Fish Immunology (1+1)

Unit 3- Nonspecific immunity

3.4. Nonspecific Cellular Defence
Cell types that are involved in the non-specific cellular defense responses of teleost fish are the phagocytic cells, monocytes/macrophages, the non-specific cytotoxic cells (NCC) and granulocytes (neutrophils). Some teleosts have both acidophilic and basophilic granulocytes in peripheral blood in addition to the neutrophils, but in others only the neutrophils has been found.
Phagocytes
Fish phagocytic cells are formed mainly in the head-kidney from stem cells, as they mature, they spread throughout the body. They are most frequent in tissues underlying epithelial barriers, their principal locations being the head kidney, blood, spleen, gut-associated lymphoid tissue, liver, atrium of heart and gills.
Phagocytes are divided into two main types,

• neutrophils and
• macrophages

These phagocytes are involved in ingestion of microbes.

Neutrophils
These cells also called polymorphonuclear leukocytes (PMNs) play a key role in the development of acute inflammation. In addition to the phagocytic nature, neutrophils also contain granules that contain acidic and alkaline phosphatases, defensins and peroxidase which necessary for successful elimination of the pathogens.
Macrophages
Macrophages termed as monocytes when present in the blood stream, these are large cells. The function of macrophages include phagocytosis and antigen presentation to T cells. Macrophages are long-lived cells. Macrophages can produce chemicals that can act as antibacte¬rial agents, peroxynitrites and hydroxyl groups
Phagocytosis
Phagocytosis is one of the most important processes in poikilothermic animals because it is the process that is least influenced by tempera¬ture. Phagocytosis is the process by which cells engulf microorganisms and particles. During phagocytosis the phagocytes are attracted towards the microbe by the chemical signals. Phagocyte attaches to the microbe with the help of either microbial sugar residues present on its surface or with the help of complement/antibody which is bound to the pathogen. Once attached the phagocytic cell engulf the microbe and form phagosome. This phagosome fuses with lysosomes to form a phagolysosome. Which leads to the destruction of the pathogen.
Natural killer (NK) cells
NK cells are large granular lymphocytes that are mainly found in the blood. These NK cells of fish are morphologically distinct from the large granular lymphocytes of mammals but they are functionally similar. They contain two unique cell surface receptors known as killer activation receptor and killer inhibition receptor.
The activation of NK cells, killer activation receptor initiates the release of cytokine molecules, while the activation of killer inhibition receptor inhibits the release of cytokine molecules. NK cells attack virally-infected cells and certain tumour cells and destroy these cells by releasing perforins and granyzymes from its granules. NK cells also secrete interferon-γ (IFN-γ), where this interferon molecule to prevent healthy host cells from becoming infected by a virus and it also increase the T cell response to other virally infected cells.