Immune responses to pathogens to which they were not previously exposed are commonly less effective in elderly people than in young adults, whereas those to agents previously encountered and overcome in earlier life may be amplified. This is reflected in the robust finding in many studies that the proportions and numbers of naïve B and T cells are lower and memory cells higher in the elderly. In addition to the "extrinsic" effects of pathogen exposure, "intrinsic" events such as age-associated differences in haematopoeitic stem cells and their niches in the bone marrow associated with differences in cell maturation and output to the periphery are also observed. In the case of T cells, the "intrinsic" process of thymic involution, beginning before puberty, further contributes to reducing the production of naïve T cells. Like memory T cell populations, innate immune cells may be increased in number but decreased in efficacy on a per-cell basis. Thus, superimposed on chronological age alone, remodelling of immunity as a result of interactions with the environment over the life course is instrumental in shaping immune status in later life. In addition to interactions with pathogens, host microbiome and nutrition, exercise and stress, and many other extrinsic factors are crucial modulators of this "immunosenescence" process. In this review, we briefly outline the observed immune differences between younger and older people, and discuss the possible impacts of behavioral variations thereon.