Macrophages were first described by Russian scientist Metchnikoff (1883) who suggested that the monocyte macrophage system plays a vital role in host defense by performing two important functions— (1) phagocytosis and (2) antigen presentation.

Monocytes/macrophages originate from bone marrow, from a separate lineage, i.e. from the granulocyte-monocyte progenitor cells.

Monocytes: They are present in blood; they are the largest blood cells measuring 12–20 µm size. They do not divide and have an average transit time of 8 hours in blood; then they migrate to tissues.

Macrophages: When monocytes migrate to tissues, they transform into macrophages (Fig. 1). Macrophages differ from monocytes in the following:

- 5–10 folds larger than monocytes

- Contain more lysozymes and cell organelles

- Produce more lytic enzymes and cytokines

- Possess greater phagocytic activity

- Have a longer life in tissues (months to years).

Most macrophages are motile, travel by amoeboid movement throughout the tissues and are called as free or wandering macrophages. While, some reside in particular tissue, become non-motile and are called fixed macrophages. Macrophages in various tissues are designated by different names (Table 1).

Fig1. Macrophage.

Table1. types of macrophages.

Secretory Products of Macrophages

Activated macrophages in turn produce a number of secretory products which mediate various functions (Table 2).

Table2. Secretory products of activated macrophages.

Functions of Macrophage

- Phagocytosis: Macrophages are the principle cells involved in phagocytosis. Macrophages from various tissues are together called as the mononuclear phagocyte system (or previously known as the reticuloendothelial system). They also remove old dying cells from the body. The steps involved in phagocytosis are as follows (Fig. 2):

* Recognition: Attachment of the microbe to the receptors present on the surface of macrophage, such as toll like receptors or immunoglobulin G (IgG)

* Engulfment: Microbe is ingested with subsequent formation of a phagocytic vacuole (phagosome)

* Fusion of lysosome with phagosome to form phagolysosome

* Killing or degradation of the ingested microbes which is accomplished largely by both:

^ Oxygen independent killing—degradation by lysosomal enzymes

^ Oxygen dependent killing by generating free radicals (Table 2).

- Antigen presentation: Macrophages also promote adaptive immunity, by acting as antigen presenting cells (APCs). Macrophages capture the antigen, process into smaller antigenic peptides and present the antigenic peptides along with the MHC class II molecules to the helper T cells; thus facilitating helper T cell activation

- Activated macrophages: On exposure to certain cytokines such as interferon-γ, macrophages become activated. The activated macrophages have greater phagocytic ability and produce many cytokines that act against intracellular bacteria, virus infected cells and tumor cells. They also express higher level of MHC class II, hence can act as efficient APCs

- Secretory products of macrophages have various biological functions (Table 2):

* Interleukin 1 (IL-1): It promotes inflammatory responses, fever, and activate helper T cells

*  IL-6 and TNF-α: They promote innate immunity, (inflammation and fever) and eliminate the pathogens

* Interferon α and β: They have antiviral activity

* TNF-α: It lyses the tumor cells (antitumor activity)

* Growth factors, such as CSF (colony-stimulating factor)—promote hematopoiesis

* Following tissue injury, various mediators are secreted from macrophage; which help in tissue repair and scar formation.

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مواضيع ذات صلة

B Lymphocytes

T Lymphocytes

Immunologic Manifestations of Infectious Mononucleosis

Peripheral Lymphoid Organs

Central Lymphoid Organs

Effector Functions of Complement

Complement Pathways

Mechanisms of innate immunity

Immunologic Manifestations of Viral, Rickettsial, and Mycoplasmal Diseases

Immunologic Manifestations of Fungal Diseases

The Structure of Antibodies

Myelomas and Monoclonal Antibodies