Inoculation, or vaccination, is a critical method in preventing disease. Vaccination dates back to 1000 CE where the Chinese utilized smallpox inoculation to prevent a future occurrence of the disease (History of Vaccines, 2017). A well-known use of vaccination was performed in 1796 where Edward Jenner inoculated a 13 year- old boy with cowpox to prevent the acquisition of smallpox (History of Vaccines, 2017). Since Jenner’s 18th centenary discovery, vaccination has been a critical method in the prevention of disease.
Vaccines create immunity to a disease by acting on the basic characteristics of immunity. The foundation of immunity to disease is the immune system, which consists of complementary organs, tissues, and cells that function in the body’s defense against pathogens. The immune system consists of two major components known as innate (non- specific) and adaptive (specific) immunity. These two components make up the three lines of defense that the body has against pathogens, which is described in the following image:
Figure Taken from Kuby, Immunology 6th edition
The first line of defense consists of mechanical, physical, and chemical defenses that are used to deter pathogens, such as saliva or the body’s acid mantel. The second line of defense consists of the complement system, fever, inflammation, and phagocytic cells. Inflammation and fever serve to deter pathogenesis and enhance the body’s immune response while the complement system and phagocytic cells serve to kill pathogens or infected cells. Finally, the third line of defense is part of the adaptive immune response and is the mechanism in which vaccines function to stimulate to create pathogenic immunity. The third line of defense can be divided into two sub- categories, which are the humoral and cell- mediated responses. In the humoral response, B- cells recognize pathogens and release antibodies. The five major functions of antibodies are the following; Agglutination, or confinement of pathogens; Optimization, or enhancing phagocytosis via the coating of pathogens by antibodies; Neutralization, or blocking the receptors on toxins or pathogens; Complement activation, which yields cell lysis and inflammation; Antibody- dependent cell- mediated cytotoxicity, which causes destruction of target cells by natural killer cells and eosinophils.
In cell mediated, cytotoxic T- cells kill infected cells by direct cell- to- cell contact and T- helper cells aid in the activation of the humoral response and enhance other immune responses. During infection by a pathogen, B- cells differentiate into memory B- cells and plasma cells. Memory cells are B- cells that are specific for the pathogen that had initiated the primary infection and can secrete antibodies against that pathogen. Thereby, memory B- cells leads to a quicker and stronger response to a secondary infection of the same pathogen. Vaccines function by activating humoral and/or cell- mediated immunity.
Vaccines can be live- attenuated, inactivated, subunit, recombinant, conjugate, DNA, or toxoid. Each of these types of vaccines have attributes that should be considered prior to vaccination. For instance, live- attenuated vaccines may not be able to be given to immunocompromised individuals, as the vaccine most closely replicates a true infection. Also, there is a possibility for the pathogen to become reverted and infect the individual. Nonetheless, vaccines serve as an important mechanism to prevent disease. Vaccines can stimulate humoral and/or cell- mediated responses resulting in the production of memory cells. The memory cells are specific for the antigen that the vaccine contains. For example, the combination vaccine known as MMR contains antigens that replicate measles, mumps, and rubella. Thereby, although the vaccine does not contain pathogens that can cause disease, the vaccine can yield the production of memory B- cells and antibodies against an infectious measles, mumps, or rubella pathogen.
- “All Timeline Overview.” Timeline | History of Vaccines, The College of Physicians of Philadelphia, 2017, http://www.historyofvaccines.org/timeline.