Biology - AQA - Unit 2 - C7. HIV and Viruses

HIV (human immunodeficiency virus) is a virus that affects the human immune system. It eventually leads to acquired immune deficiency syndrome (AIDS). AIDS is a condition where the immune system deteriorates and eventually fails. This makes someone with AIDS more vulnerable to other infections, like pneumonia.

The initial (acute) stage of a HIV infection can cause flu-like symptoms as the immune system mounts a response to the virus. This immune response is not able to destroy all of the virus though, so a small amount remains in the cells and continues to replicate.

HIV infects and eventually kills helper T-cells, which act as the host cells for the virus. Helper T-cells send chemical signals that activate phagocytes, cytotoxic T-cells and B-cells so they’re hugely important cells in the immune response.

Without enough helper T-cells, the immune system is unable to mount an effective response to infections because other immune system cells don’t behave how they should. People infected with HIV develop AIDS when the helper T-cell numbers in their body reach a critically low level.

During the 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. This is the latency period. During the latency period (which can last for years) the infected person won’t experience any symptoms.

Tip: As HIV replicates and the amount of virus increases, the helper T-cell count drops, which leads to AIDS.

People with HIV are classed as having AIDS when symptoms of their failing immune system start to appear or their helper T-cell count drops below a certain level. The length of time between infection with HIV and the development of AIDS varies between individuals but without treatment it’s usually around 10 years. People with AIDS generally develop diseases that wouldn’t cause serious problems in people with a healthy immune system.

• The initial symptoms of AIDS include minor infections of mucous membranes (e.g, the inside of the nose, ears and genitals), and recurring respiratory infections.

• As AIDS progresses the number of immune system cells decreases further. Patients become susceptible to more serious infections including chronic diarrhoea, severe bacterial infections and tuberculosis.

• During the late stages of AIDS patients have a very low number of immune system cells and can develop a range of serious infections such as toxoplasmosis of the brain (a parasite infection) and candidiasis of the respiratory system (fungal infection). It’s these serious infections that kill AIDS patients, not HIV itself.

The length of time that people survive with AIDS varies a lot. Factors that affect progression of HIV to AIDS and survival time with AIDS include:

• existing infections

• the strain of HIV they’re infected with

• age

• access to healthcare.

• The virus particle has a spherical structure.

• It’s made up of a core containing the genetic material (RNA)

• some proteins (including the enzyme reverse transcriptase, which is needed for virus replication).

• It has an outer coating of protein called a capsid

• an extra outer layer called an envelope, which is made of membrane stolen from the cell membrane of a previous host cell.

• loads of copies of an attachment protein that help HIV attach to the host helper T-cell

1. The attachment protein attaches to a receptor molecule on the cell membrane of the host helper T-cell.

2. The capsid is released into the cell, where it uncoats and releases the genetic material (RNA) into the cell’s cytoplasm.

3. Inside the cell, reverse transcriptase is used to make a complementary strand of DNA from the viral RNA template.

4. From this, double-stranded DNA is made and inserted into the human DNA.

5. Host cell enzymes are used to make viral proteins from the viral DNA found within the human DNA.

6. The viral proteins are assembled into new viruses, which bud from the cell and go on to infect other cells.

Antibiotics kill bacteria by interfering with their metabolic reactions. They target the 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 they don’t damage human cells. Makes sense.

Viruses don’t have their own enzymes and ribosomes - they use the ones in the host’s cells. So because human viruses use human enzymes and ribosomes to replicate, antibiotics can’t inhibit them because 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.

• via unprotected sexual intercourse

• through infected bodily fluids (e.g. blood from sharing contaminated needles)

• from a HIV-positive mother to her fetus. Not all babies 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.