bioInfectionAndResponse

3.1 Communicable Diseases

Communicable (infectious) diseases are caused by pathogens and spread between individuals or from animals.

3.1.1 Pathogens

Pathogens are microorganisms causing disease. Main types:

  • Bacteria

    • Single-celled prokaryotes.

    • Reproduce rapidly and produce toxins damaging cells.

    • Examples: Salmonellosis, Gonorrhoea.

  • Viruses

    • Smaller than bacteria.

    • Replicate by invading host cells, causing damage.

    • Examples: Measles, HIV, Tobacco Mosaic Virus (plants).

  • Fungi

    • Eukaryotic (single or multicellular).

    • Grow on organisms, release spores.

    • Examples: Athlete's Foot, Rose Black Spot (plants).

  • Protists

    • Single-celled eukaryotes.

    • Often parasitic, transferred by vectors.

    • Example: Malaria (Plasmodium protist, transmitted by mosquitoes).

3.1.2 Spreading Pathogens

  • Direct contact: Physical contact with an infected individual.

  • Droplet infection (airborne): Inhaling pathogens from coughs/sneezes.

  • Contaminated food/water: Consuming tainted food/water.

  • Vectors: Organisms (e.g., insects) carrying pathogens between hosts.

3.2 Principles of Disease

3.2.1 Viral Diseases

  • Measles

    • Symptoms: Fever, red skin rash.

    • Spread: Droplet infection.

    • Prevention: MMR vaccine.

    • Complications: Pneumonia, encephalitis.

  • HIV (Human Immunodeficiency Virus)

    • Symptoms: Initially flu-like, can lead to AIDS.

    • Spread: Sexual contact, shared needles, mother-to-baby.

    • Treatment: Antiretroviral drugs control virus, preventing AIDS by attacking immune cells.

  • Tobacco Mosaic Virus (TMV)

    • Affects plants.

    • Symptoms: Mosaic pattern, discolouration, stunted growth.

    • Effect: Reduces photosynthesis.

3.2.2 Bacterial Diseases

  • Salmonella

    • Symptoms: Fever, cramps, vomiting, diarrhoea.

    • Spread: Contaminated food (e.g., uncooked poultry).

    • Prevention: Vaccinating poultry, hygienic food prep.

  • Gonorrhoea

    • Symptoms: Discharge, painful urination.

    • Spread: Sexual contact.

    • Treatment: Antibiotics (resistance growing).

    • Prevention: Condoms.

3.2.3 Fungal Diseases

  • Rose Black Spot

    • Affects rose plants.

    • Symptoms: Purple/black spots on leaves, yellowing, leaf drop.

    • Spread: Wind, water.

    • Treatment: Fungicides, removing affected leaves.

3.2.4 Protist Diseases

  • Malaria

    • Symptoms: Recurrent fever, chills, sweating.

    • Spread: Mosquitoes (vector) transfer Plasmodium protist.

    • Prevention: Mosquito control, nets, antimalarial drugs.

3.3 Non-Specific Defence Systems

The body's initial barriers:

  • Skin: Physical barrier; scabs prevent entry.

  • Nose: Hairs and mucus trap pathogens.

  • Trachea and Bronchi: Mucus traps pathogens, cilia sweep them up.

  • Stomach: Hydrochloric acid kills most pathogens in food/drink.

3.4 The Immune System

White blood cells lead the immune response:

  • Phagocytes: Engulf and digest pathogens (phagocytosis). Non-specific.

  • Lymphocytes: Specific defence.

    • Produce antibodies (target specific pathogen antigens/toxins).

    • Produce antitoxins (neutralise toxins).

    • Form memory cells after infection for rapid future response (immunity).

3.5 Vaccination

  • Introduces a harmless pathogen dose (dead, inactive, weakened, or antigens) to stimulate antibody and memory cell production, providing immunity.

  • Herd immunity: Protects unvaccinated individuals by reducing disease spread when a significant proportion of the population is vaccinated.

3.6 Monoclonal Antibodies

Identical antibodies produced from a single B-lymphocyte clone (hybridoma cells).

  • Production: Mouse B-lymphocytes (producing specific antibodies) are fused with myeloma (tumour) cells to form hybridoma cells, which divide indefinitely and produce large amounts of the desired antibody.

  • Uses:

    • Diagnosis: Pregnancy tests (detect HCG), pathogen detection, blood typing.

    • Treatment: Targeted drug delivery (e.g., to cancer cells), autoimmune disease treatment, preventing organ rejection.

    • Research: Identifying specific chemicals/cells.

3.7 Development of Drugs

3.7.1 Antibiotics

  • Kill bacteria or prevent growth without harming human cells.

  • Ineffective against viruses.

  • Antibiotic resistance: Overuse leads to resistance; complete courses and avoid over-prescription.

3.7.2 Painkillers and Other Drugs

  • Painkillers: Treat symptoms (e.g., aspirin), do not kill pathogens.

  • Antivirals: Inhibit viral replication (e.g., Tamiflu for influenza, antiretrovirals for HIV).

3.7.3 Drug Discovery and Development

Drugs originate from plants, microorganisms, or chemical synthesis. Examples: Digitalis (foxgloves), Aspirin (willow bark), Penicillin (Penicillium mould).

  • Rigorous testing for efficacy (effectiveness), toxicity (safety), and dosage:

    • Preclinical testing: On cells, tissues, and animals.

    • Clinical trials: On healthy volunteers (low dose) then patients (optimum dose), compared with placebos in double-blind trials.

3.8 Plant Diseases and Defence Responses

3.8.1 Plant Diseases

  • TMV: Damages chloroplasts, reduces photosynthesis.

  • Rose Black Spot: Reduces photosynthesis due to leaf drop.

  • Aphids: Pests feeding on phloem sap, weakening plants.

  • Ion deficiencies: E.g., nitrates (stunted growth), magnesium (chlorosis).

3.8.2 Plant Defences

Plants have physical, chemical, and mechanical defences:

  • Physical barriers:

    • Cell walls: Strong barriers.

    • Waxy cuticle: Prevents water loss and pathogen entry.

    • Bark/dead cells: Protective stem layer.

  • Chemical defences:

    • Antibacterial chemicals: Deter pathogens/pests.

    • Poisons: Deter herbivores (e.g., nicotine, digitalis).

  • Mechanical adaptations:

    • Thorns/hairs: Deter animals.

    • Leaves that droop or curl: To dislodge insects.

    • Mimicry: To trick animals.