Humoral immunity Flashcards
what are the antigen-dependent responses of B cells?
occurs in secondary lymphoid tissue:
- activated by antigen divide and differentiate into plasma cells secreting soluble antibody.
- Affinity maturation and class switching may also occur via AID gene
Soluble antibodies mediate humoral immunity
what are the antigen-independent responses of B cells?
occurs in bone marrow
- Antibody genes undergo rearrangement; “naïve” B cells expressing membrane IgM +/- IgD are generated
how are immunoglobulins classified?
- by their different constant region amino acid sequence of their heavy chains
- Antibodies of different classes act in distinct location and have distinct effector functions
what is the structure of IgG?
- monomeric, m.wt 150kDa
- long hinge region, carbohydrate helps antibody be soluble
- 2 carbohydrates linked to CH2 domains to keep them apart
when is IgG most prevalent?
in secondary/memory responses
what is the difference between the primary and secondary immune response?
First time we encounter antigen, we make low amounts of IgM and IgG – primary response
Second time we encounter antigen, we make some IgM and lots of IgG – secondary/memory response
why is IgM important in primary immune responses?
IgM antibodies usually low affinity but high avidity so can bind lots of antigen at once
other classes tend to be higher affinity and bind more tightly and specifically to antigen (due to somatic hypermutation and affinity maturation)
what facilitates the transition from primary to secondary immune response?
Facilitated by class/isotype switching:
- IgM –> IgG, IgA or IgE
- T cell help (cytokines) and AID required
what are the subclasses of IgG? how are they classified?
subclasses: IgG1 (most common), IgG2, IgG3, IgG4 (least common)
- differ mainly in length and number of disulphides of hinge region
- differ in sequence in constant region of heavy chain, in the hinge region
-IgG1 and IgG3 are the most active and have the longest hinge region
what are the main functions of IgG?
- can activate complement
- binds Fc receptors on phagocytes and NK cells
- crosses placenta (binds FcRn on trophoblast) – occurs in third trimester, so premature babies do not have this protection
- long serum half-life (20-24 days) – useful for the memory response and newborns
why are IgG1 and IgG3 the more active IgG subclasses?
IgG1 and IgG3 are more active due to their amino acid composition, and that their long hinge region separates the functions of the antibody – separates Fc from Fab arms – good for innate interaction
what is FcRn?
The neonatal receptor for IgG:
- Present on trophoblast, permitting transfer of maternal IgG antibodies
- protects foetus and newborn
- also present on neonatal gut – IgG can be transferred through breast milk
FcRn is also present in adults in gut, liver and endothelial cells – binds and recycles IgG, preventing excretion (improves half-life) – prevents breakdown of IgG by sequestering it
what is the structure of IgM?
- pentamer (5 antibody subunits + J chain holds together)
- m.wt. 970,000 d, so usually serum-restricted
- no defined hinge region – has an extra pair of domains instead
- ‘functional hinge’ – some flexibility
what are the key properties of IgM?
- low affinity, but high AVIDITY (can bind up to 10 antigens)
- high valency (deca-/pentavalent) = good agglutinator of particulate antigen
what are the functions of IgM?
- can activate complement very efficiently – most potent complement activator
- important in primary antibody responses
what is the structure of IgA?
- monomer when in serum, dimer in secretions e.g. milk, tears, saliva
- secretions and at mucosal surfaces: secretory IgA = IgA dimer + J chain + secretory component wrapped around Fc regions
- Secretory component protects IgA from proteolysis
what are the subclasses of IgA?
subclasses: IgA1 and IgA2 (primates) – differ in hinge region
- evolved to deal with proteases
- IgA1 is resistant to bacterial proteases, while IgA2 is resistant to self-proteases
what are the functions of IgA?
- high valency when in secretions (bind 4 antigens at once)
- rapid catabolism – broken down quickly
- does not activate complement
- monomer, but not secretory IgA, binds Fc receptors on phagocytes
how is IgA moved from lymphoid tissue to secretions?
Specialised transport mechanism
- plasma cells in submucosa make IgA, and this must be transported to lumen
what is the process of the specialised transport mechanism of IgA?
Poly-Ig receptor – IgA binds to this for transport
- binds polymeric IgA/IgM
- member of immunoglobulin gene superfamily
IgA is internalised into a vesicle, and traverses the cell to be released on the other side – transcytosis
- During this process, IgA is cleaved to form its secretory form
- Transport system allows secretion of IgA (and IgM) into lumen
*- bacteria that penetrate mucosa can be transported back to lumen
how much IgA does mucosal lymphoid tissue produce? why?
produces 5g IgA per day because:
- IgA has high valency – good for agglutination
- Secretory component can interact with bacteria non-specifically
-IgA has a passive role in preventing adhesion/infection of pathogens - IgA coats pathogens and prevents them adhering to cells – immune exclusion
which immunoglobulin is important for immune exclusion?
IgA = coats pathogens to prevent them from adhering to cells
what is the structure of IgD?
- monomeric, m.wt 184,000
- extended hinge is glycosylated – flexible molecule so can bind antigen at different distances apart
- present as antigen receptor on many B lymphocytes, together with IgM
what is known about IgD?
- <1% serum Ig
- present as antigen receptor on many B lymphocytes, together with IgM
- Produced by B cells/plasma cells in upper respiratory tract; interacts with receptors on basophils, inducing antimicrobial, inflammatory and B cell stimulatory factors
function is unknown
what is the structure of IgE?
- monomer
- m.wt. 190,000 d
- no defined hinge – functional hinge (extra pair of domains) which confers flexibility
why are there low traces of IgE in serum?
- trace in serum (0.0003% Ig) – tiny amounts in serum
- this is because it binds with high affinity to FcR on mast cells and basophils
what is the function of IgE?
- important in allergy
- role in immune defence against large extracellular parasites e.g. helminths
- acts as a backup to IgA – provokes severe inflammatory response
how are antibodies flexible adaptors?
They have dual role (2 functions):
- Fab arms – bind antigen with high affinity
- Fc effector arms – trigger elimination by activating the innate immune responses
what is the role of the Fab arms?
Fab arms: Label pathogens
- specific binding/multivalency
- antibodies are at least divalent - binds 2 antigen
what are the functions of the Fab arms of the different Ig classes?
- Immobilise pathogens (IgM) – can stop bacteria moving by binding to flagella
- Agglutinate particles e.g. bacteria (IgM, IgA)
- Form “immune complexes” with soluble antigen - crosslinking
- Block binding of pathogens to host cells (IgG, IgA) e.g. antibodies to bacterial adhesins or viral receptors
- Neutralize toxins e.g. tetanus, diphtheria, cholera (IgG, IgA)
what is the difference between affinity and avidity?
Affinity - binding of one Fab arm to an antigen
avidity - binding of a whole antibody molecule to antigen
what is the role of the Fc regions of antibody?
Fc regions: Invoke destruction of labelled pathogens (interact with innate immunity)
what are the functions of the Fc effector regions of the different Ig classes?
- Activate complement (IgM, IgG)
- Bind Fc receptors on leukocyte surfaces (IgG, IgA, IgE)
what do Fc effector functions depend on?
- Site and type of infection – e.g. if in gut, IgA is needed, if parasite then IgE
- Stage of the immune response (primary (IgM) or secondary (IgG))
how is antibody involved in the classical activation of complement?
Requires antibody to bind to antigen, C1, C2 and C4
- C1 is made up of 3 associated proteins: C1q +C1r + C1s
- C1q r2s2
- C1s and C1r are serine proteases when activated – cleave proteins
C1q must interact with 2 adjacent Fc regions on surface of pathogen for activation to occur
- Explains why IgM is the best activator of complement
how is IgM the most potent activator of complement?
- When IgM binds antigen, it undergoes a conformational change
- Fab arms bend via hinge region to form a stable crab-like structure which can interact with complement
more likely that one IgM will bind to antigen than 2 IgG molecules adjacent to one another
which IgG subclasses are the more potent activators of complement?
For complement activation, IgG3 > IgG1 > IgG2
(IgG4 doesn’t activate complement)
what is the step-by-step process of the classical pathway of complement activation?
- When antibody binds to antigen, it interacts with C1q
- This causes conformational change in C1q, activating serine protease C1r
- When C1r is activated, it cleaves C1s to activate its protease function
- C1s cleaves C4 and C2 to form C3 convertase (C4b+C2a)
- C3 convertase cleaves C3 into C3a and C3b to generate C5 convertase (C4b+C2a+C3b)
- C5 convertase can cleave C5 to C5a and C5b
- C5b induces formation of membrane attack complex
what antibodies interact with Fc receptors on phagocytes?
IgG (IgG1=IgG3 < IgG4)
IgA
what does antibody binding to Fc receptors on phagocytes induce?
- uptake of immune complexes (soluble antigen + antibody) to present antigen to T cells
- OPSONIZATION - phagocytosis and destruction of antibody-coated pathogens)
- if a pathogen is too big to be phagocytosed, frustrated phagocytosis occurs where contents of lysosomes are secreted onto surface of the pathogen surface
what antibodies interact with Fc receptors on NK cells?
IgG (IgG1=IgG3)
what does antibody binding to Fc receptors on NK cells induce?
Mediate antibody-dependent cell-mediated cytotoxicity (ADCC):
- When cell is infected, it displays viral proteins on its surface
- NK cell can recognise antibody bound to the viral proteins
- Occurs through binding of IgG-coated targets cells to Fc-gamma receptor
- NK cells release enzymes and perforin from cytoplasmic granules and secrete granzymes to trigger apoptosis
which antibody binds to Fc receptors on mast cells?
IgE
what does antibody binding to Fc receptors on mast cells induce?
- Mediate allergy/defence against large parasites
- Mast cells are found under mucosal surfaces and secrete inflammatory mediators and cytokines
- Strong inflammatory response
- IgE binds with high affinity to receptors on mast cells and triggers immediate hypersensitivity in response to allergen
- Mast cells release inflammatory mediators via degranulation
what do B cells require to respond to pathogen?
T cell help
- thymus-dependent
what are thymus-independent antigens?
e.g. bacterial polysaccharides
- Lots of repeating epitopes that B cells can recognise and trigger differentiation
- Induce a rapid response and production of IgM antibodies only
- Memory cells are not generated and no affinity maturation
what are thymus-dependent antigens?
e.g. peptides
- Require T cells for differentiation of B cells into plasma cells.
- Long-lived memory B cells may also be generated.
- Responses can involve somatic hypermutation and class switching
