Macrophages are specialised phagocytic cells derived from the blood monocytes (one type of white blood cells) which facilitate the destruction of the extracellular antigen and other pathogenic substances by phagocytosis and also act as antigen presenting cell (APC) and processed and present peptide antigen to the naïve T-cell and activate it. Macrophages are mainly two types – Fixed macrophages and Wandering Macrophages. Function of macrophages is very important to induce our immune response.
LOCATION OF MACROPHAGES: Macrophages developed in bone marrow and migrates through blood vessels to tissue present in different body parts.
PROPERTIES: Some important properties of macrophages are-
- Macrophages contain many types of surface receptors on their plasma membrane like- complement receptors for complement protein binding.
- It also express receptors for certain classes of antibody. When an antigen is coated with the appropriate antibody, then the complexes of antigen and antibody (immune complex) bind to the antibody receptors on the macrophage membrane and enhance the phagocytosis of the antigen.
- Macrophages also contain Major histocompatibility complex (MHC) which helps to express the peptide antigen to the naïve T-cell.
FUNCTION OF MACROPHAGES: Macrophages are one of the very important immune cells which play many different roles in innate immunity and also trigger our adaptive immune response through different ways.
PHAGOCYTOSIS: Macrophages act as phagocytes and engulf the bacteria and other microbes through endocytosis (in phagosome) and after that they breakdown the trapped microbe or antigen into simple amino acids. It is done by the degradative mechanism of lysosomal enzyme after the formation of phagolysosomes (phagosome + lysosome). The macrophages participate in phagocytosis is known as inflammatory macrophages.
- ANTIGEN PRESENTATION: Macrophage also acts as antigen presenting cell. They contain MHC molecule (mainly MHC-II) and form a complex of the peptide antigen (formed after phagocytosis) and MHC-II molecule. Then they express the complex on their surface. Naïve T-cell can not recognize the free antigen, they only can recognize the antigen when they are bound with the MHC-molecule. The Ag-MHC-II complex expressed on the surfaces of the macrophages activate the naiver helper T-cell and developed it into mature and effector T-cell after the interaction between T-cell receptor (TCR) and Ag-MHC-II complex and macrophages present the antigen mainly to the T-helper cell and trigger the further immune response.
- Some macrophages are long-term residents in tissue and play an important role in regulating their repair and regeneration.
- The activated, inflammatory macrophages are more effective than the resting ones in eliminating the pathogenic antigen. They exhibit greater phagocytic activity to kill the ingested microbes and increased secretion of inflammatory cytokines and cytotoxic mediator.
NOTABLE: Monocytes are mobile white blood cells that can migrate into tissue in response to infection. They can differentiate into a variety of histological forms. The macrophage present in the central nervous system (CNS) is known as Microglial cell. The monocytes (macrophages) present in kidney is known as Kupffer cell and there are also some other macrophages called mesangial cell. There are alveolar macrophages present in the lung. The macrophages present in the bone known as osteoclast.