24.1 Cytokine receptors

Cytokine receptors can be distinguished based on their structure and consist basically of two units, one for the ligand binding and the other for signal transduction. Broad classification of cytokine receptors can be listed as under –

• •  Proteins that acts as Tyrosine kinases and function as growth factors.

•  One class of receptors is membrane-bound Guanosine triphosphate – binding proteins called as G proteins. In active state these bind GTP.

•  Channel linked receptors.

•  This class of receptor stimulates a neutral sphingomyelinase.

24.2 Cytokine regulation

The three major ways for regulating the cytokine signaling are –

• •  By specific binding proteins

•  By changes in receptor expression and

•  By cytokines that exert opposite effects.

Concept behind understanding the cytokine regulation is that as the single cell receives signals from numerous cytokine receptors, a unification of multiple signals is required to generate a coherent response.

24.3 Signal transduction and pathways

During the signal transduction the first step is the binding of cytokines to its receptors. Once the binding occurs, the receptor transmits a signal to the cell to alter its behavior. This phenomenon involving conversion of an extracellular signal into a series of intracellular events is called signal transduction. As Cell signaling needs to be precise and quick, enzyme cascade serve this function by the amplification of the responses rapidly. The important features of signal transduction include binding of cytokine to a cell surface receptor, stimulation of a transducer protein by the receptor, secondary activation of various enzymes, production of new transcription factors and ultimately gene stimulation leading to changed cell behavior.

Although there are many pathways of signal transduction but three pathways are of utmost significance to the immune system.

• •  The NF-kB pathway

•  The NF-AT pathway

•  The JAK/STAT pathway