Growth factors are proteins that bind to receptors on the cell surface, with the primary result of activating cellular proliferation and/or differentiation. Many growth factors are quite versatile, stimulating cellular division in numerous different cell types; while others are specific to a particular cell-type.
Cytokines are a unique family of growth factors. Secreted primarily from leukocytes, cytokines stimulate both the humoral and cellular immune responses, as well as the activation of phagocytic cells. Cytokines that are secreted from lymphocytes are termed lymphokines, whereas those secreted by monocytes or macrophages are termed monokines. A large family of cytokines are produced by various cells of the body. Many of the lymphokines are also known as interleukins (ILs), since they are not only secreted by leukocytes but also able to affect the cellular responses of leukocytes. Specifically, interleukins are growth factors targeted to blood cells.
- Epidermal Growth Factor (EGF) like all growth factors, binds to very specialized receptors on the surface of responsive cells. Intrinsic to the EGF receptor is tyrosine kinase activity, which is activated in response to EGF binding. EGF stimulates mesodermal and ectodermal origin, particularly of the skin and connective tissue. EGF inhibits certain carcinomas as well as hair follicle cells. EGF also has the effect of decreasing gastric acid secretion. The receptor for EGF shares a similarity for the blood type A antigen; in tests, antibodies directed against the EGF receptor also bind to the blood type A antigen as well. This could play a role in the increased incidence of certain cancers with type A, and also perhaps why stomach acid secretions are lower in blood type A over other blood types. (, , , )
- Interleukin-1 (IL-1) is one of the most important immune modifying interleukins. The predominant function of IL-1 is to enhance the activation of T-cells in response to antigen.
- Interleukin-2 (IL-2) is produced and secreted by activated T-cells and is the major interleukin responsible for T-cell proliferation. IL-2 also exerts effects on B-cells, macrophages, and natural killer (NK) cells. The production of IL-2 occurs primarily by CD4+ T-helper cells.
- Interleukin-4 (IL-4) induces B cell proliferation, eosinophil and mast cell growth and function, and IgE expression on B cells. Allergic individuals have a greater propensity to produce IL-4. Many dietary lectins induce the activation of IL-4, which implies that for many people the allergic reaction to dietary lectins may be more important than their agglutinating abilities.
- Interleukin-8 (IL-8) is an interleukin that acts as a ‘chemo-attractant’ for leukocytes and connective tissue cells. IL-8 is produced by monocytes, neutrophils, and NK cells and is acts as a homing defice for neutrophils, basophils and T-cells.
- Tumor Necrosis Factor (TNF) comes in two flavors: alpha and beta. TNF-alpha is a major immune response-- modifying cytokine produced primarily by activated macrophages. TNF-alpha increases cellular responsiveness to growth factors and induces signaling pathways that lead to proliferation. TNF-beta, produced by cytotoxic T-cells, is characterized by its ability to kill a number of different cell types and induce other to differentiate into other forms which result in their death.
- Interferon production in a majority of cases is induced in response to microbes such as viruses and bacteria and their products (viral glycoproteins, viral RNA, bacterial endotoxin, flagella, CpG DNA), as well as mitogens and other cytokines, for example interleukin 1, interleukin 2, interleukin-12, tumor necrosis factor and colony-stimulating factor, that are synthesised in the response to the appearance of various antigens in the body.
- Erythropoietin (Epo) is synthesized by the kidney and stimulates the proliferation and differentiation of immature red blood cells. When patients suffering from anemia due to kidney failure or chemotherapy are given Epo, the result is a rapid and significant increase in red blood cell count.
- Colony Stimulating Factors (CSFs) stimulate the proliferation of specific cells of the bone marrow in adults. Granulocyte-CSF (G-CSF) is specific for its effects on cells of the granulocyte lineage. Macrophage-CSF (M-CSF) is specific for cells of the macrophage lineage Epo is also considered a CSF as well as a growth factor, since it stimulates the proliferation of erythrocyte colony-forming units. IL-3 (secreted primarily from T-cells) is also known as multi-CSF, since it stimulates stem cells to produce all forms of hematopoietic cells.