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Figure 1 . There are two types of macrophage memory: trained and endowed immunity. In trained immunity, macrophages acquire an immune memory phenotype via epigenetic and metabolic reprogramming and exhibit an enhanced (training) or suppressed (tolerance) host defense response toward a secondary challenge. In endowed immunity, macrophages primed by allogeneic antigens acquire the potential ability to reject allogeneic grafts bearing the same antigen, with the assistance of helper CD4+ T cells, although lack the capacity to reject third-party grafts. In adaptive immunity, T and B cells display a highly specific immune memory subsequent to primary exposure to antigens and produce a rapid, robust, and enduring immune response.