Human cells have a finite lifespan (Hayflick limit). The existence of this Hayflick limit with regards to immune cells known as T cells, implies that infections have a long-term immunological cost (IC) to the individual because they drive immune cells towards the end of their lifespan, eventually making most of those cells unavailable to respond to other infections. The development of an accurate IC measure will lead to a better predictor of the age of the adaptive immune system than chronological age.
An important goal of research in immunology is to understand the flaws in the human immune system, such that their impacts can be effectively mitigated. Previous work documented flaws in the mechanisms by which the immune system tempers its responses to pathogens in order to avoid harming the host. As explained here, these tempering mechanisms also govern the phenomenon of the original antigenic sin, whereby an encounter with a new pathogen strain preferentially recalls less potent immune responses directed against an older, moderately different strain.