Abstract
Abstract Immunity is the state of protection against infectious disease conferred either through an immune response generated by immunization or previous infection or by other non-immunological factors. This article reviews active and passive immunity and the differences between them: it also describes the four different commercially available vaccine types [live attenuated, killed/inactivated, subunit and toxoid]: it also looks at how these different vaccines generate an adaptive immune response.
Introduction
The first article of this series reviewed those host mechanisms that protect against microbial invasion. Both limited effectiveness against particular pathogens together with pathogen evasion processes mean that certain infectious diseases are still a frequent occurrence; some are occupationally related with the risk to health care workers being particularly well documented [1,2]. Since particular occupationally transmitted infections can be prevented by immunization, this article will look at how the different vaccine types modulate adaptive responses to provide further protection. First, however, the terms active and passive immunity will be considered.
Active and passive immunity
Active immunity refers to the process of exposing the body to an antigen to generate an adaptive immune response: the response takes days/weeks to develop but may be long lasting—even lifelong. Active immunity is usually classified as natural or acquired. Wild infection for example with hepatitis A virus [HAV] and subsequent recovery gives rise to a natural active immune response usually leading to lifelong protection. In a similar manner, administration of two doses of hepatitis A vaccine generates an acquired active immune response leading to long-lasting [possibly lifelong] protection. Hepatitis A vaccine has only been licensed since the late 1980s so that follow-up studies of duration of protection are limited to