Defense Against Infectious Disease

Pathogens are disease-causing organisms: bacteria, viruses, fungi, protists, prions.

First line — physical and chemical barriers:

• Skin — keratinised; impenetrable to most microbes.
• Mucus in airways traps inhaled microbes; cilia sweep them up.
• Stomach acid (pH 2) kills most swallowed bacteria.
• Lysozyme in tears and saliva digests bacterial cell walls.
• Acidic skin secretions and competition from normal flora.

Second line — innate immune response. When pathogens breach barriers:

• Phagocytes (e.g. macrophages, neutrophils) engulf pathogens by phagocytosis: pseudopodia surround → vesicle forms → lysosome fuses → enzymes digest.
• Inflammation at injury site: histamine causes vasodilation + increased permeability → more phagocytes arrive.

Third line — adaptive immune response (slower but specific and remembered):

• Antigens are unique molecules on pathogen surfaces.
• B-lymphocytes with matching receptor are activated, multiply (clonal selection), and differentiate into plasma cells (antibody-producing) and memory cells.
• T-helper cells coordinate; cytotoxic T cells kill infected cells.

Antibodies are Y-shaped proteins (immunoglobulins) with two specific antigen-binding sites. They:

• Bind to antigens (neutralisation).
• Mark pathogens for phagocytes (opsonisation).
• Cause pathogens to clump (agglutination).
• Activate complement (membrane attack complex).

After infection, memory B and T cells persist — providing rapid, stronger response on re-exposure (immunological memory).

Antibiotics. Drugs that kill or inhibit bacteria. Examples and targets:

• Penicillin — inhibits cross-linking of peptidoglycan in bacterial cell walls. Bacteria can't maintain structure; lyse.
• Tetracycline — blocks 70S bacterial ribosomes (without affecting 80S eukaryotic).
• Ciprofloxacin — inhibits bacterial DNA gyrase.

Why antibiotics DON'T work on viruses. Viruses are not cells. They have no:

• Cell wall (penicillin target).
• Own ribosomes (tetracycline target — they use host ribosomes).
• Independent metabolism.

Treating viral infections requires antivirals (e.g. acyclovir for herpes; remdesivir for COVID).

Antibiotic resistance. Random mutations in bacteria can confer resistance (e.g. β-lactamase enzyme that breaks penicillin). Selection pressure (overuse of antibiotics in medicine and agriculture) favours resistant strains → "superbugs" like MRSA. Strategies: prescribe only when needed; complete the course; rotate antibiotics; reduce agricultural use.

Vaccination.

A vaccine introduces a harmless form of the pathogen's antigen — could be:

• Killed or inactivated pathogen.
• Weakened (attenuated) live pathogen.
• Pieces (subunit, e.g. spike protein).
• mRNA (e.g. COVID-19 mRNA vaccines) coding for an antigen the body produces briefly.

The immune system responds: makes antibodies AND memory B/T cells. On real exposure later, the memory response is rapid and strong — usually preventing illness.

Herd immunity. If a high enough fraction of the population is immune (typically 80-95%), the pathogen cannot spread effectively — protecting those who cannot be vaccinated (immunocompromised, newborns).

Examples of vaccination success: smallpox eradicated 1980; polio reduced from 350 000 cases (1988) to <100 (2021); measles outbreaks return when vaccination rates drop.

Ethics and policy. Vaccine mandates, intellectual property of vaccines, access in low-income countries, misinformation about safety — all current debates.