The immune system is the body's defense mechanism against pathogens. This system is made up of 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 the adaptive system. The roles of both types are vital in the immune system; the innate one is the rapid and non-specific primary response against any foreign material the body encounters, 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 components; these include mechanical, chemical, humoral, and cellular barriers. The main line of defense of the innate system is the cellular component; there are several types of cells involved, however, the most critical cell of all other immune cells is called a macrophage. Macrophages are phagocytic cells originating from monocytes, a form of blood leukocyte. Monocytes are produced from the bone marrow into the myeloid lineage through hematopoietic stem cells engaged in a process called hematopoiesis. Macrophages play indispensable roles in the immune system; they are involved in both innate and adaptive responses. Therefore, macrophages are always referred to as the cells that “bridge” between 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 center of the paper, carrying out antiparasitic and antitumor activity. Macrophages play an important role in defense against tumors. This defense is achieved through the rapid response, production and secretion of various cytokines that target the activation of dendritic cells (DCs) and natural killer (NK) cells. INF-γ is the main cytokine that drives NK cells to stop tumor progression and NK cells to activate T cells to respond to danger (Lamagna et al. 2006). Tissue repair is critical for the body to regenerate tissue tissue destruction during the inflammatory response. Macrophages are crucial for tissue resolution. This occurs through the secretion of the cytokine GM-CSF which is a colony-stimulating factor; proliferates and causes tissue healing. Additionally, macrophages secrete collagenase and elastase enzymes that promote tissue repair.