Antibody mediated immunity

• What do effector B cells secrete?

  Soluble forms of immunoglobulin called - Antibodies
• How are T cell receptors expressed?


  As cell-surface recognition molecules
• What type of proteins do B cells recognize?


  Native proteins
• What do T cells recognize?


  Degraded proteins bound by MHC molecules
• What are the components of T cell receptors?


  Alpha chain and beta chain
• What happens when each receptor engages with their cognitive antigen?

  B cells release their receptors as antibodies

  T cells dont
• What are the regions present in both antibodies and T cell receptors?

  • Variable region
  • Constant region
• Who produces antibodies?

  B cells
• What is the process of B cell activation?


  • Requires 2 signals:

  1. Engagement of the B cell receptor
  2. Help from T cells
• What is the role of T cells in B cell activation?


  T cells provide help for B cell activation
• What do thymus-independent (TD) antigens do?


  Stimulate B cells to make antibodiesin the absence of T cells
• What is the significance of antigen recognition in T cells?


  • T cells recognize complexes of degraded proteins
  • This recognition is crucial for immune response
• how T and B cells see the virus


  • both see the virus but different parts of it
  • These parts re called the antigenic epitote
  • B cells -recognise native state
  • T cells - recognised degraded proteins bound to MHC- give antigen to T cells
• B cell producing progeny cells process

  binding of a foreign antigen to membrane bound Ig of B cell leads to B cell activation and proliferation. B cell gives rise to progeny B cells that differentiate into plasma cells- secrete antibody o the same specificty as the membrane bound immunoglobulin.

  and memory cells for future encounters
• T and B cell collaboration during B cell activation 


  • first signal - recognition of antigen by B cell Receptor (membrane bound Ig)
  • 2nd signal- delivered by helper T cells that recognises degraded fragments of antigen as peptides bound to MHC class II molecules on the B cells surface.
  • B cell takes virus and chop it up to give to T cells via MHC.
  • Helper T cells then bind with CD40 ligands and T cell give cytokines to B cells to activate second signal
• process of B cell activation 


  • antibody binds with antigen and they undergo endocytosis, fragment broken down into small molecules - peptides.
  • peptide is given to MHC class II
  • bound peptides transported to cell surface by MHC class II.
  • on the surface the MHC class II gives the antigen to Th2 a particular T cell type. -
  • Th2 provides second signal for B cell - cytokines production due to CD40 and CD40L interaction.
  • all signals lead to B cell proliferation which differentiate into antibody secreting plasma cells
• Immune response to 1st challenge 


  • primary immune response
  • IgM produced
  • First time exposed to this antigen
• immune response to 2nd boost of previously challenged antigen 


  • upon repeated exposure - CLASS switch occurs to IgG. as it is a high affinity antibody with neutralising capabilities and longer lasting response
  • high affinity response that occurs quicker and longer lasting
• immune response to a new challenge at the same time as repeated exposure to another antigen 


  • response to new challenge is the primary immune response so IgM produced.
  • class switch to IgG for repeated exposure.
  • can distinguish between antigens.
• B cell differentiation


  T cell dependent Ab responses require activation of B cells by helper T cells that respond to the same antigen - linked recognition

  • the secretion of IL-4, IL-5 and IL-6 drives the proliferation and differentiation of B cells into antibody secreting plasma cells.
  • although T and B cells respond to the same antigen the epitopes are not the same
• what is linked recognition

  • B cell binds virus through viral coat protein as it recognises part of epitope in native state.
  • virus is internalized and degraded into small parts.
  • the inside of the virus - internal proteins - are presented to the T cell which activates the B cells by releasing produced cytokines - IL-4, IL-5, IL-6.
  • now activated B cell becomes antibody secreting plasma cells and can fight against viral coated protein.
• generation of high affinity antibodies - points


  IgM- low affinity interaction - IgG - high affinity interaction

  IgM- C‘activator - IgG- C’activator

  IgM- good agglutination - IgG- good neutraliser

  IgM- can activate complement and. Change to high affinity antibody - IgG - FcR mediated phagocytosis

  
• How do we go from IgM to IgG in primary and secondary immune response 


  • somatic mutation
  • Mechanism for introducing mutations into variable regions of activated B cells- antigen driven -thereby increasing antibody affinity.
  • Class switching due to this
  • Main purpose is that each has a special different biological function
• generation of high affinity antibodies


  • variable domain changes to produce a better fit
  • constant domain changes to have a different function
  • both due to somatic mutation
  • isotype class switching
  • difference between IgM and IgG in the variable region is there are lots of mutations
• antibody production in primary immune response.


  • production is a sign antigen is present - triggers immune response
• what drives the class switch 


  Helper T cells

  • Abs are bifunctional proteins.
  • with an immune response the effector arm (Class) of an antibody needs to change in order to mount a serious/ different challenge to harmful pathogens.
  • T cells provide the necessary cytokines needed for B cell to undergo class switching
• Cytokine regulating Ig isotype expression 


  • either induce or inhibit production of certain isotopes, most of the inhibitory effect is the result of directed switching to a different isotype.
• IL-4 inhibits and induces which Ig?


  inhibits- IgM, IgG3, IgG2a

  induces- IgG1, (IgE- depending on environment)
• IFN-y inhibits and induces which Ig?

  inhibits - IgM, IgG1,IgE

  Induces - IgG3, IgG2a
• TGF-b inhibits and induces which Ig?

  inhibits - IgM, IgG3

  induces- IgG2b, IgA
• interaction between B and T cells occur where


  in specialised tissues in lymph nodes

  • germinal centre
• germinal centre function 


  where T and B cells meet.

  allow B cells to differentiate into either antibody secreting plasma cellls or memory B cells - induce a process known as somatic hypermutation
• T -independent antigens


  • for thymus independent antigens the second signal can be delivered by the antigen itself - no T cell required.
  • this occurs in high levels of infections
  • system goes into overdrive and bypasses T cell activation
• T - independent antigens points


  • signal sufficient to induce proliferation and antibody secretion by B cells in T cell help absence
  • due to special property of some bacterial polysaccharides, polymeric proteins or lipopolysaccharides.
  • these agents activate a large number of B cells and are known as B cell mitogens.
  • downside is you only ever make IgM antibodies
• TD antigen properties


  • response in infants-
  • primes T cells
  • examples of antigen- diphtheria toxin, viral hemagglutinin
• TI-1 antigen properties 


  antibody response in infacts

  antibody production in congenitally athymic individual

  antibody response in absence of all T cells

  polyclonal B cell activation

  eg- bacterial lipo Polysaccharide
• TI-2 antigen properties

  antibody production in congenitally athymic individual

  requires repeating epitopes

  eg - salmonella