HIV (AQA A Level Biology)

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