Human Health and Diseases - Notes | Class 12 | Part 3: Allergy, Autoimmunity, AIDS



§ It is the exaggerated response of the immune system to certain antigens present in the environment.

§ Allergens: Substances causing allergy. E.g. mites in dust, pollens, animal dander, fur etc.

§ Antibodies produced against the allergens are IgE type.

§ IgE binds on mast cells to release chemicals like histamine and serotonin from them. It results in allergic reactions.

§ Symptoms: Sneezing, watery eyes, running nose, difficulty in breathing, wheezing, skin rashes etc.

§ Determination of cause of allergy: The patient is exposed to or injected with very small doses of possible allergens, and the reactions studied.

§ Treatment: Drugs like anti-histamine, adrenaline and steroids quickly reduce the symptoms of allergy.

§ Asthma is a respiratory disease due to allergy.

§ Modern-day life style and protected environment provided early in life result in low immunity and more sensitivity to allergens. So, many children in metro cities suffer from allergies and asthma.


-   In higher vertebrates, memory-based acquired immunity evolved based on the ability to differentiate foreign organisms from self-cells.

-   Sometimes, due to genetic and other unknown reasons, the body attacks self-cells resulting in damage to the body. It is called auto-immune disease. E.g. Rheumatoid arthritis.

AIDS (Acquired Immuno Deficiency Syndrome)

§ It is the deficiency of immune system.

§ Syndrome means a group of symptoms.

§ It is caused by HIV (Human Immunodeficiency Virus), a retrovirus having RNA genome.

§ AIDS was first reported in America (1981).

§ In the last 25 years, it killed over 25 million persons.


§ Sexual contact with infected person.

§ Transfusion of contaminated blood & blood products.

§ Sharing of infected needles.

§ From infected mother to her child through placenta.

High risk people of getting HIV:

§ Individuals with multiple sexual partners.

§ Drug addicts who take drugs intravenously.

§ Individuals who require repeated blood transfusion.

§ Children born to an HIV infected mother.

HIV does not spread by touch or physical contact. It spreads only through body fluids.

There is a time-lag (from few months to 5-10 years) between the infection and appearance of symptoms.

Replication of retrovirus:

Life cycle of HIV:

HIV enters body → To macrophages (acts as HIV factory) → RNA genome replicates in presence of Reverse transcriptase to form viral DNA → Viral DNA incorporates into host DNA→ Infected cells produce virus particles → HIV enters into helper T-cells (TH lymphocytes) → Replicates & produce progeny viruses → Attack other TH cells → TH cells decrease → Weaken immunity.

§ During this period, the person suffers from fever, diarrhoea and weight loss.

§ Due to deficiency of TH cells, he may be infected with Mycobacterium, viruses, fungi & parasites like Toxoplasma.

§ Diagnosis: ELISA test (Enzyme-linked immuno-sorbent Assay).

§ Treatment: Anti-retroviral drugs are partially effective. They can only prolong the life of the patient.

Prevention of AIDS:

o  Educate people about AIDS through organisations like National AIDS Control Organisation (NACO), non-governmental organisations (NGOs), WHO etc.

o  Make blood (from blood banks) safe from HIV.

o  Use disposable needles and syringes.

o  Advocate safe sex and free distribution of condoms.

o  Control drug abuse.

o  Regular check-ups for HIV in susceptible population.


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