Introduction
Depending on the types of participating cells, recognition patterns, and effector mechanisms, immunity is traditionally divided into two categories, namely, innate and adaptive immunity. The former is mediated by phagocytes, such as monocytes, macrophages, and neutrophils, as well as natural killer cells (NK cells), and is characterized by a rapid and non-specific immune response, whereas the latter is mainly mediated by T and B lymphocytes and is characterized by a slower, albeit highly specific, response. It is generally believed that the immunological memory is associated exclusively with adaptive immunity, and not innate immunity. Nevertheless, it has long been debated as to whether innate immunity also has memory feature, and the recent findings have indeed indicated that immunological memory is not an exclusive hallmark of adaptive immunity 1, 2. For example, plants and invertebrates, which lack adaptive immunity, are protected against secondary infections, and invertebrates can also exhibit transplant rejection3-6. Macrophages, which are key innate immune cell types, participate in innate immunity by phagocytizing foreign bodies, and initiate adaptive immunity by capturing and processing antigens, which are subsequently presented to lymphocytes, thereby playing a central role in immune responses 7, 8. Moreover, macrophages exhibit certain immune memory-related properties that are traditionally attributed to adaptive T and B cells. However, compared with the memory of adaptive immunity, macrophage memory remains comparatively unknown. Herein, we describe two types of macrophage memory, namely, trained immunity (innate immunological memory) and endowed immunity (adaptive immune-like memory) (Figure 1), as well as the essential roles these play in a range of diseases.