What is the difference between innate and adaptive immunity? Traditionally, innate immunity was assumed to be rapid, “non-specific”, and identical qualitatively and quantitatively each time the same pathogen was encountered. Many of the innate immune cells are considered to be short-lived, for example the lifespan of a neutrophil is estimated to be a few hours or days, making “memory” a moot concept. Conversely, the hallmarks of adaptive (also referred to as acquired) immunity are considered to include the generation of long-lived, antigen-specific cells after initial exposure to an antigen or pathogen, and these cells respond faster and more robustly on subsequent encounters with the same antigen or pathogen. Adaptive immunity has previously been considered the exclusive domain of B cells and T cells; granulocytes, monocytes, macrophages, dendritic cells, and NK cells have been delegated to the innate immune system, which also comprises epithelial cell barriers, complement, and anti-microbial peptides and other soluble factors.