A vaccine is a preparation containing antigenic material and may be:
- Whole live micro organisms,
- Dead organisms,
- Harmless version,
- Harmless form of a toxin,
- Preparation of surface antigens.
Vaccines can be given orally or injected.
Problems with vaccines are as follows:
- Vaccines are most effective if they are live and mimic infection. Dead organisms are less effective.
- Less effective vaccines may need booster injections to stimulate secondary response.
- People with defective immune systems do not respond well to vaccines - don't develop good B and T cells.
- People with protein malnutrition don't have enough protein to make antibodies or clones of lymphocytes and so don't respond well.
- Live virus may pass out in faeces and infect others (use herd immunity to solve this - vaccinate large numbers at the same time)
- Viruses / pathogens which regularly mutate and produce different antigens cannot be contained by vaccines e.g. flu. Minor changes (antigen drift) is fine, but major changes (antigen shift) make previous vaccines ineffective.
- Certain pathogens e.g. protozoa have 100s or 1000s of antigens on cell surfaces. E.g. pathogen causing sleeping sickness. It changes its antigens every 4/5 days and so immune system can't respond. Plasmodium (malaria) passes through 3 stages in its life cycle. Each stage has its own antigens therefore vaccines would be needed with antigens for all 3 stages or be specific to infective stage (short period of time between mosquito bite and liver cell infection).
- Antigenic concealment - pathogens evade attack by immune system by living inside cells e.g. parasiting T cells and macrophages, or live beyond the reach of antibodies e.g. cholera remains in intestine or conceal themselves by covering their body in host proteins e.g. some worms.