Immune System

Here’s an expanded explanation of the SL content statements for Topic C3.2: Immunity and Disease, with clear elaborations while staying within the IB SL expectations:

C3.2.1 Pathogens as the cause of infectious diseases

  • Pathogens are microorganisms that cause diseases. These include:

    • Viruses (e.g., influenza virus, HIV)

    • Bacteria (e.g., Mycobacterium tuberculosis)

    • Fungi (e.g., Candida albicans)

    • Protists (e.g., Plasmodium spp. causing malaria)

  • Archaea do not cause disease in humans.

  • Nature of Science (NOS): Observations during historical epidemics, such as Semmelweis’s work on childbed fever and John Snow’s mapping of cholera cases in London, led to major advances in disease control.

C3.2.2 Skin and mucous membranes as a primary defence

  • The skin acts as a tough physical barrier, while secretions like sebum (oil) lower pH, discouraging microbial growth.

  • Mucous membranes (e.g., in the respiratory tract) trap pathogens; some secrete lysozyme, an enzyme that digests bacterial cell walls.

C3.2.3 Sealing of cuts in skin by blood clotting

  • When skin is cut:

    • Platelets release clotting factors.

    • A cascade of reactions activates thrombin, which converts fibrinogen (soluble protein) into fibrin (insoluble fibers).

    • Fibrin traps red blood cells (erythrocytes) to form a clot.

C3.2.4 Differences between the innate immune system and the adaptive immune system

  • Innate immunity:

    • Non-specific

    • Fast response

    • No memory

    • Involves phagocytes

  • Adaptive immunity:

    • Specific to pathogens

    • Slower initially

    • Builds memory for future responses (e.g., via B- and T-lymphocytes)

C3.2.5 Infection control by phagocytes

  • Phagocytes (e.g., neutrophils, macrophages):

    • Move to infection sites by amoeboid movement.

    • Recognize pathogens using receptors.

    • Engulf pathogens via endocytosis.

    • Digest them using lysosomal enzymes.

C3.2.6 Lymphocytes as cells in the adaptive immune system

  • Lymphocytes include B-cells and T-cells.

  • Found in blood and lymph nodes.

  • Each B-cell produces a unique antibody.

  • Only a few B-cells for each antigen exist, so activation and cloning are essential.

C3.2.7 Antigens as recognition molecules

  • Antigens:

    • Typically glycoproteins or proteins.

    • Found on the surface of pathogens.

    • Recognized as "non-self," triggering an immune response.

    • Also found on red blood cells—this is relevant in blood transfusions.

C3.2.8 Activation of B-lymphocytes by helper T-lymphocytes

  • Antigen-specific B-cells must:

    • Bind their matching antigen.

    • Receive help from an activated T-helper cell recognizing the same antigen.

  • Only then do B-cells produce antibodies and become memory cells.

C3.2.9 Multiplication of activated B-lymphocytes

  • Activated B-cells divide by mitosis into clones.

  • Most clones become plasma cells that secrete large amounts of specific antibody.

  • Some clones become memory cells.

C3.2.10 Immunity as a consequence of retaining memory cells

  • Immunity: Ability to prevent re-infection.

  • Memory B- and T-cells persist long-term.

  • They respond quickly upon re-exposure, often preventing symptoms from developing.

C3.2.11 Transmission of HIV in body fluids

  • HIV is transmitted through:

    • Unprotected sexual contact

    • Contaminated blood transfusions

    • Sharing needles

    • Mother to child (birth or breastfeeding)

C3.2.12 Infection of lymphocytes by HIV

  • HIV infects and destroys helper T-lymphocytes.

  • Fewer helper T-cells = less B-cell activation = reduced antibody production.

  • Leads to AIDS: increased vulnerability to opportunistic infections.

C3.2.13 Antibiotics and their limitations

  • Antibiotics target:

    • Bacterial cell walls, ribosomes, enzymes

  • Do not affect viruses: viruses lack these structures and use host cell machinery.

C3.2.14 Evolution of resistance in bacteria

  • Misuse of antibiotics (overuse or incomplete courses) allows resistant strains to survive and multiply.

  • Leads to multi-drug resistant (MDR) pathogens (e.g., MRSA).

  • NOS: Searching chemical libraries helps discover new antibiotics.

C3.2.15 Zoonoses

  • Zoonoses: diseases transmitted from animals to humans.

  • Examples:

    • Tuberculosis (from cattle)

    • Rabies (from animal bites)

    • Japanese encephalitis (via mosquitoes from pigs/birds)

    • COVID-19 (likely from bats or intermediate host species)

C3.2.16 Vaccines and immunization

  • Vaccines contain:

    • Antigens (inactivated or weakened pathogens or parts)

    • Or genetic material (DNA/RNA) coding for antigens (e.g., mRNA COVID-19 vaccines)

  • Stimulate adaptive immunity without causing illness.

C3.2.17 Herd immunity

  • Herd immunity occurs when enough people are immune to a disease, reducing its spread.

  • Helps protect vulnerable individuals who cannot be vaccinated.

  • NOS: Scientific findings take time to validate; media may overstate early results. Vaccines have very low risks, but no intervention is risk-free.

C3.2.18 Evaluation of data related to the COVID-19 pandemic

  • Students should practice calculating:

    • Percentage change = ((new – old)/old) × 100%

    • Percentage difference = (|value1 – value2| / average of values) × 100%

  • Apply these to interpret real pandemic-related data (e.g., case numbers, death rates, vaccination rates).

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