Resident macrophages play a unique role in the maintenance of tissue function. As phagocytes, they are an essential first line defenders against pathogens and much of the initial characterization of these cells was focused on their interaction with viral and bacterial pathogens. However, these cells are increasingly recognized as contributing to more than just host defense. Through cytokine production, receptor engagement and gap junction communication resident macrophages tune tissue inflammatory tone, influence adaptive immune cell phenotype and regulate tissue structure and function. This review highlights resident macrophages in the liver and lung as they hold unique roles in the maintenance of the interface between the circulatory system and the external environment. As such, we detail the developmental origin of these cells, their contribution to host defense and the array of tools these cells use to regulate tissue homeostasis.

In a world filled with microbes, some posing a threat to our body, our immune system is key to living a healthy life. The innate immune system is made of various cell types that act to guard our bodies. Unlike the adaptive immune system that has a specific response, our innate immune system encompasses cells that elicit unspecific immune responses, triggered whenever the right signals are detected. Our understanding of immunity started with the concept of our immune system only responding to "nonself" like the pathogens that invade our body. However, over the past few decades, we have learned that the immune system is more than an on/off switch that recognizes nonself. The innate immune system regularly patrols our bodies for pathogens and tissue damage. Our innate immune system not only seeks to resolve infection but also repair tissue injury, through phagocytosing debris and initiating the release of growth factors. Recently, we are starting to see that it is not just recognizing danger, our innate immune system plays a crucial role in repair.Innate immune cells phenotypically change during repair. In the context of severe injury or trauma, our innate immune system is modified quite drastically to help repair, resulting in reduced infection control. Moreover, these changes in immune cell function can be modified by sex as a biological variable. From past to present, in this overview, we provide a summary of the innate immune cells and pathways in infection and tissue repair. This article is categorized under:Immune System Diseases > Molecular and Cellular Physiologyimmunology, innate immune cells, sterile injury | INTRODUCTIONEvery minute of every hour of every day, our body's immune cells are in a silent battle with tiny microbes in the environment around us. These microbes consist of bacteria, fungi, viruses, and parasites found in the air, water, soil, and on surfaces. In tropical climates, the range of pathogens differ from subarctic climates, but we are all faced with similar challenges daily. Immunodeficiencies, or the absence of immune cells, result in regular microbial attacks on our bodies and disease development. The concept of immunology emerged with Elias Metchnikoff's (1845-1916) identification of phagocytic cells, which engulf and destroy invading pathogens (Kaufmann, 2019). This laid the basis for innate immunity.