Activation of T cells

• Describe interaction between T cells and DCs

  Loose adhesion: DCs express ICAM-1, which weakly binds to LFA-1 on T cells and allows the T cell and DC to come close

  Strong adhesion: CD4 molecule on T cell interacts with the MHCII molecule on the DC, leading to signalling events that increases the affinity of the LFA-1 and ICAM-1 interaction.
• Describe the different complement pathways

  This is a major component of the innate immune system. Complement are made in the liver and circulate as inactive precursors.

  • Classical pathway: complement binds to antibody bound to pathogen surface
  • Alternative pathway: complement binds to pathogen surfaces
  • Lectin pathway: complement binds to mannose binding protein bound to pathogen
• Describe the classical complement pathway

  1. Pentameric IgM molecules bind to antigens on pathogen surface as well as IgG molecules
  2. C1q binds to the bound IgM and IgG on bacterial surface
  3. Activated C1 cleaves C4 to C4a and C4b, which bind to microbial surface
  4. C2 is cleaved by C1 to C2a and C2b, which form the C4b2a complex. This complex cleaves C3 to C3a and C3b
• Describe the effects of C3 cleavage in the complement pathway

  • Induces cell lysis
  • Opsonisation of pathogen
  • Induction of chemotaxis and inflammation
  • Immune complex clearance