Antibody function Flashcards
Does antibody specificity for antigen increase?
Yes; affinity maturation after subsequent immune responses in the variable region of the antibody via somatic hypermutation.
How do antibodies bind to antigens?
Fab binds to antigens complementary; the fab binds to epitope. Fab recognition region is approx 2.5 nm^2; the antibody footprint. The larger the antigen the larger number of epitopes on the antigen. Epitopes can be distinct or overlapping. Ab mostly recognise discontinuous epitopes on the antigens.
What are discontinuous (conformational epitopes)
a combination of amino acids on the antigen which would not be proximally located in the primary structure of the protein, but protein folding locates these amino acids together. Their resulting structure is what is recognised by the Fab recognition site of the antibody. These oppose to linear epitopes of a continuous polypeptide sequence
What are the Ab-Ag forces
These are all non-covalent interactions and act over a relatively short range and are reversible; hydrogen bonding, electrostatic, Van der Waals and hydrophobic. (ordered in proximity)
What is the equilibirum/affinity constant K?
K = [AbAg]/[Ab][Ag] .
What determines the intrinsic affinity of Ab?
The strength of a single Ag-Ab interaction. The higher the affinity the more efficient the antibody. As Ab are multivalent, the strenght with which an antibody binds toa multivalent agent is usually greater than the intrinsic affinity. The strength increase is termed avidity or functional avidity.
Where is multivalent binding seen?
In soluble antigens
What decreases the probability that Abs are not bound to Ags?
Binding is non-covalent, 2 arms allow for multiple binding. Binding of one arm does not influence the binding of the second Ab arm; no co-operative binding unless physically linked.
What are the characteristics of the primary immune response?
Majority Ab secretion being IgM at the beginning. Class switching then occurs in the primary response (e.g. to IgG). Memory T and B cells formed in the primary immune response.
Describe the secondary immune response.
Faster and stronger immune response due to memory cells
True/False: IgM response is the same in the secondary immune response?
True, but class switching occurs much earlier in the secondary immune response with greater proliferation of plasma cells from memory cells; The class-switched secreted Ab also increases in concentration faster.
What is the key enzyme for class switch recombination?
AID; activation-induced cytidine deaminase. Recombination of switch sequences for constant regions of Ab.
Does Cdelta have a switch sequence?
No, Cdelta is always transcribed with Cmu; causing IgM and IgD being constituitevly expressed on naive B cells.
How does the switching of BCR trasmembrane antibody occur?
As B cell develops into plasma cells, the switching occurs via endocytosing the BCR-Ag complex.
How is the switch from membrane to secreted antibodies occur?
Via the non-inclusion of hydrophobic amino acids in translation of gene segment.
What are antibodies specific function?
Neutralising bacterial toxin (perhaps by blocking cell entry) blocking viruses binding to their cellular receptors and block adherence of bacteria to mucosal surfaces.
How do antibodies usually act?
At the interface between the antigen and other immune system components via the Fc region of the antibody. Activation of effector mechanisms is brought about through multivalent presentation of Fc regions.
What are the biological effect functions of antibodies?
Complement fixation leading to direct lysis of microorganisms or opsonisation through binding to complement receptors on phagocytes.
Fc receptor-mediated opsonisation. Extracellular killing via ADCC, control of inflammatory response, complement activation.
What are the functions of IgM?
Constitutes with IgD the antigen receptor on naive B cells. Secreted IgM acts mainly in the blood, first antibody class to be produced in an immune response, activated complement, powerful agglutinin
What are the functions of IgG1?
The most abundant antibody in the blood, activates complement and enhances phagocytosis, can cross the placenta
What are the functions of IgG2?
Activates complement
What are the functions of IgG3
Activates complement, enhances phagocytosis, can cross placenta
What are the functions of IgG4?
Can cross the placenta
What are the functions of IgA1 and 2
Protect mucosal surfaces
What are the functions of IgG
Constitutes with IgM the antigen receptor on naive B cells
What are the functions of IgE?
Triggers therelease of inflammatory mediators from mast cells and is important in the protection against parasitic worms and in allergy.
