2.4 HIV Flashcards
What does HIV stand for?
The Human Immunodeficiency Virus
What is HIV?
- It causes the disease acquired immune deficiency syndrome (AIDS).
- Among contagious diseases it is a relative newcomer, having been first diagnosed in 1981
What is the structure of HIV?
- On the outside is a viral envelope derived from host cell membrane, embedded in which are peg-like attachment proteins
- Inside the envelope is a protein layer called the capsid that encloses two single strands of RNA and viral enzymes
- One of these enzymes is reverse transcriptase, so-called because it catalyses the production of DNA and RNA - the reverse reaction to that carried out by transcriptase
What does the presence of reverse transcriptase mean?
The presence of reverse transcriptase, and consequent ability to make DNA from RNA, means that HIV belongs to a group of viruses called retroviruses
How does HIV replicate?
Being a virus, HIV cannot replicate itself. Instead it uses its genetic material to instruct the host cell’s biochemical mechanisms to produce the components required to make new HIV
How does it use its genetic material to make new HIV?
- Following infection HIV enters the bloodstream and circulates around the body
- A protein on the HIV readily binds to a protein called CD4. While this protein occurs on a number of different cells, HIV most frequently attaches to helper T cells
- The protein capsid fuses with the cell-surface membrane. The RNA and enzymes of HIV enter the helper T-cell
- The HIV reverse transcriptase converts the virus’s RNA into DNA
- The newly made DNA is moved into the helper T cells nucleus where it is inserted into the cells DNA
- The HIV DNA in the nucleus creates messenger RNA using the cells enzymes. This mRNA contains the instructions for making new viral proteins and the RNA to go into the new HIV
- The mRNA passes out of the nucleus through a nuclear pore and uses the cells protein synthesis mechanisms to make HIV particles
- The HIV particles break away from the helper T cell with a piece of its cell-surface membrane surrounding them which forms their lipid envelope
What happens when someone is infected with HIV?
- Once infected with HIV a person is said to be HIV positive
- However, the replication of HIV often goes into dormancy and only recommences, leading to AIDS, many years later
How does HIV cause the symptoms of AIDS?
- HIV causes AIDS by killing or interfering with the normal functioning of helper T cells
- Attachment proteins bind to complementary CD4 receptor on Th cells
- HIV particles replicate inside Th cells, killing or damaging them
- AIDS develops when there are too few Th cells for the immune system to function
- individuals cannot destroy other pathogens and suffer from secondary diseases/infections
How many helper T cells does an uninfected person have compared to an infected person?
- An uninfected person normally has between 800 and 1200 helper T cells in each mm^3 of blood
- In a person suffering from AIDS this number can be as low as 200mm^-3
What happens if you don’t have a sufficient number of helper T cells?
- Without a sufficient number of helper T cells, the immune system cannot stimulate B cells to produce antibodies or the cytotoxic T cells that kill cells infected by pathogens
- Memory cells may also become infected and destroyed
- As a result, the body is unable to produce an adequate immune response and becomes susceptible to other infections and cancers
What do many AIDS sufferers develop?
Infections of the lungs, intestines, brain and eyes, as well as experiencing weight loss and diarrhoea. It is these secondary diseases that cause death
What does HIV do to an individual?
- HIV does not kill individuals directly
- By infecting the immune system, HIV prevents it from functioning normally
- As a result those infected by HIV are unable to respond effectively to other pathogens
- It is these infections, rather than HIV, that ultimately cause ill health and eventual death
What does ELISA stand for?
Enzyme Linked Immunosorbant Assay
How are antibodies used in the ELISA test?
- It uses antibodies to not only detect the presence of a protein in a sample but also the quantity
- It is extremely sensitive and so can detect very small amounts of a molecule.
What is the procedure of the ELISA test?
- Apply the sample to a surface, e.g a slide, to which all the antigens in the sample will attach
- Wash the surface several times to remove any unattached antigens
- Add the antibody that is specific to the antigen we are trying to detect and leave the two to bind together
- Wash the surface to remove excess antibody
- Add a second antibody that binds with the first antibody. This second antibody has an enzyme attached to it
- Add the colourless substrate of the enzyme. The enzyme acts on the substrate to change it into a coloured product
- The amount of the antigen present is relative to the intensity of colour that develops
