The dissertation is studying structural mutants of various domains of the Inducible Tyrosine Kinase (Itk), and their relation to functionality of this protein in immune cells. Using cell lines as well as primary cells from Itk-/- mice, and employing systematic functional study approach this work is exploring effects of several mutations (Y511F, BtkSH3, FYF) on immediate, and far downstream signaling of Itk under conditions of TCR induced stimulation. Structural (FRET), as well as biochemical and molecular biology techniques were used (western blotting, flow cytometry, ELISA). It was shown that Itk does not exhibit noticeable differences neither in structure, nor in localization in resting cells, but changes conformation and localization patterns under stimulation in T-cell-APC system. This event is pronounced in statistically significant manner in wild type Itk, but is disrupted to different extent in mutants. PH domain mutant FYF is the only one from those explored that has disrupted localization pattern, and surprisingly, nonfunctional activation residue mutant Y511F preserves intact localization. However, all mutants show disruption in structural pattern when compared to wild type Itk. Another novel finding is related to Itk-SLP-76 interaction, which proved to be disrupted in BtkSH3 mutant that lacks noncanonical SH3 domain interactions, pointing on important role of noncanonical SH3 interactions in signaling of Itk in complex with SLP-76, and as a result in activation of Itk. All mutants show some extent of disruption in Y511 phosphorylation, which has a direct effect on downstream events as assessed by PLC_ phosphorylation, and reflects defects in structure and localization patterns, with mutants being significantly deficient comparing to wild type. This results in even more pronounced effect in Th2 cytokine production, and this dissertation provides the first evidence of the influence of these mutants on Th2 cytokines, which are the signature cytokines of Itk. The dissertation proposes explanations of the mentioned phenomena, underlines an importance of different parts of Itk molecule in its functionality, and makes possible to apply the data acquired here to further understand the role of Itk in deeper detail with possible clinical implications in the future.