Immunological memory is a central characteristic of the immune system [(1996); (1996); (1996; (2000a)]. Nevertheless, there is no simple definition of this concept. The easiest definition is on the functional level: a host is protected more ef- ficiently against a second infection if it has previously been infected with the same pathogen, and survived the infection. Before the host has encountered a pathogen it is said to be naive. This means that the number of immune cells that are specific for this pathogen is relatively low. When the host is infected with this pathogen for the first time (primary infection), the population of specific immune cells expands, fights the pathogen, and subsequently settles around a relatively stable level that is much higher than in the naive host. This population of cells is referred to as memory cells, and memory persists in the long-term after pathogen clearance. Upon reinfection with the same pathogen (secondary infection or rechallenge), this population of memory cells can react more efficiently against the invading virus compared to a naive host. Consequently, the host suffers less or no pathology. Memory is found in all adaptive branches of the immune system: B cells (or antibody) responses, as well as T cell responses. Vaccinations rely on the generation of memory: the immune system is artificially exposed to a pathogen (or an immunogenic part of the pathogen), and this results in the generation of immunological memory and in protection against infection. While the population of memory cells is maintained in the long-term after clearance of a pathogen, it declines slowly, and this can lead to a loss of protection over time. It is unclear for how long hosts are protected against reinfection, and this may vary from case to case.
KeywordsVirus Load Memory Cell Primary Infection Antigenic Stimulation Immunological Memory
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