Adaptive immunity - Humoral immunity

Question 1

Recap the B lymphocyte generation and response

Answer 1

• aquire antigen receptors in the bone marrow → independent of antigen
  
  • somatic recombination: naive B cells not naive any more → express membrane IgM mainly
• now in the bod they will encounter antigen that they can recognise thorugh B cel lreceptors in 2ndry lymphoid tissue → divide and differentiate into plasma cells and secrete soluble antibody. undergo affinity maturation and class switching may also occur.
• Soluble antibodies mediate humoral immunity

Question 2

How many immunoglobulin classes are there? what decides which class they are and what are they called

Answer 2

• 5 classes of antibody that differ in the constant region of the heavy chains:
• different antibody classes act in distinct locations and have distinct effector functions
• so different classses have different benifits for different types of infections/places of infections etc.
  IgG (monomer), IgA (monomer/dimer), IgD (monomer), IgM (pentamer), IgE (monomer)

Question 3

Describe the structure of IgG

Answer 3

• heavy chain gamma
• Domain nomenclature
  
  • variable regions = Vl and Vh (for light and heavy chain)
  • constant regions = CH
  • CH1, 2 and 3 domains for heavy chain
  • IgG has a long hinge region
  • green = carbohydrate, helps antibody be soluble, also can be important for function
  • 2 carbohydrate molecules keep CH2 domains slightly apart.
• always occurs as a monomer
  
  • monomeric, m.wt 150,000

Question 4

Where is IgG found, what is it important in? Its characteristics

Answer 4

• main antibody in tissues and blood
• important in secondary or “memory” responses
• can activate complement
• binds Fc receptors on phagocytes and NK cells
• crosses placenta (binds FcRn on trophoblast)
• long serum half life (20-24 days)

Question 5

Describe the 4 subclasses of IgG

Answer 5

• 4 subclasses that differ in the constnat heavy chain
  
  • most of the differences reside in the hinge region (length and amino acids, and no. of disulfide bonds linking heavy chains together)
  • named to their relative abundance in the blood
  • IgG1 and IgG3 tend to be the most biologically active
    
    • also have the longest hinge region (REWATCH ~35 MIN)
    • why are they more active?
    • some of it odwn to the particualr aa in the Fc region in antibodies,
      
      • may be also down to the fact that a long hinge region is good for seperateing the 2 function of antibodies: find antigen + interact with innate elements (e.g. Fc receptors and complement)
      • a longer hinge region will facilitate interaction with innate components better
      • better at activating complement and binding to receptors on phagocytes and NK cells

Question 6

What can IgG do? i.e. what are its functions

Answer 6

• Can activate complement (via. classical pathway)
• most important function: ability to interact with receptors on innate immune cells (e.g. phagocytes and NK cells)through Fc region of antibody (Fc receptors)
• Crosses placenta (binds FcRn on trophoblast) → allows transfer of IgG maternal crossing into placenta
  
  • tends to occur in third trimester
  • premature born more succeptible to infections because they havne’t gotten mmaternal IgG
  • N = neonatal
• Long serum half life (20-24 days)
  
  • important in secondary response, as they will stay around for a long time
  • also important for new born babies

Question 7

Describe FcRn, the neonatal receptor for IgG

Answer 7

• IgA (mucusoal secretions)
• IgG can bind to the trophoblast
  
  • allows transfer of maternal IgG antibodies to the newborn
  • also present on the neonatal gut (so through breast feeding, IgG can be transferred to baby)
• FcRn also present in adults in various tissues, allowing the tissues to bind IgG and recycle it
  
  • this receptor important for giving IgG its long half life
  • helps stop it being broken down by sequestering it
  • therefor IgG FcRn interaction important when making antibodies in the lab for increasing half life

Question 8

Describe primary and secondary antibody responses- describe how class switching occurs

Answer 8

Primary response

• first encounter → always IgM
• few days later when B cells in lymphoid tissue switch class → IgG

Secondary response

• antigen staying around for long or secondary exposure happens → we see secondary response
  
  • IgM produced at same level, but IgG highly increased
  • class switching (isotype used in text books sometimes)

to get class switching you need T helper cell help, that release cytokines that switch on the AID (activation induced cytesine deaminase) gene which is required for class switching andsomatic hypermutation also for affinity maturation

primary responses tend to be low affinty, because cells haven’t undergone affinty maturation yet

IgG/IgA/IgE better affinty because they have undergone affinity maturation/hypermutation?
- Class/isotype switching:
- IgM → IgG, IgA or IgE
- T cell help (cytokines) and AID required
- IgM antibodies usually low affinity, other classes tend to be higher affinity (due to somatic hypermutation and affinity maturation).

Question 9

Describe key facts about IgM

Answer 9

• pentamer (5 antibody subunits + J chain)
• m.wt. 970,000 d
• usually serum-restricted
• no defined hinge region
• low affinity, but high AVIDITY
• high valency (deca- /pentavalent) → good agglutinator of particulate antigen
• can activate complement very efficiently
• important in primary antibody responses

Question 10

Describe what avidity and affinity means

Answer 10

• affinity = ability and strength of 1 fab arm to bind to antigen
• avididity = ability of all of the Fab arms in an antibody molecule to bind to antigen simultanesouly

Question 11

Describe the structure of IgM and where it is usually found

Answer 11

• When in serum, its found as a pentamer
  
  • pentamer forms in plasma cells and formation facilitated by an extra peptide called the J chain (J for joining) which ensure the pentamer forms correctly
  • 5 y shape antibody units joined by disulfide linkages and J chain
  • Its a huge molecule → therefor its usually only found in the blood stream

Question 12

Compare IgM to IgG

Answer 12

• IgM doesn’t really have a hinge, but has an extra domain
• i.e. no defined hinge
• the IgM does have a functional hinge that is somewhat flexible but in a different region of the antibody Fab arms
• as IgM is found mainly before somatic hyper mutation they tend to be lower affinity, but because of so many binding sites ( can bind up to 10, usually 5 because of steric hinderance) it has a high avidity
  
  • affinity = ability and strength of 1 fab arm to bind to antigen
  • avididity = ability of all of the Fab arms in an antibody molecule to bind to antigen simultanesouly
• because of high valency its good at clumping together (agglutinator) of particulate antigen
• can activate complement very efficiently
  
  • very important in primary responses
• important in primary antibody responses

Question 13

In what 2 forms can IgA be found in? and why/where + describe IgA structure

Answer 13

• In blood: monomer
  
  • do not know what it does in monomer
• In secretions: dimer
  
  • tears, saliva, mucose secretions that coat the mucosal surfaces
  • this is where most infections occur so its important
  • can be found as other polymers
  • dimer needs a J chain to form properly
  • has a secretory component (wrapped around the Fc regions of the dimer)
    
    • helps protect IgA against proteolysis, so e.g. in the gut its important for protection against degradation
    • Secretory component can also bind bacteria itsself in a fairly non-specific way
    • secretory component is a member of the immunoglublin super gene family
    • made up of 5 immunoglobulin domains that interact with Fc region of the dimer

Question 14

Describe the 2 subclasses of IgA in primates

Answer 14

• subclasses: IgA1 and IgA2 (primates)
  
  • differ in hinge region
  • IgA1 most abundant
    
    • better at being resistant to bacterial proteases
  • IgA2
    
    • better resistance to our own proteases, so found mainly in the gut

Question 15

How is secretory IgA found?

Answer 15

in secretions and at mucosal surfaces: secretory IgA = IgA dimer + J chain + secretory component

Question 16

Describe key features of IgA

Answer 16

• high valency
  
  • in secretory IgA: 4 Fab arms → can theoretically bind 4 antigens at once
  • like IgM its good at agglutination (clumping) bacteria together
• rapid catabolism
  
  • short lived???
• present in milk
  
  • important in protecting newborn against infections
  • can be taken up by newborns from mother
  • particularly important in animals like ruminants (cows, pigs, sheeps) because they don’t havep placental transfer of antibody, so they rely on IgG and IgA from theirs mothers milk to protect them against infections
• does not activate complement
• monomer, but not secretory IgA, binds Fc receptors on phagocytes
  
  • but its masked by the secretory domain, so doesn’t actually bind to phagocytes
  • however it does bind to a differnet sort of Fc receptor: one thats e.g. found on gut epithelia. which relates to how IgA gets from lymphoid tissue into where its needed in secretions →

Question 17

Describe: Specialised transport mechanism - how IgA receives its secretory component

Answer 17

as example: gut

• Underneath epithelial cells in the submucosa theres quite a lot of lymhpoid tissue surround the gut. (Payers Patches)
• in the lymphoid tissue we have plasma cells which mainly make IgA, secreted as a dimer
• to transport IgA into the lumen it binds to a receptor on the epithelial cells → whoel complex internalised → vesicle containign IgA bound to receptor traverses the cell and IgA released on the other end
• This is known as Transcytosis
• upon entering lumen, part of the IgA receptor (that was on the epithelial surface) is cleaved by proteases, some remains associated with IgA molecule which is what forms the secretory component of IgA!
• the IgA receptor is called the Poly-Ig receptor
  
  • it can bind polymers of immunoglobulins so can also bind IgM, when there’s genetic deficiencies in IgA, IgM becomes the main antibody in these mucosal secretions
  • Poly-Ig is also member of immunoglobulin gene superfamily
• If bacteria manage to penetrate the epithelium and into the submucosa → IgA can bind the bacteria and carry it back across to the lumen!
• fun fact: IgA is the antibody type most produced in the whole body. ~5g of IgA made per day!: so what’s its role?

Question 18

Describe IgA’s passive role in preventing adhesion/infection

Answer 18

• • IgA plays a passive role in pathogens adhering to body cells in the first place. Secretory component can interact with bacteria non-specifically remember!
  • e.g. in the gut you do not want an intense inflammatory response being triggered all the time as we are ingesting foreign material all the time.
  • So IgA just coats foreign antigens and stops them sticking to host cells → and are excreted.
  • known as immunic exclusion: stopping antgens getting into the body in teh first place
  • IgA can also bind to commensal bacteria but DOESN’T trigger an immune response to them

Question 19

Describe IgD

Answer 19

• monomeric, m.wt 184,000
  
  • very long extended hinge: heavily glycosylated making it heavy
• < 1% serum Ig
  
  • very low presence in serum
• Function?
• IgD gene conserved in evolution so its important but we don’t know what for, however we do know this:
  
  • it often occurs as an antigen receptor on B cells with IgM
    
    • one possibilty for its function here is that its a very flexible molecule with its extended hinge. IgM has no hinge so is constrained. Maybe on the B cell surface IgD is better at binding antigen at different distances
  • Produced by B cells/plasma cells in upper respiratory tract; interacts with receptors on basophils, inducing antimicrobial, inflammatory and B cell stimulatory factors

Question 20

Describe IgE

Answer 20

• monomer
• no defined hinge region
• has a “functional hinge”
  
  • like IgM monomers, has an extra pair of domains where you expect the hinge to be → these domains are quite flexible so the molecule can bend to some extent through this “functional” hinge
• m.wt. 190,000 d
• trace in serum (0.0003% Ig)
  
  • so present in tiny tiny amounts in blood
  • this is because → most of IgE that we make binds very tightly to receptors IgE Fc receptors on mast cells and basophils
• binds to high affinity to FcR on mast cells and basophils
• important in allergy
  
• Main role in immune defence against large extracellular parasites e.g. helminths
  
  • more important in exotic regions
  • backup to IgA e.g. if you have a tapeworm (big parasite) you need IgE to provoke a severe inflammatory response

Question 21

In general how do the immunoglobulin classes differ

Answer 21

They differ mainly in their location in the body and in their Fc effector functions (i.e. the way in which they help combat pathogens)