Trained immunity is the modification of cells in the innate immune system (the one with which an organism is born) to create a "memory" of a pathogen. Trained immunity creates no antibodies in preparation for a second encounter. Instead, the immunity is mediated mostly by epigenetic modifications, alterations in gene expression and cellular function without changes to the original DNA sequence. The resulting immunity lasts up to several months, in contrast to the classical immunological memory (which may last up to a lifetime), and is usually unspecific because there is no production of specific antibodies/receptors.