Non-receptor tyrosine kinase involved in various processes such as cell growth, development, or differentiation. Mediates essential signaling events in both innate and adaptive immunity and plays a crucial role in hematopoiesis during T-cells development. In the cytoplasm, plays a pivotal role in signal transduction via its association with type I receptors sharing the common subunit gamma such as IL2R, IL4R, IL7R, IL9R, IL15R and IL21R. Following ligand binding to cell surface receptors, phosphorylates specific tyrosine residues on the cytoplasmic tails of the receptor, creating docking sites for STATs proteins. Subsequently, phosphorylates the STATs proteins once they are recruited to the receptor. Phosphorylated STATs then form homodimer or heterodimers and translocate to the nucleus to activate gene transcription. For example, upon IL2R activation by IL2, JAK1 and JAK3 molecules bind to IL2R beta (IL2RB) and gamma chain (IL2RG) subunits inducing the tyrosine phosphorylation of both receptor subunits on their cytoplasmic domain. Then, STAT5A AND STAT5B are recruited, phosphorylated and activated by JAK1 and JAK3. Once activated, dimerized STAT5 translocates to the nucleus and promotes the transcription of specific target genes in a cytokine-specific fashion. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Interacts with STAM2 and MYO18A. Interacts with SHB (By similarity). In contrast with the ubiquitous expression of the other JAKs, JAK3 is predominantly expressed in hematopoietic tissues. Possesses two phosphotransferase domains. The second one contains the catalytic domain, while the presence of a pseudokinase domain is required for suppression of basal activity of JAK3. Autophosphorylated, leading to regulate its activity. IL2 promotes phosphorylation on tyrosine residues, including autophosphorylation on Tyr-781 (By similarity). Dephosphorylation of Tyr-976 and Tyr-977 by PTPN2 negatively regulates cytokine-mediated signaling (By similarity). Mice show a severe block in B-cell development at the pre-B stage in the bone marrow. Additionally, they possesses small thymuses revealing a defect in T-cell development. The distribution of developmental subsets is relatively normal, suggesting a block in the expansion of early T-cell progenitors. Peripheral T-cells are present at normal or increased numbers but are functionally incompetent. Belongs to the protein kinase superfamily. Tyr protein kinase family. JAK subfamily.