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
