Allergy, autoimmunity and AIDS

When defence goes too far

A girl sneezes in a dusty room, and her friend does not. The same dust is harmless to the friend. Why does it trouble the girl?

Allergy

Allergy is an exaggerated response of the immune system to a substance that is present in the environment. The substance is an allergen. Common allergens are the mites in dust, pollen and the dander of animals.

1. The first contact makes the body form IgE antibodies. They stick to the mast cells, which are cells of the tissue.
2. At the next contact the allergen attaches to the IgE on the mast cells.
3. The mast cells release histamine and serotonin.
4. These chemicals cause sneezing, watery eyes, a running nose and difficulty in breathing.

To find the cause, a doctor may expose the patient to very small amounts of the possible allergens and watch for a reaction. Anti-histamine drugs, adrenaline and steroids quickly reduce the symptoms. In rare cases an allergy is severe, and the patient needs emergency care at once.

Autoimmunity

The body can tell the self from the non-self. Sometimes, for genetic or unknown reasons, it fails and attacks its own cells. The result is an autoimmune disease. Rheumatoid arthritis, in which the immune system damages the joints, is the common example. A-Level books also give type 1 diabetes, where the immune system destroys the cells that make insulin.

AIDS

AIDS means acquired immunodeficiency syndrome. It is acquired, because it is not inherited and a person gets it after birth. It is an immunodeficiency, because the immune system becomes weak. It is a syndrome, because it shows as a group of signs.

AIDS is caused by the human immunodeficiency virus (HIV). HIV is a retrovirus: it has an RNA genome, and it makes DNA from the RNA with the enzyme reverse transcriptase.

How HIV spreads. HIV spreads through body fluids.

• by sexual contact with an infected person
• by blood transfusion with infected blood, or with blood products
• by sharing infected needles and syringes
• from an infected mother to her child, through the placenta

How HIV does not spread. It does not spread by touch, by hugging, by shaking hands, by sharing a meal or by living together. A person with HIV needs care and should not be treated unkindly. The groups at the highest risk are people with many sexual partners, people who inject drugs, people who need repeated blood transfusions and children of infected mothers.

What happens in the body.

1. The virus enters a macrophage or a helper T lymphocyte (TH cell).
2. Reverse transcriptase makes viral DNA from the viral RNA.
3. The viral DNA joins the DNA of the host cell. It directs the cell to make new virus particles.
4. A macrophage goes on making viruses and acts as an HIV factory.
5. In a helper T cell the virus multiplies, and the new viruses are released and infect other helper T cells.

The number of helper T cells falls. The patient may have fever, diarrhoea and weight loss. The immune system cannot protect the body, and the person falls to infections by bacteria such as Mycobacterium, by viruses, fungi and parasites such as Toxoplasma. Even a mild infection can be very dangerous.

The time between infection and the signs of AIDS varies from a few months to many years, usually 5 to 10 years.

Numbers. A healthy person has about 500 to 1500 helper T cells in each mm³ of blood. Doctors say that a person has AIDS when the count falls below 200 cells in each mm³, or when certain serious infections appear. NCERT does not give these numbers. They are standard values from medical references.

Diagnosis and treatment. The test used first is ELISA, which stands for enzyme linked immuno-sorbent assay. There is no cure for AIDS. Anti-retroviral drugs keep the amount of virus low and help many patients to live for many years. NCERT says that these drugs are only partially effective.

Prevention. As there is no cure, prevention is the best option. The steps are to use only blood that has been tested, to use disposable needles and syringes, to practise safe sex, to control drug abuse and to teach people about AIDS. Organisations such as the WHO and NACO (the National AIDS Control Organisation) run these programmes.

You should find that a faster yearly fall brings the count below 200 sooner. Real counts differ from person to person, and treatment changes the curve.

Try it in Python

The program reads the wells of an ELISA plate. A sample is positive if its reading is above the cutoff.

Practise with new numbers

Common mistakes

• Saying that HIV and AIDS are the same thing. HIV is the virus. AIDS is the syndrome that appears after the helper T cells are lost.
• Calling HIV a bacterium or a DNA virus. It is a retrovirus with an RNA genome.
• Saying that HIV spreads by a handshake or a mosquito. It spreads through blood, other body fluids and from mother to child.
• Mixing up the cell that HIV attacks. It attacks the helper T cells and the macrophages.
• Confusing allergy with autoimmunity. In allergy the body overreacts to an outside substance. In autoimmunity it attacks itself.

Remember these

• Allergy: allergens, IgE on mast cells, release of histamine and serotonin. Anti-histamine, adrenaline and steroids help.
• Autoimmunity: the body attacks its own cells. Example: rheumatoid arthritis.
• HIV: a retrovirus with RNA and reverse transcriptase. It attacks the helper T cells and the macrophages.
• Spread: sexual contact, infected blood, shared needles, mother to child. Not by touch.
• Test: ELISA. No cure. Prevention is the best option.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the type of antibody and the chemicals that are involved in allergy.

2. What is an autoimmune disease? Give an example.

3. Describe how HIV makes new viruses in a helper T cell.

4. Why does a person with AIDS fall to many infections?

5. Name the test for AIDS and two steps of prevention.

References

• Murphy K. and Weaver C. (2017). Janeway's Immunobiology, 9th edition, Garland Science, chapters 13 and 14.
• World Health Organization (2024). Fact sheet: HIV and AIDS.
• NCERT (2023). Biology, Textbook for Class XII, chapter 7.