The immune system is the body's defense mechanism against pathogens. This system includes many cells that work together to protect the body from foreign invaders such as bacteria, viruses, parasites and tumors. The immune system is divided into two primitive forms, the innate and adaptive systems. The roles of both types are vital in the immune system; the innate system is the rapid, non-specific primary response against any foreign object encountered by the body, while the adaptive system is a secondary response and a more specific line of defense. The initiation of adaptive and innate responses are extremely associated with each other. The innate response is made up of many elements; these include mechanical, chemical, humoral as well as cellular barriers. The main line of defense of the innate system is the cellular component; There are different types of cells involved, however, the most critical cell among all other immune cells is called a macrophage. Macrophages are phagocytic cells derived from monocytes, a form of blood leukocytes. Monocytes are produced by the bone marrow of the myeloid lineage through hematopoietic stem cells engaged in a process called hematopoiesis. Macrophages play an essential role in the immune system; they are involved in innate and adaptive responses. Thus, macrophages are still considered the cells that “connect” the innate and adaptive systems. This essay will discuss the functions and activities of the macrophage that make it the most important cell of the immune system. Monocytes are the precursors of macrophages; these monocytes migrate and differentiate into tissue macrophages once they encounter an infection. Macrophages are distributed around the middle of paper ......al, having antiparasitic and antitumor activities. Macrophages play an important role in defense against tumors. This defense is achieved by the rapid response, production and secretion of various cytokines that target the activation of dendretic cells (DC) and natural killer (NK) cells. INF-γ is the main cytokine that causes NK cells to stop tumor progression and NK cells to activate T cells to respond to danger (Lamagna et al. 2006). Tissue repair is essential for the body to regenerate tissues. tissue destruction during the inflammatory response. Macrophages are essential for tissue resolution. This is done by the secretion of the cytokine GM-CSF which is a colony stimulating factor; it proliferates and causes tissue healing. Additionally, macrophages secrete enzymes collagenase and elastase which promote tissue repair..