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Figure 16.2. B cell activation. Binding of antibody activates tyrosine kinase and tyrosine phosphorylase in the cytoplasm that then phosphorylates and dephosphorylates tyrosine residues on the Ig-α/Ig-β polypeptide. These phosphorylation/dephosphorylation reactions activate B cells by at least three different pathways: (i) Activated tyrosine residues on the Ig-α/Ig-β polypeptide cause phospholipase C activity to increase. Phospholipase C then cleaves phospholipids into inositol triphosphate and diacyl glycerol. Diacyl glycerol activates protein kinase C eventually leading to the formation of nuclear factor NF-κB. (ii) Inositol triphosphate causes Ca2+ influx from the endoplasmic reticulum and the outside environment. Increased Ca2+ concentrations activate calmodulin that in turn phosphorylates Ets-1, a DNA binding protein. (iii) Activated tyrosine residues on the Ig-α/Ig-β polypeptide activate the p21ras protein, which leads to the activation of a serine/threonine kinase. This in turn phosphorylates cJUN, another DNA binding protein. NF-κB, Ets-1 and cJUN then travel to the nucleus and cause the transcription of specific genes important in B cell activation.