The production of antibodies requires the expansion and selection of high-affinity B cell clones. This process is initiated by antigen uptake through the B cell receptor (BCR), which recognizes and binds antigen displayed on the surface of an antigen-presenting cell (APC). To acquire the antigen, B cells use myosin contractility to physically pull BCR-antigen clusters from the APC membrane. These mechanical forces influence association and dissociation rates of BCR-antigen bonds, resulting in affinity-dependent acquisition of antigen by B cells. Mechanical regulation of B cell antigen acquisition from APCs remains poorly understood, although the recent development of DNA-based force sensors has enabled the measurement of mechanical forces generated in B cell-APC contacts. In this chapter, we describe a protocol to design, synthesize, and purify DNA-based force sensors to measure B cell antigen extraction forces using fluorescence microscopy.