**Abstract**

The main line of defense that exists to eliminate foreign agents falls on phagocytic cells (neutrophils, dendritic cell, and macrophages), and it does so through phagocytosis, a complex cellular mechanism that occurs after the recognition and binding of the ligand by cellular receptors. Macrophages are part of a diverse lineage of innate immune cells. Once a macrophage receptor binds its ligand, a coordinated intracellular signaling cascade is activated to the clearance or otherwise of the foreign agent. Objects removed by macrophage phagocytosis include dead or dying host cells, cells opsonized with antibodies, and specific pathogens such as bacteria, fungi, parasites, and viruses. Currently, phagocytic macrophages have been shown to contribute to the killing of cancer cells, inflammatory bowel disease, atherosclerosis, Alzheimer's disease, and schizophrenia. For this reason, phagocytic macrophages are important in critical participation for health and disease.

**Keywords:** macrophages, phagocytosis, early phagosome, late phagosome, phagolysosome

### **1. Introduction**

Macrophages are cells distributed in all body compartments under physiological conditions, presenting various forms and functions that depend on environmental stimuli. After their origin, macrophages are distributed to different tissues, taking the name of the tissue where they are maintained or circulate in the blood as monocytes until they face a foreign body, becoming macrophages [1, 2]. It is considered that the half-life of the macrophage is 70 h, and they make up 4–10% of the total leukocytes in peripheral blood, meaning that they constitute the second cell population of the immune system.

Macrophages are essential to innate immunity since they secrete more than 100 biologically active products and present diverse functions with different phenotypes, occupying dozens of extra and intracellular receptors. Owing to their versatility, macrophages actively participate in physiological and pathophysiological processes. As previously described, they have attributed three critical activities in the host: homeostasis, immune response, and phagocytosis [3].

In this chapter, we will focus on different parts of macrophage phagocytosis.
