HIV is a single stranded RNA retrovirus that infects and replicates within the human immune system using host CD4 cells

Aetiology

Causative organism

• HIV is an RNA retrovirus
• HIV-1 group M responsible for global epidemic
• MSM is the risk group for the majority of prevalent infections and new infections in the UK

Transmission

• Sexual transmission accounts for 79% of the new infections in the UK
  • Factors increasing transmission risk: anoreceptive, trauma, genital ulceration, concurrent STI
• Parental transmission
  • Injection drug use
  • Infected blood products
  • Iatrogenic
• Mother-to-child
  • In utero/trans-placental
  • Delivery
  • Breastfeeding

Pathophysiology

Infection

• Infection of mucosal CD4+ cell
• Transport to regional lymph nodes
• Infection established within 3 days of entry
• Dissemination of virus

Immunopathogenesis

• CD4+ receptors are the target site for HIV
  • CD4 is a glycoprotein found on the surface of a range of cells, including T helper lymphocytes (CD4+ cells), dendritic cells, macrophages and microglial cells
• CD4+ Th lymphocytes are essential for induction of adaptive immune response
  • Recognition of MHC2 antigen-presenting cell
  • Activation of B-cells
  • Activation of cytotoxic T cells (CD8+)
  • Cytokine release
• Effect of HIV infection on the immune response:
  • Reduced circulating CD4+ cells
  • Reduced proliferation of CD4+ cells
  • Reduction in CD8+ T cell activation
    • Dysregulated expression of cytokines
  • Reduction in antibody class switching
    • Reduced affinity of antibodies produced
  • Chronic immune activation