Antibodies Flashcards
what are they
host proteins produced in response to pathogens or in response to PAMPs
what produce them
plasma cells
plasma cells
b cell effector- lymphoid lineage
part of the
adaptive system
are they soluble?
yes
how do they clear infection- simple
find antigen to bind to, then antigen-antibody complexes are removed from circulation through phagocytosis by macrophages (primarily)
Linus Pauling
became interested in how the polypeptide structure of an antibody moults itself around the antigen
what did Pauling suggest
that pre-formed undifferentiated ‘immunoprotein’ folded over the antigenat different locations are different haptenic groups.
- proposed that a single immunoprotein reacted with all antigens but its conformation was different in each case
who realised Pauling was wrong?
Sir Frank MacFarlane Burnet
what did Burnet propose
‘clonal selection theory’ of acquired immunity
- realised that during the immune response certain cells were selected for antibody production
-proposed that somatic mutation during embryonic life generates the random specificities of antibodies that cells possess concordant antigen receptors and that cells to self-antigens are killed
clonal selection-simple
start with large pool of antbiodies, all tested, then left with B cell capable of recognizing antigen but wont harm self tissue
what happens to b cells when infection goes
B cell populaton decreases and become memory b cells
- sit in lymphatic tissue waiting to be challenged again
induction of B cell antibody response by T cell
1) dendritic cells displaying antigen arrives at lymph nodes
2) activate CD4 specific T cells which proliferate and mature into effector cells
3) these T cells are capable of activating antigen specific B cells
4) once activated by CD4 specific T cells, the B cell proliferates to form a primary focus of antigen specific B cells
5) B cells from primary focus migrate to nearby follicles and proliferate
6) other B cells start secreting some antibodies, but eventually die (not specific to T cell)
7) specific B cells in follicle begin to proliferate rapidly- somatic mutation occurs to introduce variation into B cell receptor
8) B cells undergo selection after somatic mutation- clonal mutation
9) those which fail to bind die
strucutre
- large family of glycoproteins
- share key functional and structural features
shared functional features
able to bind both to and antigens and specialised cells and proteins of the immune system
shared structural features
COMPOSED OF ONE OR MORE COPPIES OF CHARACTERISITC UNIT THAT FORMS A Y SHAPE
antibodies bind to a specific
amino acid
any molecule or parts of molecules recognised by the variable antigen receptor of lymphocytes are
antigens
antibodies do not
recognise the whole protein, just the amino acid
if the amino acid is changed
antibody will no longer be able to recognise
the epitope
specific region of the antigen bound by the variable region of an immunoglobulin
paratope
is the antigen-binding region of an antibody
affinity
is the measure of the strength of binding of an antigen by antibody
antibody binding is
non-covalent and reversibly
the affinity of an Ab for an Ag is related to
the ratio of the rates of the forward reactions for formation of the complex to back reaction for the decay of the complex
the reaction between Ab and Ag moves backwards and forwards depending on
whether another antibody comes along with a better fit
- so if an antibody comes along and has a reasonable fit it will bind, but as the B cells go through somatic mutation to make better fitted antibodies the first antibody will lift off and allow the antibody with the better fit to bind
Each Y antibody contains
4 polypeptides
the 4 polypeptides are made up of
2 identical heavy chains (55kDa each)
2 identical light chains (22kDa each)
weight of heavy chain
55kDa
weight of light chain
22kDa
how are the 4 polypeptide chains held together
through disulphide bridges and non-covalent bonds
somatic mutation
when genes rearrange so the antibodies get a better and better fit, therefore certain clonal lineages which have the best clonal fit to the antigen
isotope switching
is a biological mechanism that changes a B cells production of immunoglobulin from one type to another
example of isotope switching
isotope IgM to isotope IgG
–> IgG becomes a better fit as it mutates
Fab domain
2 domains which carry the antigen binding site
- fragments having the antigen binding site
Fc domain
third domain
- the fragment that crystallizes
the hinge in the antibody allows
contortion
-bringing the fab domain closer to allow recognition of PAPM (epitope)- not rigid
the ability to bind antigens is determined by the
N terminal domain
- these differ considerably between antibodies of differing specificities are termed ‘variable’ or ‘v’ domains
Describe the Fab domain
made up of x 2 VL/VH/CL/CH
light chain on the
bottom
Fc domain allows
binding to cell after epitope binding and also complement binding
variable light (VL) and variable heavy (VH) chains
sequence analysis of amino acids VH and VL regions reveal small regions fo hypervariability within 4 conserved framework rgeions
which 4 conserved framework regions show hypervariability
Fr1,Fr2, Fr3, Fr4
how do these hypervariability cause variable binding sites
the hype variable regions form loops that combine together to form the principle antigen binding surfaces- determine variability within antibody structure
- these surfaces re called complementarity determining regions CDRs (CDR1 etc)
what give specific affinity and binding capability to target antigen
CDRs
5 main classes of immunoglobulins
IgG(gamma) IgM IgA(alpha) IgE and IgD (delta)
how are these 5 classes divided
on the basis of the number of Y-like units and the type of heavy chain polypeptide they contain e.g. gamma, alpha
heavy chain is always denoted by
the class of polypeptide chain - e.g. gamma, alpha, delta etc
5 different types of
heavy chain
how many types of light chain
2
types of light chain
kappa, lambda
one light chain always
associates wityh one heavy chain, so the number of light chains is always equal to the number of heavy chains
how many subclasses of IgG in humans
4 IgG1- gamma1 (heavy chain) IgG2-gamma2 IgG3-gamma3 IgG4-gamma4
IgA has how many sub-classes
IgA1 and IgA2
- these have alpha1 and alpha2 chains respectively
what did Rodney Porter
using the enzyme papain, Porter showed that gamma-globulin consisted of three factions I, II, III
- showed that I and II bound antigen without forming a precipitate
- notes the possibility that fractions 1 and II could lie either side of fraction III
today Porters fraction I and II are termed
Fab
III fraction
Fc
- this fraction could be crystalised
porter initially thought
antibodies consisted of a single polypeptide chain
Edelman showed that
IgG consisted of 4 chains, a pair of heavy and light chains to give a Y config
both Porter and Edelman worked with
IgG
papains were used to show that
antibodies are made up of 4 polypeptides
- causes fragmentation
- pepsin cm also be used
papains breaks an antibody into how many parts
3: x2 fab and 1 Fc
antibodies must be able to cross mucosal barriers in order to
reach the pathogen
what do antibodies use to cross
secretory components
secretory components
- helps IgA moveout of the bloodstram and sit on mucosal linings
J chains
a loop which holds` chains together e.g. IgM and IgA
within your gut there are m cells (sampling cells- which take info from the gut and relay to dendiritc cells- which feed info to the rest of the immune system)
antibody being released by B cell needs to move from the blood into the lumen where is can have an effect on the pathogen/toxin
B CELL CANNOT DO THIS- ONLY ANTIGEN
which is the most prevalent antibody on mucosal linings
IgA
epithelial cells in the intestine
mediate the transfer of dimeric serum IgA to the intestinal lumen via polymeric Ig receptor
the polymeric Ig receptor binds to
the secretory component, moving the antibody into the gut
- binds to antigen and prevents uptake of bacteria and toxins
trans epithelial transport of IgA
- IgA dimer with j chain attached binds to the polymeric Ig receptor- a transmembrane protein expressed at the basal surface of epithelial cells of the gut, airways and various secretory glands-binding of IgA induces transcytosis of the polymeric Ig receptor
- complex is delivered to the apical surface of the epithelial cell and into the lumen
- proteases cleave the plgR near the membrane releasing the majority of the extracellular domain still bound to IgA dimer
- the secretory component protects IgA from proteases present in the mucus and anchors IgA at the dimer
- the secretory component protects IgA from proteases present in mucus and anchors IgA a the desired location
once in the lume IgA is able to
complex with toxins and bacteria, signalling for phagocytotic cells to bind to the Fc domain of the antibody and destroy the complex
