<P> The principle behind immunization is to introduce an antigen, derived from a disease - causing organism, that stimulates the immune system to develop protective immunity against that organism, but that does not itself cause the pathogenic effects of that organism . An antigen (short for antibody generator), is defined as any substance that binds to a specific antibody and elicits an adaptive immune response . </P> <P> Most viral vaccines are based on live attenuated viruses, whereas many bacterial vaccines are based on acellular components of microorganisms, including harmless toxin components . Many antigens derived from acellular vaccines do not strongly induce an adaptive response, and most bacterial vaccines require the addition of adjuvants that activate the antigen - presenting cells of the innate immune system to enhance immunogenicity . </P> <P> Most large molecules, including virtually all proteins and many polysaccharides, can serve as antigens . The parts of an antigen that interact with an antibody molecule or a lymphocyte receptor, are called epitopes, or antigenic determinants . Most antigens contain a variety of epitopes and can stimulate the production of antibodies, specific T cell responses, or both . A very small proportion (less than 0.01%) of the total lymphocytes are able to bind to a particular antigen, which suggests that only a few cells respond to each antigen . </P> <P> For the adaptive response to "remember" and eliminate a large number of pathogens the immune system must be able to distinguish between many different antigens, and the receptors that recognize antigens must be produced in a huge variety of configurations, in essence one receptor (at least) for each different pathogen that might ever be encountered . Even in the absence of antigen stimulation, a human can produce more than 1 trillion different antibody molecules . Millions of genes would be required to store the genetic information that produces these receptors, but, the entire human genome contains fewer than 25,000 genes . </P>

Adaptive immunity is provided by lymphocytes that secrete antibodies