Explain how vaccinations prevent disease.

Vaccinations introduce the human immune system to pathogenic antibodies in a safe way. This could be in the form of the dead pathogen, a genetically engineered version or the lone antibody. The antibody activates the primary immune response. This includes the clonal expansion of lymphocytes to form plasma and memory cells. This primary response has a long latency period so if the pathogen wasn't weakened the patient would start to feel unwell. However, if or when the patient encounters the pathogen for a second time (eg infection) the memory cells enable the secondary response. This is much quicker and has a smaller latency period, the patient may not even know they were ever infected. Vaccines are most effective if they fully activate the immune system and are given to a large proportion of the population to prevent carrier or further transmission. Vaccines are also most effective for pathogens that have small numbers of strains and show little mutation for example hepatitis A. If there are many strains and lots of mutation then the vaccine will need to be regularly updated eg the flu vaccine.