Macrophages- Introductions and Functions


Macrophages are a type of white blood cells that play a vital role in the immune system. They are derived from monocytes, which circulate in the blood and migrate into tissues where they differentiate into macrophages. Macrophages are part of the mononuclear phagocytic system (MPS), which consists of monocytes, macrophages and their precursors in the bone marrow.

The main function of macrophages is to phagocytose (ingest) and destroy foreign particles, such as bacteria, viruses, fungi, parasites and dead cells. They also act as antigen-presenting cells (APCs), which means they display fragments of the ingested particles on their surface in association with molecules called major histocompatibility complex (MHC) class II. This allows them to activate other immune cells, such as helper T cells, that recognize the antigens and mount a specific immune response against them.

Macrophages are not only involved in host defense, but also in tissue homeostasis and repair. They help to clear away dead cells and debris from sites of infection or injury, and secrete various cytokines (signaling molecules) that regulate inflammation, wound healing, angiogenesis (new blood vessel formation) and fibrosis (scar tissue formation).

Macrophages are not a homogeneous population of cells, but rather exhibit a remarkable diversity and plasticity depending on their tissue location and activation state. Different types of macrophages have different names according to their anatomical location, such as alveolar macrophages in the lung, Kupffer cells in the liver, microglial cells in the brain and osteoclasts in the bone. Moreover, macrophages can be activated by various stimuli, such as cytokines, microbial components or environmental factors, and acquire different functional phenotypes that reflect their role in different immune responses. For example, classically activated macrophages (also called M1) are pro-inflammatory and microbicidal, while alternatively activated macrophages (also called M2) are anti-inflammatory and tissue-repairing.

In this article, we will discuss the differentiation of monocytes into tissue macrophages, the tissue-specific functions of macrophages, the activation of macrophages by different stimuli, and the roles of macrophages in innate and adaptive immunity. We will also highlight some of the mechanisms by which macrophages phagocytose and kill microbes, present antigens to T cells, produce cytokines and modulate tissue repair.