The circulating phagocytic immune cell is considered to be the main effector of the invertebrate defense system, involved in both immune and neuroendocrine responses, showing the functional characteristics of vertebrate macrophage. Various names have been used to define this cell in different taxa i.e., hemocyte, celomocyte, amebocyte, plasmatocyte, etc. However, regardless of the terminology, these cells perform the same immune function, and possess very similar morphology. For these reasons, it is suggested that the general term immunocyte be used to describe these cells in invertebrates.
T cells are a type of white blood cell known as lymphocyte. Lymphocytes protect the body against cancerous cells and cells that have become infected by pathogens, such as bacteria and viruses. T cell lymphocytes develop from stem cells in bone marrow. These immature T cells migrate to the thymus via the blood. The thymus is a lymphatic system gland that functions mainly to promote the development of mature T cells.
T cell lymphocytes are necessary for cell mediated immunity, which is an immune response that involves the activation of immune cells to fight infection. T cells function to actively destroy infected cells, as well as to signal other immune cells to participate in the immune response.
B cells are a lineage of lymphocytes originally discovered in the Bursa of Fabricus in birds (hence the B in B cells). B cell development in mammals was mainly found to occur in the bone marrow, as they do not have a Bursa of Fabricus. B cells are not only responsible for antibody production, but are also involved in several other areas of immunology.
Natural killer cells (also known as NK cells, K cells, and killer cells) are a type of lymphocyte (a white blood cell) and a component of innate immune system. NK cells play a major role in the host-rejection of both tumors and virally infected cells. NK cells are cytotoxic; small granules in their cytoplasm contain special proteins such as perforin and proteases known as granzymes. Upon release in close proximity to a cell slated for killing, perforin forms pores in the cell membrane of the target cell through which the granzymes and associated molecules can enter, inducing apoptosis.
Dendritic cells (DCs) are the main inducers and regulators of cytotoxic T lymphocyte (CTL) responses against viruses and tumors. One checkpoint to avoid misguided CTL activation, which might damage healthy cells of the body, is the necessity for multiple activation signals, involving both antigenic as well as additional signals that reflect the presence of pathogens. DCs provide both signals when activated by ligands of pattern recognition receptors and “licensed” by helper lymphocytes.
T helper cells are arguably the most important cells in adaptive immunity, as they are required for almost all adaptive immune responses. They not only help activate B cells to secrete antibodies and macrophages to destroy ingested microbes, but they also help activate cytotoxic T cells to kill infected target cells.
Treg cells are defined as CD4+ T cells in charge of suppressing potentially deleterious activities of Th cells. Identification of Treg cells remains problematic, because accumulating evidence suggests that all the presently-used Treg markers (CD25, CTLA-4, GITR, LAG-3, CD127 and Foxp3) represent general T-cell activation markers, rather than being truly Treg-specific. Treg-cell activation is antigen-specific, which implies that suppressive activities of Treg cells are antigen-dependent.
The monocyte is a large type of white blood cell with one large, smooth, well-defined, indented, slightly folded, oval, kidney-shaped, or notched nucleus (the cell’s control center). White blood cells help protect the body against diseases and fight infections. The number of monocytes in the blood can be detected with a test known as a complete blood count (CBC) with differential. A CBC provides important information about the kinds and numbers of cells in the blood.
A type of immune cell that is one of the first cell types to travel to the site of an infection. Neutrophils help fight infection by ingesting microorganisms and releasing enzymes that kill the microorganisms. A neutrophil is a type of white blood cell, a type of granulocyte, and a type of phagocyte.
Mast cell, tissue cell of the immune system of vertebrate animals. Mast cells mediate inflammatory responses such as hypersensitivity and allergic reactions. They are scattered throughout the connective tissues of the body, especially beneath the surface of the skin, near blood vessels and lymphatic vessels, within nerves, throughout the respiratory system, and in the digestive and urinary tracts.
Macrophages are immune system cells that are vital to the development of non-specific defense mechanisms that provide the first line of defense against pathogens. These large immune cells are present in nearly all tissues and actively remove dead and damaged cells, bacteria, cancerous cells, and cellular debris from the body. The process by which macrophages engulf and digest cells and pathogens is called phagocytosis.