HIV and viruses

• Viruses are not living things and so can only replicate/ reproduce inside the cells of another organism

  The organism they infect is called the host
• HIV = human immunodeficiency virus - affects the human immune system and eventually leads to AIDS
• AIDs = acquired immune deficiency syndrome - a condition where the immune system deteriorates and eventually fails which makes someone with AIDS more vulnerable to other infections, such as pneumonia
• HIV host cells - HIV infects and eventually kills helper T-cells, which act as host cells for the virus
• Why are helper T-cells important in the immune system?

  send chemical signals that activate phagocytes, cytotoxic T-cells and B-cells
• What happens without enough helper T-cells?

  the immune system is unable to mount an effective response to infections because other immune system cells do not behave how the should

  People infected with HIV develop AIDS when the helper T-cell numbers in their body reach a critically low level
• Initial infection:

  during initial infection period, HIV replicates rapidly and the infected person may experience severe flu-like symptoms

  After this period, HIV replication drops to a lower level - the latency period
• What is the latency period?

  The period of time when HIV replication drops to a lower level

  During the latency period (which can last for years) the infected person won't experience any symptoms
• Symptoms of AIDS:

  people with HIV classed as having AIDS when symptoms of their failing immune system start to appear/ helper T-cell count drops below certain level
• Length of time between infection with HIV and development of AIDS varies between individuals but without treatment - usually about 10 years

  people with AIDS tend to develop diseases that wouldn't cause serious problems in people with healthy immune system
• Initial symptoms of AIDS include minor infections of mucous membranes (e.g. inside of the nose, ears and genitals) and recurring respiratory infections
• As AIDs progresses - no. of immune system cells decreases further

  patients become susceptible to more serious infections including chronic diarrhoea, sever bacterial infections and tuberculosis
• During late stages of AIDS patients have very low no. of immune system cells and can develop range of serious infections such as toxoplasmosis of the brain (parasite infection) and candidiasis of the respiratory system (fungal infection) - these serious infections that kill AIDS patients not the HIV itself
• length of time people survive with AIDS varies a lot

  factors that affect progression of HIV to AIDS and survival time with AIDS include:

  1. existing infections
  2. strain of HIV infected with
  3. age
  4. access to healthcare
• Structure of HIV:

  1. virus particle has a spherical structure
  2. made up of a core containing genetic material (RNA) and some proteins (including enzyme reverse transcriptase - needed for virus replication)
  3. outer coating of protein called a capsid
  4. extra outer layer called an envelope - made of membrane stolen from cell membrane of a previous host cell
  5. sticking out from envelope are lots of copies of an attachment protein that help HIV attach to the host helper T-cell (envelope proteins)
• HIV attachment proteins are foreign antigens that can be recognised by the immune system. During replication the virus can change parts of the structure of its attachment proteins - antigenic variation - helps HIV evade destruction by immune system
• HIV replication: HIV and all other viruses can only reproduce iside cells of the organism it has infected

  HIV replicates itself inside helper T-cells

  doesn't have the equipment such as enzymes and ribosomes to replicate on its own

  so uses those of the host cell
• Process of HIV replication

  1. Attachment protein attaches to receptor molecule on host helper T-cell membrane

  2. Capsid released into cell, uncoats and releases RNA into cytoplasm

  3. Reverse transcriptase makes complementary DNA strand from viral RNA

  4. Double-stranded DNA made and inserted into human DNA

  5. Host cell enzymes make viral proteins from viral DNA in human DNA

  6. Viral proteins assembled into new viruses, which bud from cell and infect other cells
• Antibiotics and viruses

  antibiotics kill bacteria by interfering with their metabolic reactions

  target bacterial enzymes and ribosomes used in these reactions

  bacterial enzymes and ribosomes are different from human enzymes and ribosomes

  antibiotics are designed to only target the bacterial ones so the don't damage human cells
• Viruses do not have their own enzymes and ribosomes - they use the ones in the host's cells

  so bc human viruses use human enzymes and ribosomes to replicate, antibiotics can't inhibit them bc they don't target human processes
• most antiviral drugs are designed to target the few virus-specific enzymes (enzymes that only the virus uses) that exist
• e.g. HIV uses reverse transcriptase to replicate

  human cells don't use this enzyme, so drugs can be designed to inhibit it without affecting the host cell

  reverse-transcriptase inhibitors
• There is currently no cure or vaccine for HIV but antiviral drus can be used to slow down the progression of HIV infection and AIDS in an infected person
• best way to control HIV infection in a population is by reducing its spread

  HIV can be spread by unprotected sex, through infected bodily fluids (e.g. blood from sharing contaminated needles) and from an HIV positive mother to her fetus
• not all babies born from HIV-positive mothers are born infected with HIV and taking antiviral drugs during pregnancy can reduce the chance of the baby being HIV-positive