ABSTRACT

On encounter with infectious agents, the first line of protection is the innate immune system.1

This system has both humoral and cellular components and is ready to act as soon as pathogens enter an organism. Macrophages can be considered prototype cells of the innate immune system.1 They are strategically placed in sites which have contact with the external environment (e.g. lung) or in organs to help clearance of already entered pathogens (e.g. spleen, liver, peritoneum). Classifications of macrophages have changed over time and now they are considered to be a part of the mononuclear phagocyte system (MPS) together with monoblasts, promonocytes and monocytes.2 This classification has replaced the previous one (reticuloendothelial or reticulohistiocyte system) because of the lack of enough common features of cells classified in this system, such as fibroblasts or mesothelial cells, with macrophages.2 Cells belonging to the MPS have in common morphological characteristics (detected by light and electron microscopy), express enzymes such as non-specific esterase, lysosomal hydrolases and ectoenzymes (detected by histochemical staining) and are capable of non-specific uptake of particles (latex or colloidal carbon). Also, they express specific receptors, especially for the Fc portion of immunoglobulin and for complement

system components, called opsonins.3 Because none of the available methods and markers are optimal, classification of a cell population as mononuclear phagocytes must therefore be based on a combination of properties.