antigens and antibodies

IgD

1. <1% of Ig component of serum 2. present on surface of naive B cells with IgM as antigen receptor 3. long hinge region not much known

antibody effector functions via FcR

1. Ab-Ag (immune) complexes can activate the complement system 2. antibodies bound to pathogens opsonise them for phagocytosis 3. antibodies bound to cell can promote their killing by NK cells or eosinophils 4. antibodies bound to FcRs increases the cell's sensitivity to antigen; subsequent binding to antigen activates the receptor cell

how is an antigen/epitope recognised by the immune system?

1. B cells via: - cell-bound B-cell receptor (BCR) - soluble antibodies (immunoglobulins) secreted from mature B cells (plasma cells) 2. T- cells via: - Cell-bound T-cell receptor (TCR) (+MHC)

concentration order (from highest to lowest)

1. IgG1 2. IgG2 3. IgG3 4. IgG4 5. Igm 6. IgA1 7. IgA2 8. sIgA 9. IgD 10. IgE

effects of soluble antibody binding via Fab

1. high affinity IgG and IgA can neutralise bacterial toxins 2. Antibodies can neutralise and block binding of virus to host cells 3. antibodies can block adherence of bacteria to host cells 4. Ag-Ab complexes activate classical pathway of complement

IgG

1. in highest concentration in plasma and lymph of healthy individuals 2. monomer 3 2 Fab = divalent 4. four sub-classes -> IgG1, IgG2, IgG3, IgG4 5. flexibility of hinge region increases the likelihood of simultaneously binding to two epitopes 6. actively transported from blood into extracellular spaces of tissues

IgM

1. occurs as a pentamer in plasma -> multivalent 2. each heavy chain has four Ch and a Ch 3. addition of a J (joining) polypeptide chain and four oligosaccharides 4. classically shown with 'star' formation but can dislocate at Ch2 and Ch3 domains to bend and bind multiple epitopes 5. can bind complement when binding sites are exposed (at Ch3) 6. main antibody of primary immune response

IgA

1. present in serum as a monomer 2. dimeric form is the predominant class of Ab in secretions (SIgA); linked at Fc ends with an extra tailpiece and a J chain 3. secretory IgA (sIgA) produced by plasma cells in mucosal epithelium and transported across the epithelium 4. More sIgA produced daily than any other Ig 5. relatively resistant to enzymatic degradation in the gut 6. two subclasses: IgA1 and IgA2: - differ in their hinge regions: > IgA1 has a long hinge with O-linked oligosaccharides > IgA2 has a very short hinge

constant heavy three (CH3)

1. stabilises Ig molecule 2. binds protein A, rheumatoid factor

IgE

1. very low concentration in plasma 2. has extra (4th) Ch domain instead of hinge region 3. mostly strong bound to mast cells and basophils via IgE Fc receptors 4. When cross-linked, triggers an acute inflammatory reaction by activation of mast cells and release of granule contents -> wanted (immune defence) and unwanted (allergy) can result

Papain

Enzyme that cleaves an antibody at the hinge region to form two Fab and one Fc fragments

FcyR

Fc gamma receptor > FcyRI - on mononuclear macrophages - binds IgG1 and IgG3 - phagocytosis, mediator release > FcyRII - a: expresses ITAM-> binds IgG2 and IgG3 only b: expresses ITIM: when cross-linked, blocks cell activation (particularly B cells) > FcyRIII - a: on monocytes, macrophages, NK cells and some T cells moderate affinity for IgG b: on neutrophils: low affinity for IgG

FcRn

Fc receptor neonatal - tranport of IgG out of the bloodstream to extracellular spaces in tissues

attachment of antibody to Fc receptors

Fc receptors can tell other cells what to do 1. a receptor for the Fc end of the Ab molecule 2. expressed on various cell types and can be up-regulated when induced (e.g. upon activation of a cytokine) 3. cross-linking of bound BCR -> subsequent activation of ITAM (or ITIM) 4. activation via FcR can trigger - phagocytosis - antibody-dependent cell-mediated cytotoxicity (ADCC) - apoptosis - mediator release - enhanced antigen presentation (to T cells)

FcyRI binds Ch2 and IgG3

Fc receptors enable effector cells to bind to and be activated by immunoglobulins bound to pathogens

what are the two types of light chains?

Kappa and Lambda - have to be on the same end -> can recognise any of the isotopes

sIgA secretion to mucosal surface

The mucosal cell synthesise a receptor for polymeric Ig that is inserted into the basal membrane. Dimeric IgA binds to this receptor and is transported via an endocytic vacuole to the apical surface. Cleavage of the receptor releases secretory IgA still attached to part of the receptor termed the secretory component

what is an antibody?

antibodies (Abs) are soluble immune system molecules made of glycoprotein *also known as immunoglobulins (Igs) or gammaglobulins (gamma-globulins) they are found as: 1. membrane-bound receptors on the surface of B cells (as BCR) 2. soluble molecules (secreted from plasma cells) present in plasma and tissue fluids (-> humoral immunity) - through the combination of heavy and light chains, and gene rearrangement for antibody assembly, millions of possible antigenic determinants can be recognised they cannot get inside cells - each B cell and proliferated clone of B cells make only one type of antibody, with UNIQUE specificity - an antibody secreted from a mature B cell has the same binding BCR -> which binds and neutralises antigens at a distance from the originating cell

Fab fragment

antigen binding sites: determine antigen specificity and attachement - can bind to two epitopes of the same at the same time

cognate recognition

binding

constant heavy two (CH2)

binds: 1. C1q (complement trigger) 2. Fc receptors 3. proteins A, rheumatoid factor

immunogenic

capable of inducing an immune response

antibodies - general divisions and structure

composed of two different kinds of polypeptides chains -> symmetrical core structure - 2 identical heavy (H) chains -> linked to each other by disulphide bonds - 2 identical light (L) chains -> linked to H chain by disulphide bond

clonal selection theory

each B cell has the membrane-bound antibody receptors that are specific for one particular antigen, and once an antibody is selected (bound) by an antigen, the cell is stimulated to produce a clone

Variable heavy and light chains Fab regions

forms antigen binding site: paratope hinge: - most variable part of CH - the longer the hinge the more flexible the antibody and more effectively it interacts with other proteins CHO (oligosaccharide units) - number varies per Ig class - provides structural stability for effector functions

Fc region

heavy chain constant region: determines effector functions including binding to: 1. Fc receptors on host cells 2. First molecules in classical complement pathway - tells us the class and function of antibody

ITIM

immunoreceptor tyrosine-based inhibitory molecule

B cell receptors (BCR)

is assembled as the B cell develops - the diversity of Ab sequences provides millions of possible antigen-recognising combinations - contact of a specific antigen to BCR on naive B cell (cognate recognition) results in B cell signalling and activation: -> differentiation -> clonal expansion -> maturation to plasma cells -> secretion of large amounts of antibody --> (T-independent and T-dependent Abs) sustained and long-lived antibody production requires T-cell help

IgG transport

it is actively transported from the blood to extracellular spaces of tissues via the FcRn transport receptor - prolonges half-life of IgG - important in placental transfer of IgG to fetus

constant light (CL)

kappa and lambda light chains

neoantigens

modifications to an existing molecule create a new antigenic determinant

naive state

most mature form

multivalent

multiple bacteria can have lots of epitopes on it

does the C terminal (end terminal) have a disulphide bond?

no

plasma cells

release antibodies as unbound molecules -> they are the most mature form of B cells

maturation of B cells -> antibody production

soluble antibody can mediate biological and protective functions by: 1. binding epitope via the paratope (Fab) 2. binding Fc region to Fc receptor on target cells

haptens

some molecules are too small have molecular weight too low to elicit an adaptive immune response by themselves - the smaller molecule, which then is immunogenic because it is bound to the carrier molecules, is a hapten

what are the five different types of antibodies?

the heavy chain is constant 1. IgM 2. IgG ( four subclasses) 3. IgA (2 subclasses) 4. IgD 5. IgE

opsonisation

the process of coating particles to promote phagocytosis (like putting sprinkles on a cupcake)

opsonins

the substances that perform this function e.g. antibodies, complement

Antibodies - principle functions related to structure

they are flexible because of the disulphide bonds -> beneficial to stretching out to different epitopes - recognise and bind extracellular antigens (Ag) - promote the killing and/or removal of Ab-Ag complex through the activation of effector functions - composed of Fab fragment (antigen binding site and a light chain) and a Fc region (heavy chain), which are separated by a hinge region

Hypervariable regions/ Complementary determining regions (CDRs)

tips of for on antibody; unique to each antigen-specific antibody = Hv1, Hv2, Hv3 for each VL and VH - closely involved in antigen binding -> when folded, creates a unique complementary site for epitope binding - allows distinct recognition between small differences in chemical structure - supported by four beta-sheet framework regions - allows for antibody diversity

what is the job of the carbohydrate in the antibody structure?

to neutralise the overall charge and stabilise the antibody

antibody-dependent cell-mediated cytotoxicity (ADCC)

when cells with FcyRIII (CD16) eg eosinophils and natural killer (NK) cells encounter cells coating with clusters of IgG antibody, they rapidly kill the target cell via engaging FcyRIII with the Fc - ADCC is another way in which NK cells can contribute to host defense

constant heavy one (CH1)

activated C3b may attach here to solubilise aggregated Ig

Ig isotypes

- 5 classes (isotypes) of antibody -> each class has a different function in the immune response - structure of heavy chain constant regions determine class and subclass - differ in number of domains, length of hinge region, disulphide bonds, charge, amino acid sequence, carbohydrate content - naive B cells express IgM (primary immune response) - other classes formed by 'class switching' with maturation of the adaptive immune response (requires T cells)

B cell receptor complex

- all classes of Ig can be membrane bound (after class/isotype switching) -> B cell receptor complex (BCR) - associated with Igalpha and Igbeta units with short intracellular tail for cell signalling --> activation -> ITAM: immunoreceptor Tyrosine Activation Motif => phosphorylated -> turned on -> gets cell to undergo division -> 'on switch' - A 'second signal' is also required to activate the B cell -> engagement of B cell co-receptor - when BCR has bound a multi-valent antigen (cross-linking), and co-receptor is engaged, phosphrylation of tyrosine kinases in ITAM initiates intracellular signalling for B cell differentiation and proliferation

Heavy chains

- amino acids at the N sequence are unique for each each antibody --> VH (heavy chain variable region) - C terminal characteristic sequence for each Ig class -> CH (heavy chain constant region)

light chains

- amino acids at the N terminal are unique for each antibody -> VL (light chain variable region) - amino acids at the C terminal are constant for kappa or lambda --> CL (light chain constant region)

discontinuous epitopes

- can change but won't be recognised -> if lost a part it could be less immunogenic - amino acids fro different parts of a polypeptide chain are brought together with protein folding - recognised by antibodies only when in their folded conformational shape -> if opened up they wouldn't be recognised

Intracellular signalling

- cross-linking BCR activates gene transcription -> survival -> differentiation -> proliferation

Fab conformational shapes

- unique amino acid sequences create unique antibody sites via CDRs for antigen/epitope -> conformational compatibility with epitope

what are antigens

a molecule that is specifically recognised by B cell or T cell receptors - a molecule that initiates an adaptive immune response - can be: a protein, polysaccharide, lipid, nucleic acid, small chemicals, metabolites, pathogen/microbe, toxin, self-antigen

what is the most immunogenic thing in our bodies?

a protein

what is an epitpoe

a restricted part of the antigen to which an antibody binds - antibodies are specific for an epitope, not the whole antigen - antigens can have: 1. multiple different epitopes or 2. multiple identical repeated epitopes (multi-valent/ polyvalent) - antibodies have at lest two different binding sites - antibodies bind to epitopes

linear epitope

a single segment of continuous sub-units - doesn't require folding for recognition (primary structure) - T cells can recognise linear epitopes