What this chapter is about
Health is a state of complete physical, mental and social well-being, not merely the absence of disease or infirmity. This chapter explores how the human body maintains health, what happens when pathogens or abnormal conditions disrupt normal functioning, and how the immune system defends against threats. You will study common infectious diseases caused by bacteria, viruses, protozoans and helminths, along with their modes of transmission, symptoms and prevention.
Beyond infectious diseases, the chapter covers immunity in detail — the innate and acquired types, the roles of antibodies and lymphocytes, vaccination, and disorders of the immune system such as allergies and autoimmune conditions. You will also learn about AIDS, its causative virus HIV, and how it weakens immunity. The chapter addresses cancer as a disease of uncontrolled cell division, and examines the effects of drugs and alcohol on health.
After studying this chapter, you should be able to distinguish between different types of pathogens and diseases, explain how immunity develops, describe the structure and action of antibodies, and understand the biological basis of addiction and cancer.
Key ideas
- Health is influenced by genetic disorders, infections, lifestyle and environmental factors; maintaining a balanced diet, personal hygiene and regular exercise supports good health.
- Pathogens are disease-causing organisms; bacteria (typhoid, pneumonia), viruses (common cold, AIDS), protozoans (malaria, amoebiasis) and helminths (ascariasis, filariasis) cause infectious diseases that spread through air, water, vectors or direct contact.
- Innate immunity is the non-specific first line of defence present from birth, including physical barriers (skin, mucous membranes), physiological barriers (stomach acid, fever) and cellular barriers (phagocytes, natural killer cells).
- Acquired (adaptive) immunity develops after exposure to a pathogen or vaccine; it involves B-lymphocytes producing antibodies (humoral immunity) and T-lymphocytes attacking infected cells (cell-mediated immunity), and it generates immunological memory.
- Antibodies (immunoglobulins) are Y-shaped proteins with variable regions that bind specific antigens; the five classes are IgG, IgA, IgM, IgD and IgE, each with distinct functions.
- Vaccination introduces weakened or inactivated pathogens (or their antigens) to stimulate memory cells without causing disease, providing active acquired immunity; passive immunity is short-lived and obtained by receiving ready-made antibodies.
- AIDS is caused by the Human Immunodeficiency Virus (HIV), a retrovirus that infects helper T-lymphocytes (CD4⁺ cells), progressively destroying immunity and making the body vulnerable to opportunistic infections.
- Cancer results from uncontrolled cell division due to activation of oncogenes or inactivation of tumour suppressor genes; benign tumours remain localised, while malignant tumours (cancers) can metastasise to other body parts.
Formulas and facts to remember
- Definition of health (WHO): A state of complete physical, mental and social well-being, not merely the absence of disease.
- Typhoid: Caused by Salmonella typhi; spreads through contaminated food and water; diagnosed by Widal test.
- Malaria: Caused by Plasmodium species (P. vivax, P. falciparum, etc.); transmitted by female Anopheles mosquito; parasites multiply in liver and red blood cells, causing recurring fever.
- Primary and secondary immune response: Primary response is slow with low antibody titre; secondary response (on re-exposure) is faster and stronger due to memory cells.
- HIV structure: RNA retrovirus with two single-stranded RNA molecules, reverse transcriptase enzyme, and a protein coat surrounded by an envelope with glycoprotein spikes (gp120, gp41).
- CD4⁺ T-cell count: Normal count is 800–1200 cells per microlitre of blood; AIDS is diagnosed when count falls below 200 cells per microlitre.
- Carcinogens: Physical agents (UV rays, X-rays), chemical agents (tobacco smoke components, aflatoxins) and biological agents (certain viruses like HPV, hepatitis B) can initiate cancer.
- Drugs of abuse: Opioids (heroin, morphine) bind opioid receptors; cannabinoids (from Cannabis) affect cardiovascular system; cocaine stimulates the nervous system; all lead to dependence and withdrawal symptoms.
Worked examples
Example 1: Identifying the type of immunity
A newborn receives antibodies from the mother through the placenta and later through breast milk (colostrum). What type of immunity is this? Will it last throughout life?
Solution
Step 1: The baby does not produce these antibodies itself; they are transferred from the mother.
Step 2: Since ready-made antibodies are received rather than produced by the baby's own immune system, this is passive immunity.
Step 3: Passive immunity is temporary because the antibodies degrade over time and no memory cells are formed.
Answer: The newborn has passive acquired immunity. It lasts only a few weeks to months and does not provide lifelong protection.
Example 2: Analysing the malaria life cycle
A patient in a village near a stagnant pond develops high fever every 48 hours. Blood smear examination shows ring-shaped structures inside red blood cells. Identify the likely pathogen and explain why fever recurs at regular intervals.
Solution
Step 1: Ring-stage parasites inside RBCs and a history of proximity to stagnant water (mosquito breeding site) suggest malaria.
Step 2: The pathogen is Plasmodium (likely P. vivax or P. falciparum).
Step 3: After infecting liver cells, the parasite enters RBCs and undergoes asexual reproduction (schizogony). When RBCs rupture synchronously to release new parasites (merozoites), haemozoin (a toxic pigment) is released, triggering fever.
Step 4: The 48-hour cycle corresponds to the time taken for one round of multiplication inside RBCs (in P. vivax or P. falciparum).
Answer: The pathogen is Plasmodium. Fever recurs every 48 hours because RBCs rupture synchronously at the end of each erythrocytic cycle, releasing toxins that induce fever.
Example 3: Understanding drug dependence
A teenager starts using a sedative occasionally to relieve exam stress. After several months, he needs larger doses for the same effect and feels restless without the drug. Explain the biological basis of these observations.
Solution
Step 1: Repeated use leads to tolerance: the body's receptors become less responsive, so higher doses are needed for the same effect.
Step 2: The body adapts to the presence of the drug. When the drug is absent, normal functioning is disturbed, causing withdrawal symptoms (restlessness, anxiety).
Step 3: Together, tolerance and withdrawal indicate physical dependence. Psychological craving to take the drug again is psychological dependence.
Answer: Tolerance explains the need for larger doses; withdrawal symptoms explain restlessness without the drug. Both indicate the development of drug dependence.
Common mistakes
- Confusing active and passive immunity → Active immunity involves the body producing its own antibodies (takes time, long-lasting); passive immunity involves receiving pre-formed antibodies (immediate, short-lived).
- Thinking all tumours are cancerous → Benign tumours are non-cancerous and do not spread; only malignant tumours metastasise and are called cancer.
- Believing antibiotics work against viral infections → Antibiotics target bacterial cell structures; viruses use host machinery and are unaffected by antibiotics.
- Mixing up vectors and reservoirs → A vector transmits the pathogen (e.g., mosquito for malaria); a reservoir harbours the pathogen between outbreaks (e.g., humans for typhoid).
- Assuming vaccination gives passive immunity → Vaccination stimulates the body to produce its own antibodies and memory cells, so it provides active acquired immunity.
Quick revision
- Innate immunity is non-specific and present from birth; acquired immunity is specific and develops after exposure.
- B-cells produce antibodies (humoral); T-cells destroy infected cells (cell-mediated).
- HIV attacks helper T-cells (CD4⁺), weakening immunity and leading to AIDS.
- Malaria is caused by Plasmodium, transmitted by female Anopheles mosquito; fever recurs when RBCs rupture.
- Cancer involves uncontrolled cell division; oncogenes promote it, tumour suppressor genes normally prevent it.
- Drug dependence involves tolerance (need more) and withdrawal symptoms (discomfort without drug).