IB Biology HLDisease & ImmunityPaper 1 & 2~9 min read
HIV & AIDS
HIV does not make you ill by attacking your organs. It attacks the one cell that switches on your adaptive immune response, and then everything else gets in. Understanding that single fact makes the whole page fall into place.
📘 What you need to know
HIV (Human Immunodeficiency Virus) is a retrovirus. It contains RNA and the enzyme reverse transcriptase, which makes DNA in the host cell.
The virus is unable to survive outside the human body and needs host cells in order to replicate.
HIV is not spread by a vector (unlike malaria). It spreads by direct exchange of body fluids.
HIV infects and attacks T helper cells.
T helper cells are essential for antibody production, so HIV inhibits the body’s capacity to produce antibodies.
In the early stages, antibodies against HIV are produced and can be detected in blood tests — the person is HIV positive.
As the infection progresses, the person becomes prone to opportunistic pathogens. Suffering several such diseases at once is AIDS.
Anti–retroviral drugs slow the progression from HIV infection to AIDS, and many HIV positive people live full lives with a normal life expectancy.
What kind of virus HIV is
Most organisms store their genetic information as DNA and copy it into RNA. A retrovirus does it backwards, and that is where the name comes from.
Why HIV is a retrovirus
It carries RNA plus the enzyme reverse transcriptase, which is used to produce DNA in the host cell.
Compare this with the last page. The T helper cell was the cell that told B cells to activate, and it is precisely the cell HIV destroys.
How HIV is transmitted
HIV cannot survive outside the human body, so it is not carried by a vector the way malaria is carried by a mosquito. It is spread by the direct exchange of body fluids.
Sexual intercourse
Blood donation
Sharing 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
If you are asked why HIV is not spread by touching, sharing cups or insect bites, the reason is always the same: the virus cannot survive outside the human body and needs host cells to replicate. That one sentence answers a whole family of questions.
From HIV infection to AIDS
HIV infects a type of lymphocyte called the T helper cell. Because T helper cells are a key part of antibody production, HIV inhibits the body’s capacity to produce antibodies.
In the early stages of infection, antibodies against HIV itself are still being produced. These are what a blood test detects, and someone with them is described as HIV positive.
An opportunistic pathogen is one that a healthy immune system would deal with easily. It only causes serious disease when the immune system has already been weakened.
As the infection progresses, the ability to produce antibodies drops sharply. The immune system can no longer fight off other pathogens, so the person becomes prone to infection from opportunistic pathogens. When someone is suffering from several such diseases or conditions at the same time, they are said to have acquired immune deficiency syndrome, or AIDS.
🧠
HIV is the virus. AIDS is the syndrome.
The V in HIV stands for virus, and the S in AIDS stands for syndrome. A syndrome is a collection of conditions occurring together — which is exactly why AIDS is diagnosed from several diseases at once.
Treatment has changed the picture completely.Anti–retroviral drugs slow the progression from HIV infection to AIDS. Thanks to these drugs, many HIV positive people now live full–quality lives with normal life expectancies. Being HIV positive is not the same as having AIDS.
Worked examples
WE 1
Explain why HIV weakens the immune system
Explain how infection with HIV reduces the body’s ability to fight other pathogens. (3 marks)
Point 1: the target
HIV infects and destroys T helper cells, a type of lymphocyte.
Point 2: the knock–on effect
T helper cells are needed to activate B cells, so with fewer of them the body’s capacity to produce antibodies falls.
Point 3: the result
Without an effective antibody response, the person cannot fight off other pathogens and becomes prone to opportunistic infections.
Kill the T helper cells and the antibody response collapsesthe marks are in the chain — T helper, then antibodies, then other pathogens
WE 2
Distinguish between HIV and AIDS
Distinguish between HIV and AIDS. (2 marks)
Point 1: HIV
HIV is the name of the virus that infects the host and attacks T helper cells. Someone with detectable antibodies against it is HIV positive.
Point 2: AIDS
AIDS is the syndrome that the virus causes, diagnosed when a person suffers several opportunistic diseases or conditions at the same time.
One is the pathogen, the other is the condition it eventually causesnote that anti–retroviral drugs can slow or prevent the step from one to the other
WE 3
Apply it: how a blood test works
An HIV test looks for antibodies rather than for the virus itself. Suggest one advantage and one limitation of this. (2 marks)
Advantage
Antibodies against HIV are produced in the early stages of infection and circulate in the blood, so they are present in large numbers and easy to detect.
Limitation
Antibodies take time to appear after infection, so someone tested very soon after exposure could test negative while still being infected.
Easy to detect, but only once the primary response has runlink the limitation back to the primary response graph — antibodies do not appear instantly
💡 Exam tips
Say retrovirus, and explain it with RNA plus reverse transcriptase makes DNA.
Name the target cell precisely: T helper cell, not “white blood cell”.
Use the phrase opportunistic pathogens when describing AIDS.
Say HIV spreads by direct exchange of body fluids, and that it is not vector–borne.
Mention anti–retroviral drugs slowing progression — it often appears in the final mark.
Keep HIV and AIDS separate throughout your answer. Examiners look for it.
⚠ Common mistakes
Using HIV and AIDS as the same word. HIV is the virus; AIDS is the syndrome it causes.
Saying HIV kills the patient. Opportunistic infections do, once the immune system has failed.
Saying HIV attacks all white blood cells. It targets T helper cells.
Describing HIV as spread by a vector. That is malaria. HIV needs direct fluid exchange.
Saying HIV positive people cannot make any antibodies. In the early stages they do — that is what the test detects.
Forgetting that HIV cannot survive outside the body. It is the reason casual contact does not spread it.
Up next: Antibiotics. HIV is a virus, and that matters for treatment. The next page explains why antibiotics do nothing at all against it, and what they do work on instead.
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