Cells and molecules of the immune system

• Describe mannose receptors

  These are a type of C-type lectin receptor, which bind to carbohydrate structures found on pathogens, such as bacteria and parasites.
• Describe dectin-1 receptor

  These are a type of C-type lectin receptor that binds to beta-glucan structures and are important in anti fungal responses.
• Describe scavenger receptors

  These receptors bind to low density lipoproteins and sialic acid, which allows them to recognise bacteria and yeast.
• Describe complement receptors

  These receptors recognise LPS and lipophosphoglycans in bacteria and yeast.
• Describe the toll-like receptor family

  This is a family of pattern recognition receptors that recognise highly conserved structures on pathogens called PAMPs, as well as DAMPs from damaged cells. These receptors exist as homodimer or heterodimer pairs, and this defines the pattern that they can recognise.
• Describe two examples of TLRs and the patterns that they recognise

  • TLR4: homodimer receptor that recognises LPS on bacteria and activates nuclear factor kappa B via receptor interacting protein kinase family. NF-kB activates the expression of many pro-cytokine genes.
  • TLR5: homodimer receptor that recognises flagellin that also activates NF-kB, but via a complex signalling cascade.
• Describe the steps that lead to an inflammatory response progressing from local to systemic

  1. Macrophages recognise PAMPs and secrete cytokines
  2. This leads to vasodilation and enhanced vascular permeability, as well as chemotaxis of neutrophils and monocytes
  3. Oedema occurs due to the accumulation of plasma fluid and proteins
  4. Finally, immune amplification occurs when activated immune cells secrete more cytokines, creating a knock-on effect.
• Describe the action of mast cells

  These cells, when activated, degranulate to release their contents which contain:

  • Histamine
  • Heparin
  • Enzymes
  • Cytokines
• Describe the Paracrine action of some interleukins

  IL1 acts on the brain to promote fever and anorexia. IL6 stimulates liver hepatocytes to make acute phase proteins, which amplify the immune response. This Paracrine action occurs when local cytokine response is high enough.
• Describe acute phase proteins

  • Fibrinogen: clotting
  • C3b: opsonin
  • C-reactive protein: bindings to phosphoryl choline and acts as an opsonin
• Describe the complement proteins

  This is a collection of soluble proteins that circulate in the plasma in an inactive form. Once triggered they initiate a cascade:

  • Classical pathway: initiated by antibody of CRP
  • Alternative pathway: triggered by C3b
  • Lectin pathway: triggered by mannose binding lectin

  This cascade functions to induce chemotaxis (C3b), activate mast cells (C3a) or clear immune complexes (C3b).
• Describe interferons

  This is a type of inflammatory cytokine that inhibits viral replication in infected cells. They bind to cells expressing the interferon receptor and activate macrophages and natural killer cells. IFN can act via autocrine or Paracrine mechanisms.