Diseases And Immunity

Pathogens & Barriers

• A **pathogen** is a disease-causing organism that causes **transmissible diseases**.
• Pathogens spread via **direct contact** (e.g., HIV through body fluids) or **indirect contact** (e.g., airborne droplets).
• **Viruses** (e.g., cold, flu) are not living; they consist of **genetic material (RNA/DNA)** inside a **protein coat** and hijack host cells to replicate.
• **Mechanical barriers**: **skin** (physical barrier, heals with scabs), **nose hairs** (trap pathogens).
• **Chemical barriers**: **mucus** (traps pathogens, removed by coughing/sneezing), **stomach acid** (HCl kills swallowed pathogens).
• **White blood cells** defend by **phagocytosis** (engulfing pathogens) and producing **antibodies** (cause agglutination and signal destruction).

Controlling the Spread of Disease

• **Clean water supply** prevents waterborne diseases like cholera.
• **Hygienic food preparation** (washing hands, proper cooking) prevents contamination by bacteria/fungi.
• **Personal hygiene** (soap, tissues for sneezes/coughs) reduces transmission of colds and flu.
• **Waste disposal** (e.g., food waste) reduces vectors like flies.
• **Sewage treatment** removes pathogens from waste before release into environment.

Active Immunity

• **Active immunity** is the production of antibodies and **memory cells** after exposure to a pathogen.
• It can be acquired naturally (infection) or artificially (**vaccination**).
• Active immunity is **slow-acting** but provides **long-lasting protection**.

Antigens & Antibodies

• **Antigens** are molecules (e.g., proteins) on cell surfaces that are recognised as foreign by lymphocytes.
• **Antibodies** are proteins produced by lymphocytes that are **complementary in shape** to specific antigens.
• Antibodies cause **agglutination** (clumping) of pathogens, making them easier for **phagocytes** to engulf and destroy.
• After infection, **memory cells** retain antibody instructions for a faster, stronger response on re-exposure.
• Some pathogens **mutate** quickly, changing antigens so memory cells may not recognise them.

Vaccination

• Vaccines contain **dead or weakened pathogens** with specific antigens, unable to cause disease.
• Vaccination triggers an **immune response**: lymphocytes produce antibodies and **memory cells**.
• Memory cells provide **long-lasting immunity** without the risk of severe illness.
• **Herd immunity** occurs when a large percentage of the population is vaccinated, protecting the unvaccinated by reducing pathogen spread.
• Vaccination programmes aim to **eradicate** diseases (e.g., smallpox eradicated in 1980).

Preventing the Spread of Disease

• Vaccination helps control spread; not all diseases have vaccines.
• Immune individuals cannot catch or spread the disease.
• **Herd immunity** prevents epidemics/pandemics by limiting pathogen breeding opportunities.
• If vaccination rates drop, outbreaks can occur, increasing infections and deaths.

An animal cell showing organelles involved in antibody production (e.g., ribosomes, Golgi).

A bacterium, a typical prokaryotic pathogen.

General structure of an amino acid, the building block of antibodies.