LCP1 (lymphocyte cytosolic protein 1) is an actin-binding protein that functions as a critical regulator of immune cell cytoskeletal dynamics and hematopoietic cell division. As an actin filament-bundling protein, LCP1 modulates T-cell activation through TCR/CD3 and CD28 costimulation pathways, regulating cell surface expression of activation markers IL2RA/CD25 and CD69 1. Beyond T-cells, LCP1 serves as a cell-type-specific extracellular vesicle marker for microglia-like cells, suggesting roles in neuroinflammatory processes relevant to Alzheimer's disease 2. In cancer immunotherapy, LCP1 functions as a biomarker for immune-related adverse events during checkpoint immunotherapy 3 and correlates with favorable prognosis in triple-negative breast cancer through associations with M1 macrophage and memory CD4 T-cell infiltration 4. LCP1 also promotes chemotaxis via JAK2/STAT3 pathway activation and is implicated in drug resistance mechanisms in ovarian cancer 5. Clinically, germline LCP1 mutations cause severe congenital immunodeficiency characterized by neutropenia, lymphopenia, monocytopenia, and defective cytokinesis through gain-of-function effects on F-actin bundling 6. These findings establish LCP1 as both a key immune regulator and a potential therapeutic target in immunological and malignant diseases.