ZAP70 is a cytoplasmic protein tyrosine kinase that serves as a master regulator of adaptive immunity, functioning at the critical intersection of multiple signaling pathways controlling T and B lymphocyte development and activation 1. Upon T cell receptor (TCR) engagement by antigen-presenting cells, phosphorylation of TCR components CD3 and CD247 recruits ZAP70 to the plasma membrane through ITAM motifs, where Lck-mediated phosphorylation relieves its autoinhibited state 2. ZAP70 subsequently phosphorylates essential adapter proteins LAT and LCP2, initiating downstream signaling cascades that promote lymphokine production, T cell proliferation, and differentiation 2. Beyond activation, ZAP70 regulates the TCR activation switch through interactions with ubiquitin ligase CBL, controlling TCR internalization and surface expression 1. CD28 co-stimulation enhances ZAP70 recruitment and activation kinetics by accelerating Lck dynamics in TCR microclusters, potentially lowering activation thresholds 3. ZAP70 deficiency causes severe combined immunodeficiency with reduced CD8+ T cells and nonfunctional CD4+ T cells 4, while partial defects associate with autoimmunity and altered immune tolerance 1. ZAP70 polymorphisms contribute to type 1 diabetes susceptibility 5, and aberrant expression in B-cell chr2 lymphocytic leukemia correlates with poor prognosis 1, highlighting ZAP70's critical role in balancing effective immunity against pathogenic tolerance and malignancy.