(a) Vaccination

• Immunity can be developed by vaccination using antigens from infectious pathogens, so creating memory cells.
• The antigens used in vaccines can be inactivated pathogen toxins, dead pathogens, parts of pathogens and weakened pathogens.
• Antigens are usually mixed with an adjuvant when producing the vaccine.
• An adjuvant is a substance which makes the vaccine more effective, so enhancing the immune response.

(b) Herd immunity

• Herd immunity occurs when a large percentage of a population is immunised.
• Establishing herd immunity is important in reducing the spread of diseases.
• Non-immune individuals are protected as there is a lower probability they will come into contact with infected individuals.
• The herd immunity threshold depends on the type of disease, the effectiveness of the vaccine and the density of the population.
• Mass vaccination programmes are designed to establish herd immunity to a disease.
• Difficulties can arise when widespread vaccination is not possible due to poverty in the developing world, or when vaccines are rejected by a percentage of the population in the developed world.

(c) Antigenic variation.

• Some pathogens can change their antigens.
• This means that memory cells are not effective against them.
• Antigenic variation occurs in the influenza virus explaining why it remains a major public health problem and why individuals who are at risk require to be vaccinated every year.