Non-receptor tyrosine kinase involved in B-lymphocyte development, differentiation and signaling (By similarity). B-cell receptor (BCR) signaling requires a tight regulation of several protein tyrosine kinases and phosphatases, and associated coreceptors (By similarity). Binding of antigen to the B-cell antigen receptor (BCR) triggers signaling that ultimately leads to B-cell activation (By similarity). Signaling through BLK plays an important role in transmitting signals through surface immunoglobulins and supports the pro-B to pre-B transition, as well as the signaling for growth arrest and apoptosis downstream of B-cell receptor (By similarity). Specifically binds and phosphorylates CD79A at 'Tyr-188'and 'Tyr-199', as well as CD79B at 'Tyr-196' and 'Tyr-207' (By similarity). Phosphorylates also the immunoglobulin G receptors FCGR2A, FCGR2B and FCGR2C (PubMed:8756631). With FYN and LYN, plays an essential role in pre-B-cell receptor (pre-BCR)-mediated NF-kappa-B activation (By similarity). Contributes also to BTK activation by indirectly stimulating BTK intramolecular autophosphorylation (By similarity). In pancreatic islets, acts as a modulator of beta-cells function through the up-regulation of PDX1 and NKX6-1 and consequent stimulation of insulin secretion in response to glucose (PubMed:19667185). Phosphorylates CGAS, promoting retention of CGAS in the cytosol (PubMed:30356214). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Antibody-mediated surface engagement of the B-cell antigen receptor (BCR) which results in the phosphorylation of BLK on tyrosine residues, stimulates the enzymatic activity. Interacts with CBL (via SH2 domain). Interacts with CD79A and CD79B (via SH2 domain) (By similarity). Present and active in lipid rafts. Membrane location is required for the phosphorylation of CD79A and CD79B (By similarity). Expressed in lymphatic organs, pancreatic islets, Leydig cells, striate ducts of salivary glands and hair follicles. Expression is under the control of NF-kappa-B as well as the B-cell specific transcription factors PAX5 and EBF1. Phosphorylated on tyrosine residues after antibody-mediated surface engagement of the B-cell antigen receptor (BCR). Ubiquitination of activated BLK by the UBE3A ubiquitin protein ligase leads to its degradation by the ubiquitin-proteasome pathway. The disease is caused by variants affecting the gene represented in this entry. Belongs to the protein kinase superfamily. Tyr protein kinase family. SRC subfamily.