does the variability resulting in subclasses of IgG arise?
hinge region + effector function domains
Which IgG subclasses are the main activators of classical complement pathway?
IgG1 + 3
What are the frequencies of the IgG subclasses?
decrease in proportion from 1-4
What is the second most abundant monomer in blood?
IgA
What are the forms in which IgA exists in a) blood and b) secretions?
- a) blood = monomer
- b) secretions = dimer
What is the major secretory immunoglobulin?
IgA
What is the function of IgA?
major secretory immunoglobulin - protects mucosal surfaces from bacteria, viruses and protozoa
How is secretory form of IgA produced?
- joined by J chain and secretory component.
- Plasma cell secretes dimeric form without secretory component.
- This bonds to poly-Ig receptor and is endocytosed and secreted into lumen.
- The poly-Ig receptor is cleaved and becomes the secretory component
What is the function of the IgA secretory component?
protects IgA from being degraded in the lumen by proteases etc.
What is the structure of IgM?
pentameric - 5 monomers joined by J chain (10x Fab)
Where does IgM exist in the body?
mainy in the blood
What is the first Ig synthesised after exposure to antigen?
IgM - primary antibody response
What is the first Ig synthesised after exposure to antigen?
IgM - primary antibody response
What do the multiple binding sites of IgM compensate for?
low affinity
Which cells express IgD?
only exists expressed on B cell surface
What is the function of IgD?
involved in B cell development and activation
What is the function of IgE?
- produced in response to parasitic infections and in allergic diseases
- binds to high affinity Fc receptors of mast cells and basophils
- cross-linking by antigen triggers mast cell activation and histamine release
Which 2 Igs are abundant in blood?
IgG and IgM
Which Ig exist in extracellular fluid?
IgG
Which Ig is present in secretions across epithelia including breast milk?
dimeric IgA
Where does IgE exist?
with mast cells below epithelium
Which are of the body is devoid of antibodies?
brain
Which antibody is mainly responsible for neutralisation of toxins by inhibiting their toxicity?
mainly IgG
Which Ig is mainly responsible for neutralisation of viruses?
IgG
Which Ig is mainly responsible for neutralisation at body surfaces?
Secretory IgA
Which 2 antibodies are involved in agglutination i.e. Ag-Ab complexes/lattice formation?
IgM, IgG
Which Ig is mainly responsible for opsonisation to promote phagocytosis?
IgG
Which 2 antibodies are responsible for complement activation via the classical pathway?
IgM and IgG
Which Ig is responsible for mast cell sensitisation + triggering?
IgE
Which Ig is responsible for NK cell cytotoxicity/ADCC?
IgG
What is the first step of the classical complement pathway (same as for lectin pathway)?
activation fo complement C1
Which amino acid is involved in the formation of disulfide bonds, which makes Igs durable?
cysteine
