IB DP Biology- C3.2 Defense against disease - IB Style Questions For SL Paper 2 - FA 2025

(a) Structures in M. tuberculosis not present in a human cell

• Cell wall with mycolic acids: Thick, waxy outer layer containing long-chain mycolic acids, absent from human cells.
• Peptidoglycan layer: Rigid polymer (murein) forming part of the bacterial cell wall; human cells lack peptidoglycan.
• 70S ribosomes: Smaller prokaryotic ribosomes for protein synthesis, whereas human cells have 80S ribosomes.

These prokaryotic features distinguish M. tuberculosis from eukaryotic human cells and are important targets for selective antibiotic action.

(b) Production of antibodies following infection with TB

1. Antigen presentation: TB bacteria are engulfed (phagocytosed) by macrophages, which process the pathogen and present TB antigens on their surface using MHC class II molecules.
2. Activation of helper T cells: Specific helper T (Th) cells recognize the antigen–MHC II complexes and become activated.
3. B cell activation: Activated helper T cells release cytokines that stimulate antigen-specific B cells.
4. Plasma cell formation: These B cells proliferate and differentiate into plasma cells that secrete antibodies specific to TB antigens.
5. Action of antibodies: Antibodies bind to TB antigens, helping to opsonize bacteria and promote phagocytosis, or activate complement.

In tuberculosis, cell-mediated immunity (involving T cells and macrophages) plays the dominant role because M. tuberculosis often survives inside cells, but antibody production is still part of the adaptive immune response.

(c) Risks of indiscriminate antibiotic use for human populations

• Selection for resistant bacteria: Antibiotics kill susceptible bacteria, but naturally occurring resistant variants survive and reproduce.
• Spread of resistance genes: Resistance can be passed to other bacteria (e.g., via plasmids and horizontal gene transfer), leading to multidrug-resistant strains.
• Reduced treatment effectiveness: Infections such as TB become harder or impossible to treat, increasing morbidity and mortality.
• Global health threat: Resistant strains can spread in communities and across countries, undermining current antibiotics and increasing healthcare costs.

Overuse and misuse of antibiotics (for example, unnecessary prescriptions, incomplete courses, or use in livestock) accelerate the evolution of antibiotic-resistant bacteria, making infectious diseases like TB much more difficult to control.