Antibody biology Flashcards
What are B cells and what do they do?
B cells are lymphocytes central to our humoralresponse - they are responsible for production of antibodies.B cells originate and mature in the bone marrow.On activation with antigen: B cells can become antibody secreting cells (plasma cells)- differentiation, or memory B cells- basis of immunological memory.Antigen receptor on B cells = antibody the B cell is programmed to make (B cell receptor- form of antibody that the cell is programmed to make.)
What do plasma cells do?
Plasma cells secrete antibody of the same specificity as themembrane-bound immunoglobulin expressed by their B cell precursor.Membrane bound Ig has specific binding to antigen, stimulates B cell to undergo further differentiation and make daughter cells. Stimulated B cell gives rise to plasma cells- have same specificity as B cell. Programmed to affect antigen that was originally detected.
What is the structure of immunoglobulins?
All immunoglobulins are structurally similar. Made up of heavy chains (inner) and light chains (outer). Antibodies can express one of two possible light chains, λ or κ.Variable region at tips of arms. N terminal. Named VH and VL. Constant region makes up the rest of the molecule. C terminal. Named CL, CH1, CH2, CH3 etc.FAB regions are arms. Fc region is stem. Disulphide bridges between the two regions, at hinge.
What are the products of enzymatic digestion of antibodies?
Proteolytic cleavage by papain yields:FAB- Fragment antigen binding - responsible for binding antigen through tip of FAB arms.Fc- Fragment crystallisable (tends to crystallise) - responsible for effector function.
What does the Ig flexible hinge do?
Flexible hinge of IgG allows both arms to bind to many arrangements of antigens on pathogen surfaces.True for all antibodies to an extent.Flexibility at hinge region.Greater occupancy rates due to bivalent binding- greater effector function happens as a result.
What are light chains and heavy chains?
Two types of light chain - lambda (λ) and kappa (κ).All antibody classes use these same light chains (same type of chains in each antibody molecule).In any individual antibody molecule both lightchains are the same.Heavy chain determines class of antibody:IgG- γ heavy chain.IgA- α heavy chain.IgM- μ heavy chain.IgE- ε heavy chain.IgD- δ heavy chain.Domains fold up into globular domains.
How to antibodies fold into globular domains?
Antibodies fold into globular domainswith a distinctive 3D structure.Each domain comprises 2 β sheets that lie over each other, which are linked by a disulphidebridge to form a β barrel.Each sheet made up of a number of B strands. Slight difference in V and C domains- slightly more folds in V domain.Loops that join lie at either end of domain.Folded structure is known as the‘immunoglobulin fold’.
What are hypervariable loops?
Variable domains in both light and heavychains have hypervariable loops which varyextensively between different antibodies.Hypervariability- very different sequences from one antibody to the next.Framework regions-These are still variable, but much less so.Hypervariable loops form antigen binding site.Hypervariable loops are more commonly termed the complementarity-determining regions (CDRs).When VH and VL domains are paired the CDRs create theantigen-binding site.Antigen binding site hascomplementary 3D structure to that of antigenic epitopeproviding high specificity and high affinity.Loops lie at one end of domain.6 loops brought close together in binding- form binding site.CDR- bind to antigen through complementarity of structure eg. hand in glove, key in lock etc.High specificity binding- they fit together in a specific way- one one type of antigen will bind into one type of site.Adjust loop sequence give subtle structural changes- changes which antigen can fit in site.
What are linear and discontinuous epitopes?
Epitopes (Regions recognised by antigen.AKA antigenic determinants) in protein antigens may comprise a single stretch ofpolypeptide chain = linear or continuous epitope. Binds in a linear sequence.Or may depend on 3D folded structure = conformational ordiscontinuous epitope. Doesn’t bind in a linear way.
How do epitopes bind to antibodies?
Antibody recognises structural ‘epitopes’ or ‘antigenicdeterminants’ on the antigen.Epitopes can bind to pockets, grooves, extended surfaces orprojections in antigen binding sites.
Where are epitopes located?
Epitopes for antibodies are exposed on pathogen surfaces.Sites need to be accessible for antibody binding.
How is antibody diversity achieved?
Almost any substance can elicit an antibody response.Responses to even simple substances are diverse.Each individual has an antibody repertoire (i.e. available antigenspecificities) of approximately 10^11- Need to recognise 10^11 different antibody structures.To achieve this, Ig genes are organised differently to other genes.In all cells except B cells, Ig genes are in fragmented form thatcannot be expressed.Ig heavy and light chain loci consist of families of gene segments,arrayed sequentially along the chromosome- This is why they are in fragmented form.They are inherited in this form through egg and sperm (germline)Therefore their arrangement is termed “germline configuration”.In developing B cells Ig gene segments must be rearranged toassemble functional light and heavy chain gene.
What are the gene segments and how are they arranged?
V = V gene segment encodes most of the V domain (95-101 aa)J = ‘Joining’ segment (encodes up to 13 aa)D = ‘Diversity’ segmentVH domain is encoded by V, D and J gene segments.VL domain is encoded by V and J gene segments.Lambda light chain locus- J segments upstream of corresponding constant region.Kappa light chain locus- just Vk and Jk genes- no D genes.Heavy chain locus- Have around 40 VH segments arranged one after other. DH (diversity segment). 23 joining segments.
When do Ig gene rearrangements occur?
Ig gene rearrangements occur at definedpoints in B cell development.Heavy chain Ig rearranges first- D-J rearranging at early pro-B cell stage. V-DJ rearranging at late pro-B cell stage.Light chain Ig reararnges later- V-J rearranging at small pre-B cell stage.
How to pro-B cells rearrange heavy chain gene segments?
Pro-B cells randomly recombine heavy chain gene segments.Works through alternative splicing.D gene segment is joined to a J gene segment.V gene segment joined to DJ gene segment.These events are called somatic recombination because they occur in cells other than germ cells.DNA that is spliced out is lost- irreversible step.VH-CH1-CH2-CH3-CH4: Transcript to produce IgM- first antibody that is usually produced.Codes for IgM heavy polypeptide chain (VH domain).