Immunobiography and the Heterogeneity of Immune Responses in the Elderly: A Focus on Inflammaging and Trained Immunity

Owing to its memory and plasticity, the immune system (IS) is capable of recording
all the immunological experiences and stimuli it was exposed to. The combination of
type, dose, intensity, and temporal sequence of antigenic stimuli that each individual is
exposed to has been named “immunobiography.” This immunological history induces a
lifelong continuous adaptation of the IS, which is responsible for the capability to mount
strong, weak or no response to specific antigens, thus determining the large heterogeneity
of immunological responses. In the last years, it is becoming clear that memory
is not solely a feature of adaptive immunity, as it has been observed that also innate
immune cells are provided with a sort of memory, dubbed “trained immunity.” In this
review, we discuss the main characteristics of trained immunity as a possible contributor
to inflammaging within the perspective of immunobiography, with particular attention to
the phenotypic changes of the cell populations known to be involved in trained immunity.
In conclusion, immunobiography emerges as a pervasive and comprehensive concept
that could help in understanding and interpret the individual heterogeneity of immune
responses (to infections and vaccinations) that becomes particularly evident at old age
and could affect immunosenescence and inflammaging.