Human Health and Disease

In this chapter: common diseases and pathogens; malaria life cycle; prevention; innate and acquired immunity; antibodies; active and passive immunity; vaccination; allergies; autoimmunity; lymphoid organs; AIDS; cancer; drugs, alcohol and tobacco.

Health and disease

Early ideas (Hippocrates, Ayurveda) linked health to a balance of body "humours". William Harvey, who discovered blood circulation and used the thermometer to show normal body temperature, disproved the "good humour" idea. Diseases are infectious (spread from person to person) or non-infectious (cancer is a major non-infectious cause of death).

Common diseases in humans

Other bacterial diseases include dysentery, plague and diphtheria. Dengue and chikungunya are spread by Aedes mosquitoes.

Life cycle of Plasmodium

1. An infected female Anopheles bites a person and injects sporozoites (the infectious form).
2. They multiply first in liver cells, then attack RBCs, causing them to rupture.
3. RBC rupture releases a toxin, haemozoin, which causes chills and high fever recurring every 3 to 4 days.
4. Sexual stages (gametocytes) develop in the RBCs.
5. A female Anopheles biting an infected person takes up gametocytes; fertilisation and development occur in the mosquito's gut.
6. Sporozoites form and are stored in the mosquito's salivary glands, ready to infect the next person.

So Plasmodium needs two hosts: humans and mosquitoes. The female Anopheles is also the vector.

Prevention and control

• Personal hygiene: clean body, clean drinking water, food, vegetables and fruit.
• Public hygiene: proper disposal of waste and excreta; periodic cleaning and disinfection of water reservoirs, pools, cesspools and tanks; hygiene in public catering.
• Airborne diseases (pneumonia, common cold): avoid close contact with infected people and their belongings.
• Vector-borne diseases (malaria, filariasis, dengue, chikungunya): avoid stagnant water; clean household coolers; use mosquito nets and wire mesh; introduce fish like Gambusia, which eat mosquito larvae; spray insecticides in ditches and swamps.
• Vaccines have eradicated smallpox and largely controlled polio, diphtheria, pneumonia and tetanus.

Immunity

Innate immunity (non-specific, present at birth)

Acquired immunity (pathogen-specific, with memory)

• The first encounter produces a low-intensity primary response; later encounters give a highly intensified secondary (anamnestic) response.
• B-lymphocytes produce antibodies into the blood: the humoral immune response. T-lymphocytes do not secrete antibodies but help B-cells produce them, and mediate cell-mediated immunity (CMI).
• An antibody has four polypeptide chains: two light and two heavy (H₂L₂). Types: IgA, IgM, IgE, IgG.
• Graft rejection is due to CMI, since the body distinguishes self from non-self. Tissue and blood group matching are needed before transplants, and patients take immunosuppressants for life.

Active and passive immunity

Vaccination and immunisation

Vaccination is based on immune memory. A vaccine (antigenic proteins or inactivated/weakened pathogen) produces antibodies and memory B and T cells. When a quick response is needed (tetanus, snake bite), preformed antibodies or antitoxins are injected: passive immunisation. Recombinant DNA technology makes antigenic polypeptides in bacteria or yeast, allowing large-scale production; the hepatitis B vaccine is produced from yeast.

Allergies and autoimmunity

• Allergy: an exaggerated immune response to environmental antigens (allergens) such as dust mites, pollen and animal dander. Antibodies of the IgE type are produced; mast cells release histamine and serotonin, causing sneezing, watery eyes, running nose and difficulty in breathing. Anti-histamines, adrenaline and steroids reduce symptoms. Protected early-life environments in cities may increase sensitivity.
• Autoimmunity: due to genetic and other reasons, the body attacks its own cells. Example: rheumatoid arthritis.

The immune system

• Primary lymphoid organs: bone marrow and thymus, where immature lymphocytes become antigen-sensitive. Bone marrow produces all blood cells. The thymus is large at birth and shrinks with age, becoming very small by puberty.
• Secondary lymphoid organs: spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix, where lymphocytes meet antigens and multiply into effector cells.
• Spleen: bean-shaped; filters blood by trapping blood-borne microbes; a large reservoir of erythrocytes.
• Lymph nodes: trap microbes and antigens in lymph and tissue fluid, activating lymphocytes.
• MALT (mucosa-associated lymphoid tissue) in the linings of the respiratory, digestive and urogenital tracts makes up about 50% of lymphoid tissue.

AIDS

• Acquired Immuno Deficiency Syndrome, first reported in 1981; has killed more than 25 million people worldwide.
• Caused by HIV, a retrovirus with an envelope around an RNA genome.
• Spread by sexual contact, contaminated blood and blood products, shared needles (intravenous drug users), and from an infected mother to her child through the placenta. Not spread by touch or casual contact, only through body fluids.
• The gap between infection and symptoms ranges from a few months to many years (usually 5 to 10 years).

How HIV works

1. HIV enters macrophages; its RNA is copied into DNA by reverse transcriptase.
2. Viral DNA joins the host DNA and directs production of new viruses, so macrophages act as an "HIV factory".
3. HIV enters helper T-lymphocytes (T_H), replicates and attacks more T_H cells, steadily reducing their number.
4. The person has bouts of fever, diarrhoea and weight loss, then infections normally easily fought off: Mycobacterium, viruses, fungi and parasites like Toxoplasma.

Diagnosis: ELISA (enzyme-linked immunosorbent assay). Anti-retroviral drugs only partially help; they prolong life but cannot prevent death. Prevention is the best option: safe blood, disposable needles and syringes, free condoms, controlling drug abuse, safe sex and regular check-ups in susceptible groups. In India, NACO (National AIDS Control Organisation) and NGOs lead awareness. 1 December is World AIDS Day.

Cancer

• Normal cells show contact inhibition; cancer cells have lost it and divide uncontrollably.
• Benign tumours stay in their original location and cause little damage. Malignant tumours grow rapidly, invade surrounding tissue and starve normal cells. Cells that break away travel through blood and start new tumours elsewhere: metastasis, the most feared property.
• Carcinogens: ionising radiation (X-rays, gamma rays), non-ionising radiation (UV), chemicals in tobacco smoke (a major cause of lung cancer), and oncogenic viruses with viral oncogenes. Normal cells carry proto-oncogenes (cellular oncogenes, c-onc), which can cause cancer when activated.
• Detection: biopsy and histopathology; blood and bone marrow tests (leukaemia); radiography (X-rays), CT (X-rays, 3-D image) and MRI (strong magnetic fields and non-ionising radiation); antibodies against cancer-specific antigens; molecular tests for inherited susceptibility.
• Treatment: surgery, radiation therapy, chemotherapy (side effects such as hair loss and anaemia) and immunotherapy. Tumour cells can evade the immune system, so patients may be given biological response modifiers such as α-interferon.

Drugs, alcohol and tobacco

• Adolescence is the period of 12 to 18 years, the bridge between childhood and adulthood, and the age most vulnerable to experimenting with drugs.
• Withdrawal syndrome: anxiety, shakiness, nausea and sweating when regular use stops.
• Effects of abuse: reckless behaviour, vandalism, violence; high doses can cause coma and death by respiratory failure, heart failure or cerebral haemorrhage. Injecting drugs spreads AIDS and hepatitis B. Chronic use damages the nervous system and liver (cirrhosis), and harms the foetus in pregnancy.
• Anabolic steroids misused in sport: in females, masculinisation, aggressiveness, mood swings, abnormal menstrual cycles, excess facial hair, enlarged clitoris and deep voice; in males, acne, aggressiveness, smaller testicles, less sperm, kidney and liver problems, breast enlargement, early baldness and enlarged prostate; in adolescents, premature closure of growth centres of long bones (stunted growth).
• Prevention: avoid undue peer pressure; education and counselling; help from parents and peers; watching for danger signs; professional and medical help.

Common traps: (1) Common cold does not infect the lungs; pneumonia does. (2) Haemozoin causes the malarial fever; fertilisation of Plasmodium occurs in the mosquito. (3) Filariasis is spread by Culex; dengue by Aedes. (4) Colostrum gives passive immunity (IgA). (5) HIV attacks helper T cells, using macrophages as a factory. (6) Cocaine affects dopamine transport; heroin is a depressant.

NEET focus

• Disease-pathogen-vector table; Widal test; Typhoid Mary; Gambusia.
• Plasmodium life cycle order.
• Barriers of innate immunity; humoral vs CMI; antibody structure; active vs passive; hepatitis B vaccine from yeast.
• IgE, mast cells, histamine; primary vs secondary lymphoid organs; MALT 50%.
• HIV mechanism and ELISA; cancer terms and detection methods; drug sources and effects.

Practice questions