Figure 2.

 Bivalent linear complex formation could be the molecular clue to superagonistic CD28 stimulation. Only CD28 superagonists (red), but not conventional anti-CD28 antibodies (yellow) or B7 molecules (green), bind to a lateral motif of the CD28 molecule (A).⁴ Therefore, only CD28 superagonists are capable of forming linear complexes with CD28 molecules (B, C). In these complexes activating signalling components, presumably, get aggregated, which is sufficient to surpass the threshold for T cell activation.