Immunity, pathogen evasion and antibiotic resistance

What you’ll learn

• Classify active and passive immunity.
• Explain resistance as an evolutionary process.
• Interpret antibiotic comparisons and infection-control evidence.

Understand the idea

Active immunity develops when a person’s own immune system responds to antigen and forms memory cells: infection is a natural route and vaccination an artificial route. Passive immunity supplies ready-made antibodies, naturally through maternal transfer or artificially through an antibody preparation. It can act quickly but does not itself create antigen-specific memory in the recipient and declines as the antibodies are broken down.

Pathogens and hosts impose reciprocal selection pressures. Antigenic variation can reduce recognition by existing antibodies; hiding inside cells or persisting in a latent state can also aid evasion. Bactericidal antibiotics kill susceptible bacteria, whereas bacteriostatic antibiotics inhibit their multiplication. Resistance can arise through mutation or acquisition of resistance genes; exposure changes relative reproductive success and increases the frequency of resistant variants. Antibiotics do not target viral replication as though viruses were bacterial cells.

Hospital infection prevention combines hand hygiene, effective cleaning, aseptic care and appropriate isolation with evidence-based antibiotic prescribing. A supervised antibiotic comparison must control bacterial strain, inoculum, medium, exposure and measurement; inhibition-zone size is affected by diffusion as well as susceptibility, so zones for different compounds cannot automatically rank clinical effectiveness. Use supplied classroom data and approved safety procedures rather than culturing unknown environmental or patient samples.

Worked example

A person receives ready-made antibodies after an exposure and is also offered a vaccine. Explain why the two interventions have different time courses.

1. The supplied antibodies can bind their target immediately, so passive protection starts rapidly.
2. Those antibodies are gradually degraded and do not themselves establish memory cells.
3. A vaccine requires activation and expansion of the person’s lymphocytes, which takes longer but can establish memory.

Answer: Passive protection is rapid and temporary; vaccine-induced active immunity develops more slowly and may last longer.