This paper seeks to explore the key affordances influencing patients’ engagement with online communities on Twitter. Drawing on a socio-technical perspective of affordances, the paper presents the qualitative analysis of a sample of tweets from a patients’ community chat on Twitter. The paper makes the following contributions. First, it draws on the concept of composite affordances to show the influence of affordance relationships on patients’ engagement with online communities on Twitter. Second, it extends our knowledge about the implications of the attributes of Twitter digital objects for the realization of affordances that influence engagement in patients’ online communities. The paper furthers our understanding of the main factors that influence aggregation and interaction within communities of patients on Twitter.