Class switching and generating diversity and memory

Light and heavy chains

• composed of repeating Ig domains

N terminal domains of both chains

• variable domains

• Remainder is constant domains

Antigen binding site

• V L and V H domain interactions

Heavy chain structures

• differ considerably

IgM heavy chain

• many disulfide bonds between Ig domains

• H chain is very n-glycosylated

N-glycans

• complex carbs

• Added to asparagine residues during folding before secretion in ER

• Large

• Hold domains apart to expose functional motif

IgG heavy chains

• different domains have different functions

• Binding complement components , and their tail receptors on different cells

IgE heavy chain

• many N glycine make it a stiff rigid model

• Good for targeting large pathogens

• Can’t cross link small targets

IgA heavy chain

• very flexible

• High valency

• Good cross linker

Class switching

• pre B cells express membrane bound IgM in bone marrow

• During maturation, IgM is also expressed

• All selected by antigen, undergo clonal selection

• Mature b CELLS SWITCH to other Ig classes while maintaining same specificity for an antigen

• Same V h domain (antigen binding component) on a different heavy chain

Segmenting h chain

• class switching

• Ig heavy chain encodes a variable domains , and all constant regions separated by introns

• Somatic recombination of DNA means introns are excised to allow the variable region to be expressed with certain domains for class switching

Alignment and deletion of domains

• genomic DNA is looped

• Allows unusual DNA recomb between switch regions using specialised proteins

• Cutting and joining of DNA leads to loop being excised and class switching occurs

• Variable heavy domain retained so same specificity retained

Expression of class switching

• rearranged gene is transcribed

• Segments encoding the variable region and other domains are fused in frame at RNA level

• Caused by excision of intron to generate the MRNA

Clonal selection theory

• antigen only activates lymphocytes already committed to the response

• Meaning it displays receptors for the antigen even if its never encountered it before

B cell receptors

• membrane bound antibodies

Clonal selection

• after infection, clones are selected by antigens based on how well the antigen and receptor fit

• Pathogen specific lymphocytes selected from pools of B and T cells

Clonal expansion

• selected clones undergo mitosis , proliferate and differentiate

• Forms effector cells

Clonal deletion

• lymphocytes that react with self antigens are destroyed

Cause of pre existing diversity in adaptive immune system

• antigen specific receptors are encoded by unusual segmented genes

• Assembled from a series of gene segments via somatic gene recombo

• Used for class switching but slight modification leads to massive pre existing diversity

Generating primary antibody diversity

• 3 antibody genes

• 2 classes of light chains, inc diversity

• All about the variable domains - encode antigen specificity

Generating diversity w 2 LC classes

• Same variable heavy chain domain can be partnered w variable domains from 2 classes

• Increases repertoire of binding site possibilities

Generating diversity with constant domains

• multiple gene segments encoding variable regions

• Can be combined w constant domains via somatic recombination

Somatic recombination

• v complex and sophisticated

• Involves selection of small pieces of diversity and joining DNA that link V and C domains

• Means 1 heavy chain and 2 light chains genes can generate 10^14 different binding sites

Affinity maturation

• antibodies improve specificity and affinity over time

Cause of affinity maturation

• accumulation of point mutations in V domains after coding sequences have been assembled from segmented genes

Location of affinity maturation

• occurs in lymph nodes

• Some of the activated b cells proliferate in lymphoid follicles , forming germinal centres

• High mutation rate at germinal centres

• Confined to gene segments encoding V domains

Somatic hypermutation

• very high mutation rate in germinal centres (proliferated B cells in lymphoid follicles)

• Confined to gene segments encoding V domains

Affinity maturation step 1

• clonal expansion

• Class switching allows pre existing diversity in antigen binding sites to be transferred to other Ig , PROVIDING xtra immune function

Affinity maturation step 2

• proliferation of some b CELLS from expanded population into germinal centres

• Undergo somatic hypermutation

• Generates antibodies w altered V domain specificity

Affinity maturation step 3

• most hypermutation clones are worse than original so wont be stim by original antigen and die

• Rarely, they’ll have higher affinity for the antigen and so proliferate

• Repeated cycle = evolution of high affinity antibody response

Affinity maturation and Darwin

• Darwinian process

• Survival by selective advantage

Antibody secondary response

• greater and more efficient

• Short lag , showing memory of immune system

• More specific and greater due to class switched antibodies that have undergone somatic hypermutation

Immunological memory

• generated by primary response some antigen stim cells multiply and differentiate into memory cells

• Can be induced to become effectors by subsequent antigen stim

• Memory cellls dont die after immune response

Effector B cells

• secrete antibody

Effector T cells

• kill infected cells

• Influence response of other cells

T cells in memory

• multiple classes of memory T cell exist

• Some carry surface markers characteristic of T h cells

• Others carry surface markers characteristic of T c cells

• They all migrate to tissue

B cells - memory

• memory b cells

• Already switched from IgM to more mature isotopes

• Circulate through secondary lymphoid compartments that contain naive B cells

Memory failure

• some pathogens avoid being remembered

• Avoids innate defences

• Dismantles adaptive defence