HIV (AQA A Level Biology)

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