The germinal centre(GC) reaction supports affinity-based B-cell competition and generates high-affinity bone-marrow plasma cells (BMPCs)(1,2). How follicular T-helper (T-FH) cells regulate GC selection is not clear(3,4). Using competitive mixed chimaera, we show here that, beyond the role in promoting T-FH development(5-7), ICOSL (inducible T-cell co-stimulator ligand, also known as ICOSLG) is important for individual B cells to competitively participate in the GC reaction and to develop into BMPCs. Using intravital imaging aided by a calcium reporter, we further show that ICOSL promotes an 'entangled' mode of T-FH-B-cell interactions, characterized by brief but extensive surface engagement, productive T-cell calcium spikes, and B-cell acquisition of CD40 signals. Reiterated entanglement promotes outer-zone co-localization of outcompeting GC B cells together with T-FH cells, affording the former increased access to T-cell help. ICOSL on GC B cells is upregulated by CD40 signals. Such an intercellular positive feedback between contact-dependent help and ICOSL-controlled entanglement promotes positive selection and BMPC development, as evidenced by observations that higher-affinity B-cell receptor variants are enriched in the ICOSLhigh fraction, that numerically disadvantaged ICOSL-deficient GC B cells or BMPCs exhibit strong affinity compensation in competitive chimaera, and that when GC competition proceeds without ICOSL, selection of high-affinity variants in otherwise normal GC reactions is impaired. By demonstrating entanglement as the basic form of GC T-FH-B-cell interactions, identifying ICOSL as a molecular linkage between T-B interactional dynamics and positive selection for high-affinity BMPC formation, our study reveals a pathway by which T-FH cells control the quality of long-lived humoral immunity.