AQA A Level Biology复习笔记2.6.3 HIV



Transmission of HIV

  • Human Immunodeficiency Virus is a retrovirus
  • The HIV virus is not transmitted by a vector (unlike in malaria)
  • The virus is unable to survive outside of the human body
  • The virus is spread by intimate human contact and can only be transmitted by direct exchange of body fluids
  • This means HIV can be transmitted in the following ways:
    • sexual intercourse
    • blood donation
    • sharing of needles used by intravenous drug users
    • from mother to child across the placenta
    • mixing of blood between mother and child during birth
    • from mother to child through breast milk


Structure of HIV

  • The HIV virus is made up of several key components:
    • Two RNA strands
    • Proteins (including the enzyme reverse transcriptase)
    • A protein coat (capsid)
    • A viral envelope consisting of a lipid bilayer and glycoproteins
      • The lipid bilayer is derived from the cell membrane of the host helper T cell that the particle escaped from


    • Attachment proteins



Structure of the HIV virus


Replication of HIV

  • When the virus enters the bloodstream it infects a certain type of lymphocyte - helper T cells
    • Normally lymphocytes seek out and destroy pathogens that enter the body, producing antibodies that attach to pathogens and enhancing phagocytic activity
    • HIV avoids being recognised and destroyed by lymphocytes by repeatedly changing its protein coat


  • The virus uses the cell machinery of helper T-cells to multiply:
    • Viral RNA enters the cell
    • Viral reverse transcriptase enzymes produce a DNA copy of the viral RNA
    • The DNA copy is inserted into the chromosomes of the cell
    • Each time the cell divides it copies the viral DNA
    • The infected cells remain normal as the viral DNA is inactive


  • At this stage, the individual is HIV positive and will have antibodies against the virus
  • After a period of time (possibly years) the viral DNA within the host cells becomes active
    • It takes control of the helper T cell
    • More HIV particles are produced
    • This causes the helper T cell to die
    • As a result, thousands of new HIV particles are released which are able to infect other helper T cells


  • Gradually the virus reduces the number of helper T cells in the immune system
    • B cells are no longer activated
    • No antibodies are produced


  • This decreases the body’s ability to fight off infections, eventually leading to AIDS (Acquired immune deficiency syndrome)



HIV attached to T lymphocytes and uses their cell machinery to replicate. This leads to decreased lymphocyte number which then affects the body's ability to respond to infection