Do we have memory of danger as well as antigen?

Recognition of specific protein antigens leads to immunological memory of antigen, whereas recognition of danger signals by the innate immune system determines the size, nature and longevity of a response. Recent data indicate that recognition of danger might have long-lasting effects on CD8 memory T-cell populations, specifically enhancing early cytokine release and thus altering the nature of subsequent immune responses. Here, a modified model of immune regulation is proposed in which a nonspecific immunological memory of danger accumulates during maturation of the immune system, allowing more potent cell-mediated immunity to develop in dangerous environments. Such a mechanism could be involved in phenomena leading to the ‘hygiene hypothesis’.