The immune system is the body's defense mechanism against pathogens. This system comprises of many cells that work together to protect the body from foreign invaders such as bacteria, viruses, parasites as well as tumors. The immune system is divided into two primitive forms, the innate and adaptive systems. The roles for both types are vital in the immune system; the innate is the rapid, non-specific primary response against any foreign material the body encounters, whereas the adaptive system is a secondary response and a more specific line of defense. The adaptive as well as the innate responses initiation are hugely associated with each other. The innate response consists of many components; these include mechanical, chemical, humoral as well as cellular barriers. .
The main line of defense in the innate system is the cellular component; there are different types of cells involved, however, the most critical cell of all other different immune cells is called a macrophage. Macrophages are a phagocytic cells originated from monocytes, a form of blood leukocytes. Monocytes are made by bone marrow in the myeloid lineage through committed haematopoietic stem cells in a process called haematopoiesis. Macrophages carry out indispensable roles in the immune system; they are involved in both of innate and adaptive responses. Thus, macrophages are always referred as the cells that 'bridge' the innate and adaptive systems. This essay will discuss the functions and activities of the macrophage that make it the most important cell in the immune system. Monocytes are the precursors for macrophages; these monocytes migrate and differentiate into tissue macrophages once they encounter an infection. Macrophages are distributed around the whole body; they either act as resident or inflammatory macrophages that explore the tissues. Resident macrophages are found in many body organ systems; these include the liver kupffer's cells, alveolar macrophages in lungs, histiocytes in connective tissues and many others.