Most pathogens have to get past the immune system. HIV does something cleverer and far more damaging: it attacks the immune system itself, and it goes for the exact cell that holds the whole adaptive response together.
📘 What you need to know
HIV is the Human Immunodeficiency Virus, and it is a retrovirus.
It contains RNA and the enzyme reverse transcriptase, which produces DNA in the host cell — that is what makes it a retrovirus.
The virus cannot survive outside the human body and needs host cells to replicate.
It is not spread by a vector. It spreads by the direct exchange of body fluids.
HIV infects and attacks T helper cells.
Because T helper cells are needed for antibody production, HIV reduces the body’s capacity to produce antibodies.
Early on, antibodies against HIV can be detected in blood tests — the person is HIV positive.
When antibody production has fallen far enough that several opportunistic infections take hold at once, the person is said to have AIDS.
Anti-retroviral drugs slow the progression from HIV infection to AIDS, and many HIV positive people now live full lives with a normal life expectancy.
How HIV is transmitted
Because the virus cannot survive outside the body, transmission requires the direct exchange of body fluids. Unlike malaria, no vector such as a mosquito is involved.
Sexual intercourse
Blood donation
Sharing of needles 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
The flip side of that list is just as examinable. Because HIV cannot survive outside the body, it is not spread by coughing, sharing cutlery, swimming pools or ordinary social contact.
What “retrovirus” actually means
In a normal cell, information flows from DNA to RNA to protein. A retrovirus runs part of that backwards.
Human cells have no need to turn RNA back into DNA, so they have no reverse transcriptase. That is why a drug attacking this enzyme can hit the virus without wrecking your own cells.
Why attacking T helper cells is so serious
Go back to the activation chain from the previous page. The T helper cell is the link between the antigen-presenting cell and the B cell. Take it out and the chain breaks in the middle.
Compare this with the activation chain on the previous page. HIV attacks step three, and everything downstream of it stops working.
HIV positive is not the same as AIDS
Term
What it means
HIV
The name of the specific virus that is infecting the host
HIV positive
Antibodies against HIV can be detected in the person’s blood, showing they are infected
AIDS
Acquired immune deficiency syndrome — the collection of diseases and conditions that develop once antibody production has fallen far enough
In the early stages of infection the immune system does produce antibodies against HIV, which is exactly what a blood test looks for. As the infection progresses, the ability to produce antibodies falls significantly, leaving the person open to opportunistic pathogens — ones that a healthy immune system would deal with easily. When several such diseases or conditions are present at once, the person is said to have AIDS.
The distinction to get right
HIV is the virus • AIDS is the syndrome the virus eventually causes
Progression from HIV infection to AIDS can be slowed a great deal with anti-retroviral drugs. Thanks to these, many HIV positive people now live full lives with normal life expectancies. If a question asks about treatment, that is the point to make — and note that it slows progression rather than curing the infection.
Worked examples
WORKED EXAMPLE
Explain why an HIV positive person eventually becomes vulnerable to a wide range of infections. [4]
1. The targetHIV infects and destroys T helper cells.2. What T helper cells doThey activate B lymphocytes by releasing signalling proteins, which is needed for antibody production.3. The lossWith fewer T helper cells, fewer B cells are activated, so far fewer plasma cells and antibodies are produced.4. The consequenceThe adaptive response to any pathogen is weakened, so opportunistic pathogens that a healthy person would fight off cause disease.Wide vulnerability because the damage is to the system, not to one defencethe phrase “opportunistic pathogens” is worth writing explicitly
WORKED EXAMPLE
A blood test for HIV detects antibodies rather than the virus itself. Suggest one advantage and one limitation of this. [2]
AdvantageAntibodies against HIV are produced in the early stages of infection and circulate in the blood, so infection can be detected before symptoms appear.LimitationIt takes time after infection for enough antibodies to be produced, so a very recently infected person could test negative.Early detection, but not immediate detection“suggest” questions accept sensible reasoning built on what you know
💡 Exam tip
Name the cell: T helper cell. “White blood cells” is too vague for full marks.
Name the enzyme: reverse transcriptase, and say it makes DNA from the viral RNA.
Say HIV cannot survive outside the body — it explains the whole transmission list in one sentence.
Keep HIV and AIDS separate throughout your answer. Marks are regularly lost for using them interchangeably.
Say anti-retroviral drugs slow progression. They are not a cure.
⚠ Common mix-up
Saying HIV is spread by a vector. It is not — that is malaria. HIV needs direct exchange of body fluids.
Writing that HIV kills the patient directly. Opportunistic infections do the killing.
Saying HIV attacks B cells. It attacks T helper cells; B cells fail because they are never activated.
Treating HIV positive and AIDS as the same stage. Being HIV positive can precede AIDS by many years, especially with treatment.
Calling HIV a bacterium. It is a virus, so antibiotics have no effect on it.
Up next: Antibiotics — why these drugs work brilliantly against bacteria, and why they do absolutely nothing for a virus like HIV.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
