<p>This study examines how Human Milk Banking Association of North America (HMBANA) biobanks foster solidarity among donors, volunteers, and communities, transforming individual prosocial behaviors into collective action. I define solidarity as mutual recognition of common goods based on shared similarities that unites individuals through intentional participation, distinguishing between rhetorical claims and practical expressions of solidarity. Through digital ethnography and visual analysis, I investigate the relationships between belongingness, solidarity, and maternal identity in milk donation contexts. My findings reveal two primary solidarity-building mechanisms: solidarity through anonymous sharing and solidarity through community-driven partnerships. The research demonstrates how individual acts of volunteering evolve into expressions of solidarity within a maternal community, cultivating belongingness among donors who share a common identity and purpose. HMBANA’s volunteer-dependent model reinforces this solidarity through community engagement, shifting our understanding of human milk donation from an individual altruistic act to one shaped by collective action and shared experiences. This framework highlights how nonprofit milk biobanks foster communal bonds and promote collective well-being, offering insights for enhancing community engagement in healthcare initiatives. By examining these dynamics, this study contributes to broader conversations about prosocial behavior transformation into meaningful collective action through institutional practices and shared values.</p>