20 - antibodies Flashcards
what are antibodies?
immunoglobulins
large Y shape secreted glycoproteins
structure of antibodies
- 2 identical heavy chains
- 2 identical light chains
- joined by non-covalent interactions and disulphide bridges
- N terminal variable region
- C terminal constant region
- can be secreted or membrane bound
where does enzymatic cleavage of immunoglobulins (Ig) occur?
at the hinge region
Fab (fragment antigen binding) contains antigen-binding region
Fc (fragment crystallisable) interacts with Fc receptors on cells with C1q
what is Fab?
fragment antigen binding
the arms of the antibody
what is Fc?
fragment crystallisable
the bottom of the antibody
main purposes of antibodies
bind specifically to epitopes on the pathogen/antigen that elect the immune response
• neutralisation and opsonisation
recruit cells and molecules to destroy the pathogen/antigen once the antibody is bound to it
what region of the antibody is involved in effector functions?
the constant or C region (Fc)
where does antigen binding occur?
macromolecules
• proteins
• carbohydrates
• nucleic acids
surfaces of intact protein molecules
other components on pathogen surfaces
what is the epitope?
part of an antigen to which an antibody binds
antigenic determinant
what are the 2 types of epitope?
linear - continuous
conformational - discontinuous
what are antibody-antigen interactions made by?
non-covalent forces • van der Waals forces • hydrophobic interactions • hydrogen bonds • electrostatic forces
what is affinity?
strength of binding between a single antibody binding site and epitope
why are multivalent antigens stronger?
more than 1 binding site employed
less likelihood of dissociation from both sites simultaneously
what is avidity?
strength of binding between antibody and antigen
• affinity of an dauber of antigen binding sites
• greater than single site affinity
• many bacteria have multiple repeated epitopes
6 functions of antibodies
- neutralisation
- opsonisation
- cell recruitment - ADCC
- agglutination
- cell recruitment - degranulation
- activate complement
neutralisation
bind to bacteria/virus surface, bacterial toxin
prevent interaction with cell receptors
opsonisation
direct - binding to receptors for antibody constant region (Fc receptors)
indirect - increasing complement deposition on pathogen and binding to complement receptors
cell recruitment - ADCC
detection of antibodies by FcR on several cell types induces cell activation
agglutination
cause bacteria/virus to clump together
cell recruitment - degranulation
degranulation via Fc receptors (FcR)
• degranulation of mast cells
• killing by NK cells and eosinophils (ADCC)
activate complement
classical pathway
antigen-antibody complex binds C1qrs
act on C4 and C2 to generate C3 convertase
how can we use antibodies in the lab?
- enzyme linked immunosorbent assay (ELISA)
- flow cytometry
- immunoprecipitation
- fluorescence microscopy
- western blot
