NCERT Class 12 Biology: Chapter – 8 Human Health and Diseases Part 8

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Question 15:

Diseases like dysentery, cholera, typhoid etc. , are more common in over-crowded human settlements. Why?


Diseases like dysentery, cholera, typhoid etc. are mainly contaminated water based diseases and are more common in overcrowded human settlements because in case of open excretion, the water gets mixed with the infected excreta and the contaminated water spreads the disease further.

Question 16:

From which plant cannabiniods are obtained? Name any two cannabinoids. Which part of the body is effected by consuming these substances?

Answer: Cannabinoids are the chemical substances that mainly act on the cannabinoid receptor in the brain.

Obtained from: Inflorescence of Cannabis Sativa.

Example of Cannabinoids: Marijuana, Ganja and Charas etc.

Body part affected: Cardiovascular System is he most affected.

Question 17:

In the metropolitan cities of India, many children are suffering from allergy/asthma. What are the main causes of this problem? Give some symptoms of allergic reactions.

Answer: In the metropolitan cities of India, many children are suffering from allergy/asthma because:

1. All these cities are highly polluted due to the increased use of vehicles, factories and construction which releases pollutants and allergens like dust, smoke in the air.

2. This increased pollution lowers immunity in children and make them more sensitive towards the allergens.


The general symptoms include:

1. Breathing Difficulty.

2. Sneezing

3. Runny Nose

4. Burning and watery eyes.

Question 18:

What is the basic principle of vaccination? How do vaccines prevent microbial infections? Name the organism from which hepatitis B Vaccine is produced.

Answer: 1. Vaccination is the process of injecting weak or inactive pathogen of a specific disease in the body of an individual.

2. This makes the body generate an immune response by releasing antibodies with memory B and T cells.

3. So, when a vaccinated person is attacked by the same pathogen again, naturally the body remembers and recognise the antigen and release the same antibodies and lymphocytes against it.

Question 19:

What is cancer? How is a cancer cell different from the normal cell? How do normal cells attain cancerous nature?

Answer: Mutation in the genes may lead to uncontrollable and rapid division of the cells forming a mass or tumour. This mass is known as cancer. A cancer cell is different than the normal cell because they metastise that is move from one place to another through lymphatic system and the bloodstream and are highly invasive.

Normal cells attain cancerous nature when the normal genes goes through mutation which is the alteration in the nucleotide sequence. These mutation occurs due to exposure to UV rays, genetic make-up or other radiations.

Question 20:

A person shows strong unusual hypersensitive reactions when exposed to certain substances present in the air. Identify the condition. Name the cells responsible for such reactions. What precaution should be taken to avoid such reactions?

Answer: This unusual hypersensitive reaction is allergy, caused due to exposure towards allergens. Mast cells are responsible for such reactions. When an allergen binds to the IgE receptors on the mast cell a response is seen.


1. Avoid exposure to such allergens.

2. Take medicines as prescribed.

Question 21:

For an organ transplant, it is an advantage to have an identical twin. Why?

Answer: Transplantation is the procedure of replacing the unhealthy organ with a healthy one in an individual. Sometimes these replaced organs get rejected by the body’ s immune system as they are considered as a foreign object by the body. In response to this, cell mediated immune response takes place by T-Lymphocytes. These lymphocytes try to differentiate between self-cells and foreign cells because of which sometimes the graft gets rejected. To avoid this, blood groups and tissue matching is done. In the case of identical twins the chances of graft rejection is less because blood group and tissue match is easy.