Adaptive Immunity: B-Cell Response

• What are the functions of B cells?

  -Protect against infection by the production of antibodies

  
  

  -Present antigen to CD4 T cells through class II MHC

  
  

  -Regulation by the production of cytokines: B cell derived cytokines are predominantly IL-10 and TNFa (IL-10 producing suppressive B cells are known as Breg)
• What does a B cell receptor comprise?

  An antibody molecule of 2 identical heavy chains & 2 identical light chains

  
  

  The antibody molecule has a transmembrane domain but no independent signalling capability

  
  

  So B cell receptor signalling is dependent on IgA and IgB

  
• What does an antibody look like?

  The heavy chains are joined together by disulphide bonds identified by the dotted oval

  
  

  The light chains are joined to the heavy chains by disulphide bonds identified by dotted circles

  
  

  The ovals that are joined together to form the heavy and light chains structures are called domains

  
• What is B cell receptor signalling dependent on?

  Ig-alpha & Ig-beta (aka CD79a and b)

  
• What do antibodies recognise on the surface of antigens?

  Epitopes

  
• What is an epitope?

  the part of an antigen molecule to which an antibody attaches itself
• What do the antibodies do when they recognise the epitopes on the surface?

  Binding occurs via the tips of the Y shapes that are highly variable between B cells
• How can an antibody bind to the antigen?

  Binding can be physical such as lock and key, or physicochemical such as hydrophilic/ or electrostatic attractions
• Can a single antigen have multiple epitopes?

  Yes, and it can be recognised by antibodies expressed by, or secreted by different B cells

  
  

  E.g in this diagram, there are two antibodies (blue and red) that recognise the two epitopes.

  
• What are the hinge, Fab & Fc portions of an antibody?

  The hinge is what allows the antibody to bend & some have a longer hinge allowing them to bend more

  The part of the antibody above the hinge is sometimes referred to as the Fab

  The part of the antibody comprising the hinge and below is sometimes referred to as the Fc &some cells, including macrophages and neutrophils, have receptors for antibody that bind the Fc (Fc receptors)
• What is the term Fc often followed by?

  The Greek letter for the class of antibody that the Fc receptor recognises

  
  

  Eg. A Fc receptor that binds to IgE is an Fce
• What are the 5 different immunoglobulin 'isotypes' or 'classes'?

  IgG, IgM, IgD, IgA & IgE

  
• What are the 4 subclasses of IgG in humans?

  IgG1, IgG2, IgG3, IgG4, numbered by their abundance in human blood so IgG1 is the most abundant subclass
• Which other Ig isotype has subclasses?

  IgA in humans has 2 subclasses that differ in their constant region sequences that are encoded by different gene segments

  
  

  IgA in blood is mostly IgA1 whereas IgA in gut can be either IgA1 or IgA2

  
  

  BUT IgA1 and IgA2 subclasses can be dimeric or monomeric . Therefore there are 4 different types of IgA.
• What property does IgA & IgM have?

  They can be polymeric

  
  

  IgM in blood is pentomeric. IgM in blood comprises of 5 units of IgM, held together by a molecule called 'J-Chain'.

  
  

  IgA can be monomeric or dimeric, When IgA is dimeric, the 2 units are help together by J chain.

  
• What are the 6 functions of antibodies?

  1. Blocking

  2.Complement fixation

  3.Opsonisation

  4.ADCC (Antibody-dependent-cellular-cytotoxicity)

  5.Agglutination

  6.Mast Cell degranulation
• What is blocking?

  Where antibodies bind to a target to prevent it binding to a receptor

  
• What is complement fixation?

  IgG and IgM can 'fix' complement when they bind to a surface

  
  

  This initiates the complement cascade that results in the production of proinflammatory molecules and formation of the membrane attack complex

  
• What is opsonisation by antibodies?

  - The binding of IgG to surface of a pathogen = makes pathogen more visible to phagocytes such as macrophages and neutrophils.

  
  

  -Bound IgG can be recognised by Fc receptors of phagocytes, which facilitates phagocytosis.

  
  

  -The antibody can be referred to as an opsonin & the antibody coated target is said to be opsonised

  
• What is ADCC?

  Antibody-dependent cellular cytotoxicity

  1. IgG Antibody binds antigens on the surfaces of target cells

  2. Antibody bound to a target cell can be recognised bynatural killer (NK)cells via their Fc-gamma receptors

  3. The NK cells with cross-linked Fc receptors cansecrete cytotoxic granulesinto the synapse

  4. resulting in the death of the target cell

  
• What is agglutination?

  Polymeric antibodies with multiple binding arms can cross link pathogen resulting in formation of a lattice.

  
  

  This can 'disarm' the pathogen
• Which immunoglobulin isotype is especially good at agglutination and give an example?

  IgA- polyclonal IgA antibodies

  
  

  -can be actively secreted at mucosal sites such as the gut, eyes and mouth and are at relatively high concentration in mucus, saliva and tears compared to other antibodies

  
  

  -IgA antibodies are transported across epithelia by secretary component that binds to the J chain
• What does IgA, therefore, have a specific role in?

  Regulating microbial populations due at least in part to its ability to coat and agglutinate bacteria
• What is mast cell degranulation?

  Exocytosis of granules from mast cells with release of mediators of inflammation and bronchoconstriction

  
• How does mast cell degranulation occur?

  - resting mast cell contains granules containing histamine and other inflammatory mediators. However, IgE is not cross linked so the granule contents will not be releases.

  
  

  -Activated mast cell contains cross linking of IgE, so mast cell will release granules resulting in an allergic reaction

  
• What is the antibody variable region & what does it comprise?

  -The tip of the antibody Y-shape is the variable region

  
  

  -The variable region of the heavy chain is comprised of 3 segments called:

  
  

  -Variable (V), Diversity (D) and Joining (J)

  
• Where are the three segments that comprise each variable region assembled from?

  From multiple alternatives during B cell development to form a unique receptor sequence.

  
• What is the maths behind BCR variability?

  -When the segments join together, there is variation in the junctions = junctional diversity

  
  

  -Also, the heavy & light chain genes rearrange independently & when these are paired, you also have additional variation

  
• What is the difference between T-dependent & T-independent B cell responses?

  -T-dependent B-cell responses require the antigen to have a protein component

  
  

  -T-independent B-cell responses depend on antigens having repeat subunit structures so that they can cross-link the B cell receptor

  
• What sequence of events result in affinity maturation?

  Affinity maturation = Generation of high affinity antibodies.

  
  

  Essentially, centrocytes re-express antibodies and if their antibody has sufficient affinity, they are able to sample antigen from follicular dendritic cells. This is important to be able to process and present it to T follicular helper cells.

  
  

  If they cannot do this, the centrocytes die.

  
  

  This help us to generate only high affinity antibodies.

  
• What does the germinal centre response generate?

  Memory cells & plasma cells