Class 12 Biology Notes

Unit VIII: Biology and Human Welfare · Chapter 7

Human Health and Disease

Common diseases in humans

Disease Germ (pathogen) How it spreads Main symptoms and special facts
Typhoid Salmonella typhi (bacterium) Contaminated food and water High fever (39–40 °C), weakness, stomach pain. Widal test confirms it. Famous carrier: "Typhoid Mary"
Pneumonia Streptococcus pneumoniae, Haemophilus influenzae (bacteria) Droplets from coughs and sneezes; sharing glasses Alveoli fill with fluid, so breathing is hard. In severe cases, lips and fingernails turn grey to bluish
Common cold Rhinoviruses Droplets; touching contaminated things Blocked nose, sore throat, cough. Lasts 3–7 days. Affects the nose and airways, not the lungs
Malaria Plasmodium (protozoan): P. vivax, P. malariae, P. falciparum (most dangerous) Bite of the female Anopheles mosquito Chills and high fever every 3–4 days
Amoebiasis Entamoeba histolytica (protozoan) Houseflies carry it to food and water Constipation, cramps, stools with mucus and blood
Ascariasis Ascaris (roundworm) Eggs in soil, water, vegetables Internal bleeding, anaemia, blocked intestine
Filariasis (elephantiasis) Wuchereria bancrofti, W. malayi (worms) Bite of the female Culex mosquito Swelling of the legs (and genital organs)
Ringworm Microsporum, Trichophyton, Epidermophyton (fungi) Towels, combs, soil Dry, scaly, itchy patches on skin, nails, scalp. Grows in warm, moist body folds

The malaria parasite's life cycle

  1. An infected female Anopheles bites → sporozoites enter the blood.
  2. Sporozoites reach the liver and multiply.
  3. They then attack red blood cells and multiply there.
  4. RBCs burst and release haemozoin → chills and high fever.
  5. Some parasites become gametocytes (male and female).
  6. A mosquito bites the patient and sucks up the gametocytes.
  7. Fertilisation happens in the mosquito's stomach.
  8. New sporozoites form and move to the mosquito's salivary glands, ready to infect the next person.
  • Humans are the intermediate host (asexual reproduction happens in us).
  • The mosquito is the definitive host (sexual reproduction happens there).

Preventing infectious diseases

  • Personal hygiene: clean body, clean drinking water, clean food.
  • Public hygiene: proper disposal of waste and sewage.
  • Control mosquitoes: no stagnant water; spray insecticides; use mosquito nets and screens.
  • Put the fish Gambusia in ponds. It eats mosquito larvae.
  • Vaccines and antibiotics.

Immunity: the body's army

  • Innate immunity (inborn, non-specific):
    • Physical barrier: skin, mucus
    • Physiological barrier: stomach acid, saliva, tears
    • Cellular barrier: neutrophils, monocytes, natural killer cells, macrophages
    • Cytokine barrier: interferons
  • Acquired immunity (specific, has memory):
    • Humoral: B cells make antibodies
    • Cell-mediated: T cells attack directly

Innate immunity: 4 barriers

  • Physical barrier: skin, and the mucus lining of the nose, gut and airways, which traps germs.
  • Physiological barrier: stomach acid, saliva and tears kill germs.
  • Cellular barrier: white blood cells like PMNL-neutrophils, monocytes, natural killer cells and macrophages eat or kill germs.
  • Cytokine barrier: virus-infected cells release interferons, which protect nearby cells from the virus.

Acquired immunity

  • It is specific (one germ at a time) and has memory.
  • Primary response: the first time we meet a germ, the response is slow and weak.
  • Secondary response: the next time, the response is fast and strong, thanks to memory cells. This is also called the anamnestic response.
B-lymphocytes T-lymphocytes
Make antibodies that float in the blood Don't make antibodies; they help B cells and kill infected cells
Humoral immunity Cell-mediated immunity (CMI)
Responsible for graft (organ transplant) rejection

Structure of an antibody

  • Each antibody has 4 chains: 2 heavy + 2 light, written as H₂L₂.
  • Types of antibodies: IgA, IgM, IgE, IgG (and IgD).

Active vs passive immunity

Active immunity Passive immunity
Your body makes its own antibodies Ready-made antibodies are given
After an infection or a vaccine Colostrum (mother's first milk), antibodies from mother to baby through the placenta, anti-tetanus injection, snake anti-venom
Slow to start, long-lasting Fast, but short-lived

When immunity goes wrong

  • Allergy: the body over-reacts to harmless things (allergens) like dust, pollen or animal hair.
    • Involves IgE antibodies and chemicals like histamine and serotonin from mast cells.
    • Treated with antihistamines, adrenaline and steroids.
  • Autoimmunity: the body attacks its own cells. Example: rheumatoid arthritis.

Lymphoid organs

Primary lymphoid organs Secondary lymphoid organs
Where lymphocytes are made and mature Where lymphocytes meet germs and multiply
Bone marrow and thymus Spleen, lymph nodes, tonsils, Peyer's patches (in the small intestine) and appendix
  • MALT (mucosa-associated lymphoid tissue) makes up about 50% of all lymphoid tissue in the body.

AIDS

  • AIDS = Acquired Immuno Deficiency Syndrome. It is caused by HIV, a retrovirus (it has RNA as its genetic material).
  • Spreads through: unsafe sex, infected blood, sharing needles, and from an infected mother to her baby.
  • Does NOT spread by: touching, hugging, sharing food or mosquito bites.

How HIV destroys the immune system

  1. HIV enters the body and gets into a macrophage.
  2. Reverse transcriptase makes viral DNA from the viral RNA.
  3. The viral DNA joins the host DNA; the macrophage keeps making new viruses (an "HIV factory").
  4. The new viruses attack helper T-cells.
  5. The number of helper T-cells keeps falling.
  6. The immune system fails → infections such as TB (Mycobacterium) and Toxoplasma.
  • Test: ELISA (enzyme-linked immunosorbent assay).
  • Treatment: anti-retroviral drugs only slow down the disease; they don't cure it.
  • Awareness: NACO (National AIDS Control Organisation) and NGOs educate people. World AIDS Day is 1 December.

Cancer

Benign tumour Malignant tumour
Stays in one place Spreads to other body parts (metastasis)
Usually less harmful Dangerous: this is what we call cancer
  • Causes (carcinogens):
    • Physical: X-rays, gamma rays (ionising radiation) and UV rays (non-ionising)
    • Chemical: tobacco smoke and other chemicals
    • Biological: oncogenic viruses. Genes called proto-oncogenes (c-onc) in normal cells can also turn into cancer genes.
  • Detection: biopsy, blood tests, X-ray, CT scan, MRI, and antibodies against cancer antigens.
  • Treatment: surgery, radiation therapy, chemotherapy and immunotherapy (e.g. α-interferon, which boosts the immune system).

Drugs and alcohol abuse

Drug type Examples Source Effect
Opioids Heroin ("smack") = diacetylmorphine Latex of the poppy plant (Papaver somniferum) Depressant: slows body functions
Cannabinoids Marijuana, hashish, charas, ganja Cannabis sativa Affect the heart and blood vessels
Coca alkaloid Cocaine ("coke", "crack") Erythroxylum coca Interferes with dopamine; causes excitement, then hallucinations
Hallucinogens — Datura, Atropa belladonna Hallucinations
  • Barbiturates, amphetamines, benzodiazepines and LSD are sometimes used as medicines, but are often misused.
  • Tobacco: contains nicotine, which makes the adrenal gland release adrenaline, raising blood pressure and heart rate. Smoking increases carbon monoxide in the blood, so less oxygen reaches the body.

Why teens are at risk, and how to stay safe

  • Adolescence (about 12–18 years) is a time of curiosity, stress and peer pressure.
  • Warning signs: falling grades, skipping school, losing interest in hobbies, mood swings, changes in sleep and eating.
  • How to prevent it:
    1. Avoid undue peer pressure. It's OK to say NO.
    2. Education and counselling.
    3. Talk to parents and friends.
    4. Watch for danger signs.
    5. Get professional and medical help.

30-second revision

  • Typhoid → Salmonella typhi, Widal test. Malaria → Plasmodium, female Anopheles, haemozoin.
  • Filariasis → Wuchereria, Culex. Ringworm → fungi.
  • Gambusia fish eats mosquito larvae.
  • Innate barriers: physical, physiological, cellular, cytokine (PPCC).
  • Antibody = H₂L₂. B cells → antibodies; T cells → CMI and graft rejection.
  • HIV → macrophage → helper T cells ↓. ELISA test. 1 December.
  • Cancer: contact inhibition lost; malignant tumours spread (metastasis).