L1 Antibody structure & function Flashcards
Describe general antibody structure and understand how this relates to function. Distinguish structural features of antibodies that are related to antigenicity. Understand and explain clonal selection and the generation of antibody diversity. Compare and contrast all antibody class structure/functions.
What is the primary function of an antibody?
Detect and neutralise foreign substances/invaders.
Which three cells interact to imitate an antibody response?
Macrophages, T cells and B cells.
Which cells are the APCs?
Macrophages, monocytes, langerhan cells, dendritic cells & B cells themselves are all antigen presenting cells.
Briefly how are B cells activated?
B-cell binding with Ag induces signal to increase expression of MHC-II & B7.
Th cell recognises same antigen on MHC-II.
Th cell then expressed CD40-L & B7-CD28 interaction is co-stimulation. B cell beings to express cytokine receptors and binding of cytokines causes up regulation of DNA synthesis in B cell = proliferation.
Immunoglobulins (antibodies) are expressed in two forms, what are they?
Secreted form = produced by plasma B-cells as antibodies.
Membrane bound = present on B-cell surface as antigen specific receptor.
What four polypeptide chains come together to form an antibody?
2 Heavy chains & 2 Light chains.
What three fragments of an antibody were discovered when mercaptoethanol (reducing agent) disrupted the disulphide bonds?
Fragment antigen binding region 2x (Fab region).
Fragment crystallisable (Fc, constant region).
How many amino acids are found at tip of antigen binding site?
100-130
In the constant domain of the antibody, which chain gives rise to the class?
The heavy chain
What is the role of the hinge part of the antibody?
The Hinge connects the Fab and Fc regions of the antibody, via disulphide bonds.
What are two classifications of light chain?
kappa or lambda (but not useful for class/subclass definition)
Describe characteristics of IgG
Most common antibody, 80%. Major Ig in blood. Can cross placental barrier.
Standard 2H and 2L chains.
Key player in humour immune response, able activate complement, opsonise, activate phagocytes and neutralise toxins.
Describe characteristics of IgA
Found as monomer in circulation but dimerises (active) at mucosal surfaces. Requires secretory component to dimerise and give resistance to enzymatic cleavage.
Good neutralising, provides defence at mucosal surfaces (saliva, tears, mucus, sweat, gastric fluid)
Is IgA good at activating complement?
No, very weak, its involved in clearance, and preventing pathogens invading epithelial surfaces. Does not induce bacterial cell lysis via complement as this is very inflammatory. As its targets are not found ‘within’ body, no great need for large inflammatory response.
Describe characteristics of IgM
IgM forms a pentamer via linking of J chains between 5 monomers.
Largest antibody and cannot enter tissues. Expressed on B cell surface as monomer as B cell antigen receptor.
Indicated primary response, good for removal of bacteria, good at activating complement and good for agglutination.
Which antibody is first to be produced during immune response?
IgM, indicates primary response to antigen.
Describe the chraracteristics of IgE?
Involved in hypersensitive reactions (allergies) Fc binds to mast cells and basophils causing release of amines (histamines).
Protects against parasites via releasing mediators that attract eosinophils.
Describe IgD characteristics
The only known function is as the B-cell receptor. Found on naive B-cells. Once mature, replaced by other antibodies.
May play a protective role in severe infection.
What part of the antibody dictates the effector functions, upon interaction with what receptor on which groups of cells?
Fc region interaction with cognate Fc receptors (FcR) which are found on macrophage/neutrophil/monocytes /mastcells/basophils/etc
Class of antibody dictates its effect via binding to target specific cells, e.g. IgE binding to FcR’s specific only to mast cells/basophils
Protein A and G are important in antibody technology for what reasons?
Important in binding IgG in beads. Able to pull antibodies out of a mixture as they bind to the Fc region.
What is the difference between polyclonal and monoclonal antibodies?
Polyclonal antibodies are a pool of antibodies that recognise multiple epitopes (usually on one antigen).
Monoclonal antibodies are a pool that only recognise one specific epitope.
Which three genes are rearranged during gene rearrangement of antibody?
Variable (V) diversity (D) and joining (J). Form the blueprint for the variable region of the antibody.
What is isotope class switching?
The switching of the class of an antibody (e.g. IgM-IgG)
What is the paratope?
Binding site on the antibody.
What is the epitope?
Binding site on the antigen.
What are the hyper variable region?
The HVR are regions of aa’s 10 in length. There are three of them on each chain (6 per binding site) complenentarity-determining regions (CDR) CDR1, CDR2 and CDR3.
They line the antibody binding site and make up the paratope. ~15% of variable region involved in binding. 4/6 minimum involved in binding.
Which chains CDR’s make more contact with epitope?
Heavy chain CDRs make more contact than those on light chain (HDCR3 always involved)
What four foreign substances can bind to the Fab region?
Protein, Carbohydrate, Nucleic Acid, Peptide.
Is paratope-epitope binding covalent?
Binding is non-covalent, meaning that it is reversible and means antibodies can be used to extract and purify their targets from solutions. Basis for antibody technology.
What bonds are involved in antibody-antigen binding?
Hydrophobic bonds drive molecules together.
Direct contact ionic bonds, hydrogen bonds and Van der Waals forces.
