Human Health and Disease | CBSE Class 12 Biology Notes

This note covers NCERT Class 12 Biology Chapter 7, Human Health and Disease: what health is, common infectious diseases and their prevention, immunity and its types, vaccination, allergies and autoimmunity, the lymphoid organs, AIDS, cancer, and drug and alcohol abuse among adolescents. The diseases are set out with their pathogen, how they spread and their symptoms.

What is health, and what affects it?

For a long time health was considered a balance of certain "humors", as early Greeks like Hippocrates and the Indian Ayurveda system asserted. It was thought, by pure reflective thought, that persons with "black bile" had a hot personality and would have fevers.

The discovery of blood circulation by William Harvey using the experimental method, and the demonstration with a thermometer that persons with black bile had normal body temperature, disproved the "good humor" hypothesis.

Later, biology showed that the mind influences our immune system through the neural and endocrine systems, and that the immune system maintains our health. So the mind and mental state can affect health.

Definition: Health is not simply the absence of disease or physical fitness. It is a state of complete physical, mental and social well-being.

Health is affected by three things:

1. genetic disorders, that is deficiencies with which a child is born and defects inherited from parents;
2. infections; and
3. life style, including the food and water we take, rest and exercise, and the habits we have or lack.

Healthy people are more efficient at work, which increases productivity and brings economic prosperity. Health also increases longevity and reduces infant and maternal mortality.

A balanced diet, personal hygiene, regular exercise and yoga help to maintain good health, as do awareness of diseases, vaccination, proper disposal of wastes, control of vectors, and hygiene in food and water.

When one or more organs or systems are adversely affected, with various signs and symptoms, we have a disease. Diseases are broadly infectious, easily transmitted from one person to another (some, like AIDS, are fatal), or non-infectious, of which cancer is the major cause of death.

Which pathogens cause the common diseases in humans?

Disease-causing organisms are called pathogens. They include bacteria, viruses, fungi, protozoans and helminths. Most parasites are pathogens, because they harm the host by living in or on it. Pathogens have to adapt to life within the host; for example, those that enter the gut must survive the low pH of the stomach and resist digestive enzymes.

Bacterial and viral diseases

A classic case is that of Mary Mallon, nicknamed Typhoid Mary, a cook who was a typhoid carrier and continued to spread typhoid for several years through the food she prepared. Dysentery, plague and diphtheria are some other bacterial diseases.

Protozoan, helminth and fungal diseases

What is the life cycle of Plasmodium?

Malignant malaria, caused by Plasmodium falciparum, is the most serious form and can even be fatal. The malarial parasite requires two hosts, human and mosquito, to complete its life cycle, and the female Anopheles mosquito is also the vector (transmitting agent).

1. Plasmodium enters the human body as sporozoites (the infectious form) through the bite of an infected female Anopheles mosquito.
2. The parasites initially multiply within the liver cells.
3. They then attack the red blood cells (RBCs), resulting in their rupture. The rupture of RBCs releases a toxic substance, haemozoin, which is responsible for the chill and high fever recurring every three to four days.
4. When a female Anopheles mosquito bites an infected person, the parasites enter the mosquito's body and undergo further development.
5. The parasites multiply within the mosquito to form sporozoites, which are stored in its salivary glands.
6. When the mosquito bites a human, the sporozoites are introduced into the body, and the cycle begins again.

A cycle drawn around a human host and a mosquito host. A mosquito bite injects sporozoites, which reach the liver through the blood. The parasite reproduces asexually in liver cells, bursting the cells and releasing parasites into the blood. It reproduces asexually in red blood cells, bursting them and causing cycles of fever. Sexual stages, male and female gametocytes, develop in red blood cells. A female mosquito takes up the gametocytes with a blood meal. Fertilisation and development take place in the mosquito's gut, and the mature sporozoites escape from the gut and migrate to the salivary glands, ready to be injected at the next bite.

See Fig. 7.1 in your NCERT textbook

Note: Keep the order of the two human stages: the liver first, then the red blood cells. The fever comes from haemozoin released when the red blood cells rupture, not from the liver stage.

How can infectious diseases be prevented and controlled?

Vector control can be achieved by avoiding stagnation of water around residential areas, regular cleaning of household coolers, the use of mosquito nets, introducing fishes like Gambusia in ponds to feed on mosquito larvae, spraying insecticides in ditches, drainage areas and swamps, and providing doors and windows with wire mesh.

The use of vaccines and immunisation programmes has enabled us to completely eradicate a deadly disease like smallpox. Polio, diphtheria, pneumonia and tetanus have been controlled to a large extent by vaccines, and antibiotics and other drugs have enabled us to treat infectious diseases effectively.

What is immunity, and how does innate immunity work?

Definition: Immunity is the overall ability of the host to fight the disease-causing organisms, conferred by the immune system.

We are exposed to a large number of infectious agents every day, but only a few exposures result in disease, because the body defends itself against most of them. Immunity is of two types: innate and acquired.

Innate immunity is a non-specific type of defence that is present at the time of birth. It is accomplished by different barriers to the entry of foreign agents.

How does acquired immunity work?

Acquired immunity is pathogen specific and is characterised by memory. The first encounter with a pathogen produces a primary response of low intensity. A subsequent encounter with the same pathogen produces a highly intensified secondary or anamnestic response, because the body appears to remember the first encounter.

These responses are carried out by two special types of lymphocytes in the blood: B-lymphocytes and T-lymphocytes.

Antibodies

Each antibody molecule has four peptide chains: two small light chains and two longer heavy chains. An antibody is therefore represented as H₂L₂. Different types of antibodies are produced in our body, including IgA, IgM, IgE and IgG.

A Y-shaped molecule. The two long heavy chains form the stem and continue up into the arms. A shorter light chain lies along the outer side of each arm. The tips of the two arms are labelled antigen binding site, and the chains are held together by disulphide (S-S) bonds. The N end is marked at the tip of one arm, and the C ends at the base of the stem.

See Fig. 7.4 in your NCERT textbook

Organ transplantation

When organs such as the heart, eye, liver or kidney fail, transplantation may be the only remedy. Grafts cannot be taken from just any source, such as an animal, another primate or any human being, because they would be rejected sooner or later. Tissue matching and blood group matching are essential before any graft, and even then the patient has to take immuno-suppressants all his or her life. The body can differentiate "self" and "non-self", and the cell-mediated immune response is responsible for graft rejection.

What are active and passive immunity, and how do vaccines work?

Vaccination and immunisation

The principle of vaccination is based on the memory of the immune system.

1. A preparation of antigenic proteins of the pathogen, or the inactivated or weakened pathogen (the vaccine), is introduced into the body.
2. The body produces antibodies against these antigens, which would neutralise the pathogenic agents during an actual infection.
3. The vaccine also generates memory B-cells and T-cells.
4. On subsequent exposure these recognise the pathogen quickly and overwhelm the invaders with a massive production of antibodies.

When a quick immune response is required, as in tetanus, preformed antibodies or an antitoxin (a preparation containing antibodies to the toxin) are injected directly. The injection given for snakebites also contains preformed antibodies against the snake venom. This is passive immunisation.

Recombinant DNA technology allows antigenic polypeptides of a pathogen to be produced in bacteria or yeast, which allows large-scale production and greater availability of vaccines, for example the hepatitis B vaccine produced from yeast.

What are allergies and autoimmunity?

Definition: Allergy is the exaggerated response of the immune system to certain antigens present in the environment. The substances to which such a response is produced are called allergens.

• The antibodies produced against allergens are of the IgE type.
• Common allergens are mites in dust, pollens and animal dander.
• Symptoms include sneezing, watery eyes, running nose and difficulty in breathing.
• Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.
• To find the cause, the patient is exposed to or injected with very small doses of possible allergens and the reactions are studied.
• Drugs like anti-histamine, adrenalin and steroids quickly reduce the symptoms.

Modern-day life style has somehow lowered immunity and increased sensitivity to allergens. More and more children in metro cities of India suffer from allergies and asthma, possibly because of the protected environment provided early in life.

Autoimmunity

Memory-based acquired immunity evolved in higher vertebrates based on the ability to differentiate foreign organisms from self-cells. Sometimes, due to genetic and other unknown reasons, the body attacks self-cells, which damages the body. This is called an auto-immune disease. Rheumatoid arthritis is an auto-immune disease.

What are the lymphoid organs?

The human immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies. Lymphoid organs are the organs where the origin and/or maturation and proliferation of lymphocytes occur.

What is AIDS, and how does HIV damage the immune system?

AIDS stands for Acquired Immuno Deficiency Syndrome: a deficiency of the immune system acquired during a person's lifetime, not a congenital disease. "Syndrome" means a group of symptoms. AIDS was first reported in 1981, and in the following twenty-five years or so it spread all over the world, killing more than 25 million persons.

AIDS is caused by the Human Immuno deficiency Virus (HIV), a retrovirus, which has an envelope enclosing an RNA genome. Transmission generally occurs by:

• sexual contact with an infected person,
• transfusion of contaminated blood and blood products,
• sharing infected needles, as by intravenous drug abusers, and
• from an infected mother to her child through the placenta.

HIV/AIDS is not spread by mere touch or physical contact; it spreads only through body fluids. So infected persons should not be isolated from family and society. There is always a time-lag between infection and the appearance of AIDS symptoms, from a few months to many years (usually 5 to 10 years).

How HIV causes immune deficiency

1. HIV enters macrophages, where its RNA genome replicates to form viral DNA with the help of the enzyme reverse transcriptase.
2. The viral DNA gets incorporated into the host cell's DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus, acting like an HIV factory.
3. Simultaneously, HIV enters helper T-lymphocytes, replicates and produces progeny viruses.
4. The progeny viruses released into the blood attack other helper T-lymphocytes, and this is repeated, leading to a progressive decrease in their number. The person suffers bouts of fever, diarrhoea and weight loss.
5. With fewer helper T-lymphocytes, the person suffers from infections that could otherwise have been overcome, such as those due to bacteria (especially Mycobacterium), viruses, fungi and parasites like Toxoplasma.

A retrovirus with a viral protein coat and a viral RNA core infects a normal animal cell. The viral RNA is introduced into the cytoplasm, and viral DNA is produced from it by reverse transcriptase. The viral DNA incorporates into the host genome in the nucleus. New viral RNA is produced by the infected cell, new viruses are produced, and they leave to infect other cells. A note says that the infected cell can survive while viruses are being replicated and released.

See Fig. 7.6 in your NCERT textbook

A widely used diagnostic test for AIDS is the enzyme linked immuno-sorbent assay (ELISA). Treatment with anti-retroviral drugs is only partially effective: it can prolong the life of the patient but cannot prevent death.

Prevention of AIDS

As AIDS has no cure, prevention is the best option. In India the National AIDS Control Organisation (NACO) and non-governmental organisations educate people about AIDS, and WHO has started programmes to prevent its spread.

The steps include making blood from blood banks safe from HIV, using only disposable needles and syringes, free distribution of condoms, controlling drug abuse, advocating safe sex and promoting regular check-ups for HIV in susceptible populations. HIV infection should not be hidden, and infected people need help and sympathy instead of being shunned.

What is cancer, and how is it caused, detected and treated?

Cancer is one of the most dreaded diseases and a major cause of death all over the globe. More than a million Indians suffer from cancer, and a large number of them die from it annually.

In the body, cell growth and differentiation are highly controlled and regulated. Normal cells show contact inhibition: contact with other cells inhibits their uncontrolled growth. Cancer cells appear to have lost this property, so they continue to divide, giving rise to masses of cells called tumours.

Metastasis is the most feared property of malignant tumours.

Causes of cancer

Agents that transform normal cells into cancerous neoplastic cells are called carcinogens. They may be physical, chemical or biological.

Detection and treatment

Tumour cells avoid detection and destruction by the immune system, so patients are given biological response modifiers such as α-interferon, which activate the immune system and help to destroy the tumour.

Which drugs are commonly abused, and what are their effects?

The drugs commonly abused are opioids, cannabinoids and coca alkaloids. Most are obtained from flowering plants, and some from fungi.

• Other well-known plants with hallucinogenic properties are Atropa belladona and Datura.
• Barbiturates, amphetamines, benzodiazepines and similar drugs, normally used as medicines for mental illnesses like depression and insomnia, are often abused. Morphine is a very effective sedative and painkiller for patients who have undergone surgery.
• Taking such substances for a purpose other than medicinal use, or in amounts or frequency that impair physical, physiological or psychological functions, constitutes drug abuse.

Tobacco

Tobacco has been used for more than 400 years and is smoked, chewed or used as snuff. It contains nicotine, an alkaloid, which stimulates the adrenal gland to release adrenaline and nor-adrenaline, raising blood pressure and heart rate.

Smoking is associated with increased incidence of cancers of the lung, urinary bladder and throat, bronchitis, emphysema, coronary heart disease and gastric ulcer; tobacco chewing with cancer of the oral cavity. Smoking increases carbon monoxide in the blood and reduces haem-bound oxygen, causing oxygen deficiency. Smoking also paves the way to hard drugs.

Why are adolescents vulnerable, and how can drug abuse be prevented?

Adolescence means both "a period" and "a process" during which a child becomes mature in attitudes and beliefs for effective participation in society. The period between 12 and 18 years of age may be thought of as adolescence, a bridge between childhood and adulthood and a very vulnerable phase of mental and psychological development.

Addiction and dependence

The effects of abuse include reckless behaviour, vandalism and violence; coma and death from respiratory failure, heart failure or cerebral haemorrhage with excessive doses; AIDS and hepatitis B from sharing needles; damage to the nervous system and liver (cirrhosis) with chronic use; and harm to the foetus during pregnancy. Sportspersons who misuse narcotic analgesics, anabolic steroids, diuretics and certain hormones risk many side effects, some permanent with prolonged use.

Prevention and control

Prevention is better than cure. Parenting that combines high levels of nurturance with consistent discipline has been associated with a lowered risk of substance abuse. The following measures help:

1. Avoid undue peer pressure: a child should not be pushed unduly to perform beyond his or her threshold limits.
2. Education and counselling: to face problems and stresses, accept disappointments and failures, and channel energy into sports, reading, music, yoga and other activities.
3. Seeking help from parents and peers, including close and trusted friends.
4. Looking for danger signs, by alert parents, teachers and friends.
5. Seeking professional and medical help from psychologists, psychiatrists, and de-addiction and rehabilitation programmes.

Glossary

• Health — A state of complete physical, mental and social well-being, not simply the absence of disease.
• Pathogen — A disease-causing organism, such as a bacterium, virus, fungus, protozoan or helminth.
• Widal test — The test used to confirm typhoid fever.
• Haemozoin — The toxic substance released when red blood cells rupture in malaria, causing chill and recurring high fever.
• Vector — A transmitting agent, such as the female Anopheles mosquito for malaria.
• Innate immunity — Non-specific defence present at birth, provided by physical, physiological, cellular and cytokine barriers.
• Anamnestic response — The highly intensified secondary response on a later encounter with the same pathogen, due to memory.
• Interferons — Proteins secreted by virus-infected cells that protect non-infected cells from further viral infection.
• Colostrum — The yellowish fluid secreted by the mother in the initial days of lactation, rich in IgA antibodies.
• Allergen — A substance, such as dust mites or pollen, to which an exaggerated immune response is produced.
• MALT — Mucosa-associated lymphoid tissue, in the lining of the major tracts; about half the body's lymphoid tissue.
• Reverse transcriptase — The enzyme with which HIV forms viral DNA from its RNA genome.
• Contact inhibition — The property of normal cells by which contact with other cells stops their uncontrolled growth.
• Metastasis — The spread of cells from a malignant tumour through the blood to distant sites, where they start new tumours.

Common errors and misconceptions

• Misconception: The common cold infects the lungs. Correct: Rhino viruses infect the nose and respiratory passage but not the lungs. Pneumonia infects the alveoli of the lungs.
• Misconception: Plasmodium attacks red blood cells as soon as it enters the body. Correct: It first multiplies within the liver cells, and then attacks the red blood cells.
• Misconception: Houseflies cause amoebiasis. Correct: Entamoeba histolytica causes amoebiasis. Houseflies are only mechanical carriers.
• Misconception: T-cells secrete antibodies. Correct: B-lymphocytes produce antibodies. T-cells help B cells to produce them, and T-lymphocytes mediate cell-mediated immunity.
• Misconception: A vaccine is an example of passive immunity. Correct: Vaccination induces active immunity. Injecting ready-made antibodies, as for tetanus or snakebite, is passive immunisation.
• Misconception: The antibodies in allergy are IgG. Correct: The antibodies produced to allergens are of the IgE type.
• Misconception: HIV spreads by touching or living with an infected person. Correct: HIV spreads only through body fluids, not by mere touch or physical contact.
• Misconception: Benign tumours spread through the body. Correct: Benign tumours remain confined to their original location. Malignant tumours show metastasis.

Exam-style questions with model answers

Q1. Expand MALT, CMI, AIDS, NACO and HIV. [1 mark]

1. MALT is mucosa-associated lymphoid tissue; CMI is cell-mediated immunity; AIDS is Acquired Immuno Deficiency Syndrome; NACO is the National AIDS Control Organisation; HIV is the Human Immuno deficiency Virus.

Q2. Name the primary and secondary lymphoid organs. [2 marks]

1. The primary lymphoid organs are the bone marrow and the thymus, where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
2. The secondary lymphoid organs are the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix, where lymphocytes interact with antigens.

Q3. How is a cancerous cell different from a normal cell? What is metastasis? [2 marks]

1. Normal cells show contact inhibition, so contact with other cells stops their uncontrolled growth; cancer cells have lost this property and keep dividing to form tumours.
2. Metastasis is the spread of cells sloughed from a malignant tumour through the blood to distant sites, where they start new tumours.

Q4. Differentiate between innate and acquired immunity, with examples. [3 marks]

1. Innate immunity is non-specific and present at birth. It works through physical barriers such as skin and mucus, physiological barriers such as stomach acid and tears, cellular barriers such as neutrophils and macrophages, and cytokine barriers such as interferons.
2. Acquired immunity is pathogen specific and characterised by memory: a primary response of low intensity on first encounter, and an intensified secondary response on the next.
3. It is carried out by B-lymphocytes, which produce antibodies (humoral response), and T-lymphocytes, which mediate cell-mediated immunity; examples are the antibodies made against a pathogen and the rejection of a grafted organ by the cell-mediated response.

Q5. Differentiate between active and passive immunity, with examples. [3 marks]

1. Active immunity arises when the host is exposed to antigens and produces its own antibodies; it is slow to give its full response. Examples are immunisation and natural infection.
2. Passive immunity is given by ready-made antibodies; it acts at once.
3. Examples of passive immunity are IgA in colostrum, antibodies passed through the placenta, and injected antitoxin for tetanus or antibodies against snake venom.

Q6. How are amoebiasis, malaria and pneumonia transmitted? [3 marks]

1. Amoebiasis: drinking water and food contaminated by faecal matter; houseflies carry the parasite from faeces to food.
2. Malaria: the bite of an infected female Anopheles mosquito, which introduces sporozoites.
3. Pneumonia: inhaling droplets or aerosols released by an infected person, or sharing glasses and utensils with one.

Q7. Describe the life cycle of Plasmodium in the human and the mosquito. [5 marks]

1. Plasmodium enters the human body as sporozoites, the infectious form, through the bite of an infected female Anopheles mosquito.
2. The parasites first multiply within the liver cells, and then attack the red blood cells, which rupture.
3. The rupture releases haemozoin, which causes the chill and high fever recurring every three to four days.
4. When a female Anopheles bites an infected person, the parasites enter the mosquito and develop further; they multiply to form sporozoites, which are stored in the salivary glands.
5. When this mosquito bites a human, the sporozoites are introduced and the cycle repeats. The parasite needs two hosts, and the mosquito is also the vector.

Q8. Explain the mechanism by which HIV causes deficiency of the immune system. [5 marks]

1. HIV, a retrovirus with an RNA genome, enters macrophages, where its RNA is copied into viral DNA by the enzyme reverse transcriptase.
2. The viral DNA is incorporated into the host cell's DNA and directs the cell to produce virus particles, so the macrophages act like an HIV factory.
3. HIV also enters helper T-lymphocytes, replicates, and releases progeny viruses into the blood, which attack other helper T-lymphocytes.
4. This is repeated, so the number of helper T-lymphocytes falls progressively, and the person suffers bouts of fever, diarrhoea and weight loss.
5. The person becomes so immuno-deficient that infections by bacteria such as Mycobacterium, viruses, fungi and parasites such as Toxoplasma can no longer be overcome.

Key takeaways

• Health is a state of complete physical, mental and social well-being, affected by genetic disorders, infections and life style.
• Typhoid, pneumonia, common cold, malaria, amoebiasis, ascariasis, filariasis and ringworm are caused by bacteria, viruses, protozoans, helminths and fungi.
• Plasmodium enters as sporozoites, multiplies in the liver and then in red blood cells, and haemozoin released on their rupture causes the recurring fever.
• Innate immunity is non-specific and uses physical, physiological, cellular and cytokine barriers; acquired immunity is specific and has memory.
• B-lymphocytes produce antibodies (H₂L₂), and T-lymphocytes mediate cell-mediated immunity, which causes graft rejection.
• Vaccines give active immunity through memory cells; ready-made antibodies such as antitoxins and colostrum give passive immunity.
• HIV destroys helper T-lymphocytes after copying its RNA into DNA with reverse transcriptase; AIDS is diagnosed by ELISA and has no cure.
• Cancer cells lose contact inhibition; malignant tumours metastasise, and carcinogens may be physical, chemical or biological.

Test yourself

Which test confirms typhoid fever?

Typhoid fever caused by Salmonella typhi is confirmed by the Widal test.

Which species of Plasmodium causes malignant malaria?

Plasmodium falciparum causes malignant malaria, the most serious form, which can even be fatal.

Which fish is introduced into ponds to control mosquitoes?

Gambusia fishes are introduced into ponds because they feed on mosquito larvae.

How many chains does an antibody molecule have?

An antibody has four peptide chains, two light and two heavy, so it is represented as H₂L₂.

Which chemicals released from mast cells cause allergy?

Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.

Give an example of an auto-immune disease.

Rheumatoid arthritis is an auto-immune disease, in which the body attacks its own cells.

What is the usual time-lag between HIV infection and AIDS symptoms?

The time-lag may vary from a few months to many years, usually 5 to 10 years.

From which plant is cocaine obtained, and what does it interfere with?

Cocaine is obtained from the coca plant Erythroxylum coca, and it interferes with the transport of the neurotransmitter dopamine.