Macrophage under a microscope
Macrophages are cells carrying a protective function, primarily by means of phagocytosis of large particles. The name "Macrophage" comes from them. However, phagocytosis, being very important,it is the only function of these cells. Current studies define them as polyfunctional macrophage cells. Macrophages are formed from blood monocytes when they exit the bloodstream. Macrophages are characterized by structural and functional heterogeneity depending on the degree of maturity, the field of localization, as well as on their activation by antigens or lymphocytes.
Macrophages are divided into fixed and mobile. The macrophages of connective tissue are mobile or wandering, those are histiocytes. There are also macrophages of serous cavities (peritoneal and pleural), alveolar, macrophages of the liver, macrophages of the central nervous system: glial macrophages and osteoclasts. All of these different forms of macrophages combined form a mononuclear phagocytic system.
On the functional state macrophages are divided into the residual (inactive) and activated. Depending on this functional state, their intracellular organization is differentiated. The most characteristic structural feature of macrophages is expressed lysosomal apparatus, that is, their cytoplasm containe many lysosomes and phagosomes. Histiocytes feature the presence of many folds, invaginations and pseudopodia on their surface, reflecting the movement of cells or histiocytes' capture of a variety of particles. In plasmolemma of macrophages there is a variety of receptors, through which they recognize several antigenic particles, as well as biologically active substances.
The defensive function of macrophages is manifested in various forms:
1. nonspecific protection. The protection through phagocytosis and intracellular digestion;

2. The ejection into extracellular environment of lysosomal enzymes and other substances.

3. specific or immunologic protection, participation in a variety of immune responses.
Absorbing antigenic substances, macrophages eject, concentrate and then bring their active chemical groups to cytolemma and then transmit them to lymphocytes. This function is called antigen-presenting. With the help of this function, macrophages trigger an immune response. It is known that the majority of antigenic substances are unable to trigger an immune response by themselves, that is to act directly on receptors of lymphocytes. Besides this, activated macrophages eject several biologically active substances, which have a regulating influence on various aspects of immune reactions. Finally, macrophages are involved in the final stages of immune responses as the humoral and cellular immunity. In humoral immunity, they absorb immune complexes. In cellular immunity macrophages acquire killer properties and may destroy the alien, including tumor cells. Thus, while not being immune cells, macrophages are actively involved in immune reactions.
Macrophages also synthesize and release into the intercellular environment of around a hundred different biologically active substances. Therefore, macrophages can be attributed to the secretory cells.

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