Wellmune

In general, the primary cells of the innate immune system are phagocytic and fall into three main groups: granulocytes, macrophages, and dendritic cells. The granulocytes, which include neutrophils, are usually the first cells to enter a site of inflammation. Neutrophils attack pathogens in large numbers. These cells are short-lived in that they die after engulfing and killing pathogens. Macrophages are slower to respond to infections, usually arriving after the granulocytes, but macrophages are larger, live longer, and have the capacity to enhance the function of the adaptive immune system. Both granulocytes and macrophages enter into sites of infection via the blood. Dendritic cells, similar to granulocytes and macrophages, also engulf pathogens, but usually are present at typical sites of entry for pathogens (mucosal tissue, such as lung and gut, and skin). Like macrophages, dendritic cells also help with the activation of the adaptive immune system. Another type of innate immune cell, which is not a phagocyte, is the natural killer (NK) cell. These cells can directly recognize pathogen infected host cells, especially virus-infected cells, and kill the infected host cell. Although lymphoid in origin, NK cells are innate immune cells (i.e. respond quickly to pathogen invasion).