Infection and Response

1. What are Pathogens?

  • Pathogens: Microorganisms that cause infectious diseases.

    • They reproduce rapidly inside the body and produce toxins or damage cells.

  • Types of Pathogens:

    • Viruses: Much smaller than bacteria. Reproduce only inside living cells, causing cell damage. Examples: Measles, HIV, Tobacco Mosaic Virus (TMV).

    • Bacteria: Single-celled microorganisms that can reproduce rapidly. May produce toxins that damage cells and tissues. Examples: Salmonella, Gonorrhoea, Crown gall (plants).

    • Fungi: Single-celled (e.g., yeast) or multi-cellular (e.g., mushrooms) organisms. Some are pathogenic, causing diseases like Athlete's foot or Rose black spot (plants).

    • Protists: Single-celled eukaryotic organisms. Some are parasites, needing a vector to transfer them to a host. Example: Malaria (caused by a protist spread by mosquitoes).

2. How are Diseases Spread?

  • Direct contact: Physical contact with an infected person (e.g., touching, sexual contact).

  • Indirect contact:

    • Droplet infection (airborne): Pathogens spread in tiny droplets when an infected person coughs or sneezes (e.g., Measles, Flu).

    • Water: Drinking contaminated water (e.g., Cholera).

    • Food: Contaminated food (e.g., Salmonella).

  • Vectors: Organisms that carry pathogens from one host to another (e.g., mosquitoes carrying Malarial protists).

3. How to Prevent and Control Disease Spread?

  • Hygiene: Handwashing, using disinfectants, cooking food thoroughly.

  • Destroying Vectors: Killing mosquitoes to prevent malaria.

  • Isolation: Separating infected individuals.

  • Vaccination: Protecting individuals and communities (see section 5).

4. Human Defence Systems
4.1 Non-Specific Defences (First Line)

  • Physical barriers:

    • Skin: Acts as a barrier, produces antimicrobial secretions.

    • Nose hair/Mucus: Trap pathogens.

    • Cilia: Small hairs that sweep mucus (and trapped pathogens) up to be swallowed.

  • Chemical barriers:

    • Stomach acid: Kills most ingested pathogens.

    • Tears: Contain enzymes that kill bacteria (lysozyme).

4.2 Specific Immune Response (Second Line)

  • White Blood Cells (Phagocytes):

    • Phagocytosis: Ingest and destroy pathogens.

  • White Blood Cells (Lymphocytes):

    • Identify specific pathogens by their unique surface proteins (antigens).

    • Produce antibodies: Specific proteins that lock onto antigens, destroying pathogens. Antibodies are specific to each pathogen.

    • Produce antitoxins: Neutralise toxins released by pathogens (e.g., bacteria).

    • Memory cells: After an infection, some lymphocytes remain as memory cells, providing immunity. If the same pathogen enters the body again, memory cells rapidly produce antibodies, preventing illness.

5. Vaccination

  • How it works:

    1. A small amount of a dead or inactive pathogen (or its antigens) is introduced into the body.

    2. The immune system is stimulated to produce antibodies and memory cells against that pathogen.

    3. This provides immunity without causing the disease.

  • Herd immunity: When a large proportion of the population is vaccinated, it reduces the spread of the pathogen, protecting even unvaccinated individuals.

6. Antibiotics and Painkillers

  • Antibiotics:

    • Medicines that kill bacteria or prevent their growth.

    • Do NOT work against viruses because viruses reproduce inside host cells making it difficult to kill them without damaging host cells.

    • Antibiotic resistance: Bacteria can evolve resistance to antibiotics. Overuse or misuse of antibiotics ($\text{e.g., not completing the full course}$$) can accelerate this. MRSA is an example of an antibiotic-resistant bacterium.

  • Painkillers: Treat the symptoms of disease (e.g., pain, fever) but do not kill pathogens.

7. Discovering and Developing Drugs

  • Early medicines: Many derived from plants (e.g., Digitalis from foxgloves for heart disease, Aspirin from willow bark for pain).

  • Modern drugs:

    • Are synthesised in laboratories.

    • Must be tested for toxicity, efficacy, and dose.

  • Stages of drug testing:

    1. Pre-clinical testing: Drugs are tested on cells, tissues, and live animals to check for toxicity and efficacy.

    2. Clinical trials (on humans):

      • Low doses given to healthy volunteers to check for side effects.

      • Tested on patients to find the optimum dose and efficacy.

      • Placebo: Some patients are given a placebo (a medicine with no active drug) for comparison, especially in double-blind trials where neither the patient nor the doctor knows who received the drug or the placebo, to prevent bias.

  • Monoclonal antibodies (Higher Tier):

    • Produced from a single clone of B lymphocytes.

    • Highly specific to particular target antigens.

    • Uses: Pregnancy tests, diagnosing diseases, treating some cancers (delivering drugs directly to cancer cells).

8. Plant Diseases

  • Signs of plant disease:

    • Discoloured leaves (e.g., yellowing in rose black spot).

    • Stunted growth (e.g., in TMV).

    • Spots on leaves.

    • Malformed stems or leaves.

    • Rotting or decay.

  • Physical plant defences:

    • Waxy cuticle: Prevents entry of pathogens.

    • Cell walls: Provide a physical barrier.

    • Layers of dead cells (e.g., bark): Act as a barrier.

  • Chemical plant defences:

    • Produce antibacterial or antifungal chemicals.

    • Produce compounds that deter herbivores (which could otherwise cause damage, allowing pathogen entry).