Mutations in Leucine-Rich Repeat protein Kinase 2 (LRRK2) are associated with Parkinson's disease (PD) and Crohn's disease (CD), but the regulation of LRRK2 during inflammation remains relatively unexplored. Here we describe the development of a flow cytometry-based assay to assess LRRK2 activity in individual cells and the generation of an EGFP-Lrrk2 knock-in reporter mouse to analyse cell-specific LRRK2 expression. Using these tools, we measured LRRK2 levels and activity in murine splenic and intestinal immune cells and in human blood. Anti-CD3 induced inflammation increases LRRK2 expression and activity in B cells and monocytes, while in mature neutrophils, inflammation stimulates activity but reduces LRRK2 expression. A kinase-activating PD-associated LRRK2-R1441C mutation exacerbates inflammation-induced activation of LRRK2 specifically in monocytes and macrophages. We identify IL-4 as a novel T-cell-derived factor that upregulates LRRK2 expression and activity in B cells, replicating inflammatory effects observed in vivo. Our findings provide valuable new insights into the regulation of the LRRK2 pathway in immune cells, crucial for understanding LRRK2 and its therapeutic potential in inflammatory diseases such as CD.