Diseases and Immunity Revision Notes

Pathogens and Transmissible Diseases

Definition of a Pathogen: A pathogen is defined as a disease-causing organism.
Transmissible Diseases: Because pathogens are passed from one host to another, the diseases they cause are classified as transmissible diseases.
Methods of Transmission: - Direct Contact: The pathogen is transferred directly from one host to another through the transfer of body fluids. - Examples of fluids: Blood or semen. - Examples of diseases: HIV, gonorrhoea, hepatitis B, and hepatitis C. - Indirect Contact: The pathogen leaves the host and is carried by an intermediate medium or a vector. - Intermediate media/vectors: Contaminated surfaces, air (via droplets or aerosols), food, water, or organisms such as insects. - Specific insect examples: Mosquitoes carrying malaria or dengue fever.

The Body’s Defense Systems

The body utilizes three primary mechanisms to defend itself against disease: mechanical barriers, chemical barriers, and specialized cells.

1. Mechanical Barriers: These are physical structures that make it difficult for pathogens to enter the body. - Skin: Covers almost all parts of the body. If the skin is cut or grazed, it immediately begins to heal by forming a scab to prevent infection. - Hairs in the Nose: These trap pathogens to prevent them from being inhaled further into the lungs.
2. Chemical Barriers: These are substances produced by body cells to trap or kill pathogens. - Mucus: Produced in various locations; it traps pathogens which are then removed by coughing, blowing the nose, or swallowing. - Stomach Acid: Contains hydrochloric acid ( $HCl$ ). It is strong enough to kill pathogens caught in swallowed mucus or those consumed in contaminated food and water.
3. Cells: Different types of white blood cells prevent pathogens from reaching areas where they can replicate. - Phagocytosis: The process of engulfing and digesting pathogenic cells. - Antibody Production: Lymphocytes produce antibodies that clump pathogenic cells together (agglutination) and release chemicals to signal other cells for destruction.

Controlling the Spread of Disease

The simplest way to prevent disease is to stop pathogens from spreading through hygiene, sanitation, and waste disposal.

Clean Water Supply: Ensures drinking water is free of pathogens to prevent waterborne diseases like cholera.
Hygienic Food Preparation: Includes washing hands with soap, using correct cooking methods, and hygienic preparation to avoid bacterial and fungal contamination.
Personal Hygiene: Using soap for washing and tissues for coughs and sneezes (disposing of them immediately) reduces the transmission of colds and flu.
Waste Disposal: Proper disposal of food waste reduces pests like flies that act as vectors.
Sewage Treatment: Removes harmful pathogens from waste before environmental release, reducing the risk of spreading disease via contaminated water.

Active Immunity: Antigens and Antibodies

Active Immunity: This involves the body making its own antibodies and developing memory cells for future responses. It is slow-acting but provides long-lasting protection. - Triggers for Active Immunity: Infection by a pathogen or vaccination.
Antigens: Molecules (usually proteins) projecting from cell membranes. Different individuals have different antigens on their cell surfaces.
Antibodies: Proteins produced by lymphocytes that are complementary in shape to specific antigens on a pathogen.
The Role of Antibodies and Phagocytes: - Antibodies attach to antigens, causing agglutination (clumping). - Agglutination prevents pathogens from moving easily and allows phagocytes to engulf many pathogens at once. - Chemicals are released to signal phagocytes to move toward the infection site to destroy the pathogens.
Terminology Distinctions: - Antigen: A molecule on the surface of a cell. - Antibody: A protein made by lymphocytes that is complementary to an antigen, causing clumping and facilitating destruction. - Antitoxin: A protein that neutralises toxins produced by bacteria.
Memory Cells: After the initial encounter (which may take a few days and lead to illness), lymphocytes produce memory cells. If reinfection occurs with the same pathogen, antibodies are produced more quickly and in greater quantities, destroying the pathogen before symptoms appear.
Antigenic Mutation: Some microorganisms mutate quickly, changing their surface antigens. In these cases, existing memory cells will not recognize the new antigens, and immunity is lost.

Vaccination and Herd Immunity

Mechanism of Vaccination: A dead or altered form of a pathogen (containing specific antigens) is introduced into the body. It cannot cause illness but provokes an immune response.
Result: Lymphocytes produce complementary antibodies and memory cells, providing long-term immunity without the risk of the actual disease.
Herd Immunity: When a large percentage of a population is vaccinated, it protects the entire population. There are fewer hosts for the pathogen to breed in, protecting even the unvaccinated.
Consequences of Low Vaccination Rates: If vaccination rates drop, the risk of mass infection and death increases as the pathogen spreads more easily through the population.
Eradication: Some vaccination programs aim to completely eliminate diseases. - Example: Smallpox was officially eradicated in 1980 following a World Health Organisation (WHO) program that began in the mid-1950s.

Passive Immunity and Breastfeeding

Passive Immunity: A fast-acting, short-term defense acquired from another individual.
Process: The body does not produce its own antibodies or memory cells.
Breastfeeding: Antibodies pass from mother to infant via breast milk. This is critical for helping young infants fight infections while their own immune systems are still developing.

Cholera and Diarrhoea

Diarrhoea: The loss of watery faeces from the anus. Severe or prolonged diarrhoea can lead to death due to the significant loss of water and ions, which causes organ failure.
Treatment: Oral Rehydration Therapy, which is a drink consisting of a small amount of salt and sugar dissolved in water.
Pathogenesis of Cholera: - Caused by the bacterium Vibrio cholerae. - The bacteria are ingested via infected water or food and reach the small intestine. - Step-by-Step Process of Diarrhoea in Cholera: 1. Bacteria attach to the wall of the small intestine. 2. They produce a specific toxin. 3. The toxin stimulates lining cells to release chloride ions ( $Cl^-$ ) into the intestinal lumen. 4. Chloride ions accumulate, lowering the water potential in the lumen. 5. Water moves out of the cells and into the intestine by osmosis (moving from higher water potential to lower water potential). 6. Large quantities of water leave the body as watery faeces. 7. This results in the blood containing too few chloride ions and too little water.