11 General Introduction growth factors [35,36]. Monocytes reside in the blood until they migrate into tissues upon inflammatory stimuli [37]. In tissues, monocytes will differentiate into macrophages or dendritic cells to perform immune surveillance and protect against pathogens [37]. The most important macrophage subsets during wound healing are the pro-inflammatory macrophages (M1) and macrophages that support wound healing (M2) [38,39]. NK cells are cytotoxic lymphocytes that are involved in tissue homeostasis and killing of stressed or infected cells [40]. Based on their cytotoxicity and cytokine and marker expression they can be classified as either NKdim cells (less cytokine production, more cytotoxic) and NKbright cells (more cytokine production, less cytotoxic) [41]. T cells and B cells are part of the adaptive immune system and generate tailored responses to pathogens through specific effector T cells and antibody production [42]. After stimulation, naïve T cells can differentiate into specific subtypes with different effector functions: Th1, Th2, T9, Th17, Th22, Tfh cells or regulatory T cells (Tregs) [43]. The Th phenotype will influence other immune cells and the direction and duration of the overall immune response. These immune cells produce and are influenced by inflammatory mediators (Table 1). 1
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