CB5 — Health, Disease and the Development of Medicines

Communicable diseases and pathogens

Pathogens are microorganisms that cause disease. Types:

Transmission routes: direct contact, droplets (coughing/sneezing), contaminated food/water, sexual contact, vectors (mosquitoes for malaria).

Malaria: caused by Plasmodium protist; transmitted by Anopheles mosquito vector. Prevention: mosquito nets, insecticides, antimalarial drugs.

HIV: virus. Destroys helper T cells → AIDS (Acquired Immune Deficiency Syndrome) → immune system too weak to fight infections. Transmitted: unprotected sex, sharing needles, mother to child. Treatment: antiretroviral drugs (suppress but not cure).

The immune system

Physical barriers: skin (prevents pathogen entry), mucus (traps pathogens in respiratory tract), cilia (sweep mucus away), stomach acid (pH 1.5–2 kills pathogens).

Non-specific immune response:

• Phagocytosis: phagocytes (neutrophils, macrophages) engulf and destroy pathogens — non-specific (works against any pathogen).
• Inflammation: increased blood flow, more phagocytes recruited.

Specific immune response (lymphocytes):

• B lymphocytes: produce antibodies (proteins that bind specifically to antigens on pathogens → neutralise, agglutinate, or flag for destruction). Each B cell clone produces one type of antibody specific to one antigen. After infection, memory B cells persist → faster, larger response on re-exposure (immunological memory).
• T lymphocytes: T helper cells coordinate the immune response; T killer cells destroy infected cells.

Antigens: molecules (usually proteins) on the surface of pathogens that trigger an immune response.

Antibodies: Y-shaped proteins. The variable region is complementary to a specific antigen. Antibodies can: neutralise toxins, agglutinate bacteria, activate complement.

Vaccines

A vaccine contains a dead, weakened, or antigen fragments of a pathogen. It is introduced to the body to stimulate an immune response (antibody production + memory cell formation) WITHOUT causing disease. On future exposure to the live pathogen, the immune system responds rapidly.

Herd immunity: if a large proportion of the population is vaccinated, even unvaccinated individuals are protected because the pathogen cannot spread.

Examples: MMR (measles, mumps, rubella); polio; HPV (cervical cancer prevention); flu vaccine (annual — virus mutates).

Antibiotics and antivirals

Antibiotics kill or inhibit bacteria ONLY. They work by disrupting bacterial cell walls (penicillin), inhibiting ribosome function (streptomycin), or disrupting cell membranes. They do NOT work against viruses (viruses have no cell wall, use host ribosomes).

Antivirals inhibit viral replication (e.g., oseltamivir / Tamiflu for influenza; antiretrovirals for HIV).

Drug development and testing

New medicines go through a rigorous pipeline (Edexcel Paper 2 topic):

1. Discovery: identify potential compound (plant-based, synthetic, or from microorganism). Computer modelling.
2. Pre-clinical testing: tested on cells in culture (in vitro) and on animals (in vivo) to assess safety and efficacy. Checks for toxicity, dosing, side effects.
3. Clinical trials — Phase I: small group of healthy volunteers. Safety, pharmacokinetics, dose range.
4. Clinical trials — Phase II: larger group of patients with condition. Efficacy and side effects.
5. Clinical trials — Phase III: large randomised controlled trial (RCT). Double-blind placebo-controlled. Statistical significance.
6. Regulatory approval: data reviewed by regulatory authority (e.g., MHRA in UK). Approval for use.
7. Phase IV (post-marketing surveillance): monitor for rare side effects in wider population.

Double-blind trial: neither patient nor doctor knows who received the drug or placebo — prevents bias. Placebo: inactive substance given to the control group to account for the placebo effect. Randomised: patients randomly assigned to groups — controls for confounding variables.

CP7 — Microbial cultures

CP7: investigate the effect of disinfectants/antibiotics on microbial growth.

• Prepare agar plates with bacteria (spread evenly with sterile spreader — aseptic technique).
• Place antibiotic discs or disinfectant-soaked discs on surface.
• Incubate at 25°C (school setting — below body temperature to reduce risk from pathogens).
• Measure zone of inhibition (clear area around disc where bacteria have not grown).
• Larger zone = more effective antibiotic/disinfectant.
• Aseptic technique throughout: flame inoculating loop, work near Bunsen burner, sterilise bench with disinfectant, seal plates with tape (not completely — to prevent dangerous anaerobes).