humoral immune response

• molecular mast ca 150kDA (for IgG)

• only preset in vertebrates

• humoral component of the adaptive immune system

  • => secreted antibodies against immune system

• produced by diffentiated b cells (plasma cells)

• in fluid

• ca 25kDa each

• 2 identical chains

• Either k- lamda-chain (no difference in function)

• constant and variable domains

  • CL / VL

  • VL with AG binding - paratope

  • CL epitope

• ca 50 kDa

• 2 identical chains

• constand and variable domains

  • one VL, different number of CL depending on AB domains

• 5 main classes/isotypes of HC:

• IgA, IgD, IgE, IgG, IgM

  = different AB isotypes

  =different functions

Fab region

• responsible for AG binding

• composition CH1, VH, CL1, VL

• paratope at N-terminut - binds to AB

• VH and VL from the variable regions

• epitope

FC region

• composition 2-3 constand domains (depending on Ig isotype)

• Guides the effector functions of Ig isotypes

• Binds to Fc receptors, complement proteins -> opsonization, degradation of mast cells etc.

Three loops of the VH and VL from the AG binding pocket (= paratope)

the paratope (lock) interacts with the epitope (key) of the respective AG

Epitope - thing what is recognised by AB - Paratope binds to epitope

Loop regions form the AGbinding pocket - where epitope would be bound

IgA, IgD, IgE, IgG, IgM

different heavy chains: alpha (igA), sigma (IgD), epsilon (igE), gamma (IgG), µ (IgM)

different structure, properties and effector functions

B cell activation by AG and TH cell -> AB secretion by plamsa cells -> 3 ways

1. Neutralization - AB prevents bacterial adherence - bind of pathogen -> prevents interfering

2. opsonization - AB promotes phagocytosis - mark by phagocytosis

3. Complement activation - AB activates complement, which enhances opsonization and lyses some bacti -> complement cascade = activation - direct lysis opsonisation

   -> classical pathway

IgA - Main AB in the mucosa (lung, intestine, urogenital tract) -> external protection

IgD - AG receptor on mature B cells. -function unknown (IgM - surface, immuno globuline)

IgG - mainly AB in body liquids -> internal protection (highest presentation of Ig)

IgM - AG receptor on B cells, First produced isotype during a primary immune response -> primary protection, strong complement-activating isotype - innate immunity, pentamere = increasing binding by 10 binding sites = adaptive immune response

IgE - activates effector cells (mast cells and eosinophils) during a helminth infection; responsible for allergic reactions (type 1 hypersensitivity: asthma, allergic rhinitis

IgM - MW -970kDa; IgM forms pentamers; activates the classical complement pathwy; all B cells are IgM+ in early stages; heavy chain µ. primary! the higher, the fresher the infectionn

IgA - MW -160kDa; IgA 1&2 can be present in monomeric and dimeric fashion; sIgA protects mucosal surfaces; heavy chain a

IgD - MW -180 kDa; acts as AG receptor on mature B cells; heavy chain sigma

IgG - MW - 150 kDa; four IgG subclasses (IgG1-4); 1 is present in highest concentration with long half-life (-20 d); activates the classical complement pathway; placenta transfer - for almost 1 year; heavy chain y

IgE - MW - 188 kDa; very low plasma concentration; IgE binds to mast cells with high affinitc and mediated allergic responses; havy chain epsilon

affinity vs avidity

• Agglutination of bactieria

• neutralisation - virus partical

• opsonization - mark them

• complement activation

• AB-dependent cell-mediated cytotoxicity (ADCC)

• Activation of neutrophils, basophils and mast cells - particially IgE

  • control of B cell responses and AB production by binding to inhibitory Fc receptors (feedback inhibition) -> thereby inhibit own production -> if enough AB are there.

• feedback inhibition

cross-linking of neighboring bacteria (particularly by IgM, but also IgG)

=> facilitates uptake by phagocytosis (macrophages, neutrophils)

• igM leads to agglutination (best, but also IgG can do it and form a net) - agglutination-immune-complex

• proble - complexes lead to thrombosis

• big problem in the kidneys

• binding of AB to toxins => neutralization

• binding of AB to bacterial of viral surfaces => blockade of pathogen attachement to host cell surface

• Particularly IgG (internal protection) and IgA (External protection) are capable of pathogen neutralization

uptake of pathogen

• binding of ABs to pathogen surface => marking for uptake by phagocytes

• Fc receptor expressed by phagocytes recognize constant region of Abs => endocytosis

• particularly IgM (first reaction) & IgG

• partially overlapping

FCe R1 - high affinity IgE receptor for allergies

FCy R3

FCy R2-B1 - inhibitory FC receptor on B cells, if binding - inhibition of further production of ABs - FEEDBACK inhibition!

epsilon binging to IgE

alpha binding to IgA

gamma inding to IgG

• binding of Abs to pathogen surfaces => complement activation

• recruitment of phagocytes (via complemt receptors)

• formation membrane-attack complex => lysis of the pathogen (direct or opsonisation)

• IgM & IgG

AB binds AG (viral proteins) on the surface of target cells (Virus infected cells, NK cann kill via missing self) —> Fc receptor on NK cells recognize bound AB (as soon as they bind -> activated and kill by apotosis)—> Cross-linking of Fc receptors signals the NK cell to kill the target cell —> target cell dies by apoptosis

• ADCC antibody dependent cell mediated cytotoxicity

• binding of AB to Fc receptors on NK cells => release of cytotoxic granules

• mainly mediated by IgG

bone marrow: rearrangement of gene segments encoding for the variable region. -> maturation of b cells

germinal center: Class switch -> as soon as they are activated (in the lymph node) - irreverisble - classes of IG. fragments on DNA level - cut ut! recombination event. Change of AG isotype

• delta cross out

why IgM is always the first - µ havy chain - most 5’!

somatic bc not in germ cells! in somatic cells recombination is rare

IL5 - key eosinophil activating cytokine - IgA- eosinophils - helminth infection - in external tissue (gut etc)

IFN - key cytokine for TH1 cells - for diff IgG subclasses

• AG presentation by B to T cell + costimulation

• T cell activation and up-regulation of CD40 ligand expression

• B cell activation by interaction between CD40/CD40L and cytokines

• Clonal expansion; class switch; somatic hypermutation; affinity maturation

tcell help - provides help mediated by CD40Ligand and CD40 on the Bcell - costimulatory signal for Bcell activation. Tcell help for b cell = Co-stimulation

IL-4 classical B cell activating cytokine + a bit IL-6 and contribute T cell help

active B cell ->somatic hypermutation of immuniglobulin V regions in rapidly proliferating GC B cells ->2 ways (IL4 and IL6) :

1. GC B cell with low-affinity surface immunoglobulin -> BCR is not cross-linked and B cell cant present AG to T cell —> B cell dies by apoptosis (B cell loses signal -> apoptosis) if there is not TH2 help anymore, AB with lower affinity -> dead by apoptosis

2. GC B cell with high-affinity surface immunoglobulin (B cell goes around randome code muation. -> endocing for AG binding) -> Tcell hel and BCR cross-linking sustain B cell proliferation and maturation (B cell will survive) ->1. Memory B cell (long living) and 2. plasma cell. ->what is induced by vaccination

exceptionally high mutation rate - only in DNA regions encoding for variable regions - hypERmutations

CDR complementary determining region - loop regions

black line - single point mutations - most in loop regions - which are responsible for AG binding (at least the ones which are found)

affinity maturtion - selection process

somatic hypermutation - point mutation in this region

primary focus - bcell becomes clone

how to descriminate naive b cell from plasma cell bc resting b cell — high level of surface Ig - naive bcells dont produce ABs

resting b cell can proliferate,…

plamsa cells cant bc they have already done it and undergone hypermutation

phenotypicalls descrimination. plasma cells also mcuh larger with much more ER and Golgi

B cell develop in the bone marrow -> mature B cells travel to the lymph node via the bloodstream and leave via the efferent lymph -> B cells that encounter AG form primary foci from which proliferating cells migrtae to the primary follicle, forming a secondary follicle with a germinal center -> plasma cells migrate to the medullary cords or leave via the efferent lymphatics -> plasma cells migrate to the bone marrow

lymphocytes and lymph return to blood via mesenteric lymph nodes and thoracic duct; naive lymphocytes enter musocal tissue from blood => AGs from infectious agents taken into submucosal lymphoid tissues

effector lymphocytes disseminate to mucosal surface in lung, tonsil, adenoids, gut and genitourinary tract

ducuts euracicus - connection between lymphtic and blood system

M cells are interspersed between enterocytes and in close contract with subepithelial lymphocytes and DCs -> M cells take up AGs from the gut lumen by endocytosis (trans cytosis) ->AGs are released beneath M cells and taken up by AG-presenting DCs

sample AG and transport it from the lumen to the lamina (downside of epithelium)

Pias patches / MELT/MALT / GALT -> lymph nodes

how come that IgA is on the outside? -> because IgA gets transported via transcytosis

Polymeric IgA is transported into the gut lumen throuh epithelial cells at the base of the crypts -> Polymeric IgA binds to the mucus layer overlying the gut epithelium -> IgA in the gut neutralizes pathogens and their toxins

IgA present on epithelium, dimer consisting of 2 molecules

Binding of IgA to receptor on basolateral face of epithelial cell -> Endocytosis (and then transcytosis through the cell to the lumen) -> Transport to apical face of epithelial cell -> release of IgA dimer at apical face of epithelial cell

The colon is colonized by large numbers of commensal bacteria -> Antibiotics kill many of these commensal bacteria -> (colonisation) Clostridium difficile gains a foothold and produces toxins that cause mucosal injury -> neutrophils and red blood cells leak into gut between injured epithelial cells -> why Antibiotics shouldn’t be suscribes without reason