In this article we will discuss about the properties of cytokines (explained with diagram).
1. Cytokines are a group of low-molecular- weight regulatory proteins secreted by WBC and other cells in the body.
2. Cytokine secretion is very specific and self- limited event as because they are not usually stored as performed molecules. Cytokine synthesis is initiated by new gene transcription as a consequence of cellular activation. Once synthesized, cytokines are rapidly secreted, resulting in a burst of release when needed.
3. After secretion, cytokines (almost 60 different types of cytokines) regulate immune and inflammatory reactions (Fig. 8.1A, B).
4. Cytokines bind to specific receptors on the membrane of target cells, triggering signal- transduction pathways that ultimately alter gene expression in the target cells (Fig. 8.2). The nature of the target cell for a particular cytokine is determined by the presence of specific membrane receptors. Cytokines are so specific due to their high affinity for which Pico molar concentrations of cytokines can mediate a biological effect.
5. Cytokine actions may be local or systemic.
(i) Most of the cytokines act close to where they are produced, A particular cytokine when binds to receptors on the membrane of the same cell from where it has been secreted is called autocrine action (Fig. 8.3A).
(ii) When secreted cytokines bind to receptors on a target cell in close proximity to the producer cells, it is called paracrine action (Fig. 8.3B).
(iii) In most of the cases, cytokines act on cells that are in contact with the cytokine producers but when cytokines are produced in large amounts, it may enter the circulation and act at a distance from the site of production (Fig. 8.3C). This is called endocrine action.
6. Cytokines often influence the synthesis and actions of other cytokines and different immune cells. The ability of one cytokine to stimulate production of others, leads to cascade in which a second or third cytokine may mediate the biological effects of the first. Cytokines regulate the intensity and duration of the immune response by stimulating or inhibiting the activation, proliferation and differentiation of various cells.
7. The cytokines secreted by a single lymphocyte following antigen-specific activation can influence the activity of various cells involved in the immune response. For example, cytokines produced by activated TH cells (T-helper cells) can influence the activity of B-cells, Tc cells, natural killer cells (NK cells), macrophages, granulocytes and haematopoietic stem cells. This means that it activates an entire network of interacting cells.
8. Cytokines exhibit the attributes of pleiotropy, redundancy, synergy and antagonism.
Pleiotropism refers to the ability of one cytokine to act on different cell types which action; fewer exhibit endocrine action allows a cytokine to mediate diverse biological effects (Fig. 8.4A).
Redundancy refers to the property of multiple cytokines i.e. two or more cytokines having the same functional effects. This property of cytokine makes it difficult to explain a particular activity to a single cytokine (Fig. 8.4B).
Synergism exhibits to the phenomenon when the combined effect of two cytokines on cellular activity is greater than the additive effects of the individual cytokines (Fig. 8.4C).
Antagonism indicates the property that is just opposite to the synergism as in this case, the effects of one cytokine inhibit or offset the effects of another cytokine (Fig. 8.4D).
9. Cytokine activity is also being regulated by external signals, the expression of cytokine receptors vary and also the responsiveness of cells to cytokines. As for e.g., stimulation of T or B-lymphocytes by antigens leads to increased expression of cytokine receptors.
10. Many of the changes in gene expression induced by cytokines result in differentiation of T and B-lymphocytes and activation of effector cells such as macrophages.
11. Besides activation, cellular responses to cytokines can also include feedback inhibitory signals to the cytokine activity. These mechanisms include cytokine induction of gene encoding inhibitors of the cytokine receptors. These inhibitors may inhibit the function of cytokine receptors expressed on the cell surface, molecules that block interactions of signalling kinases, phosphatases.
