In the humoral immune response, B cells that produce high-affinity antibodies are generated through a Darwinian evolutionary process called affinity maturation. Affinity maturation, which takes place in germinal centers (GC), is driven by cycles of selection (mediated by T cells), division, and hypermutation of B cells. Recently, it was found that the T cells themselves also exhibit dynamic behavior throughout the course of the GC reaction, undergoing affinity-dependent proliferation. A model suggests that this reciprocal stimulation enforces a robust “homeostatic” regulation of the relative number of B cells and T cells in the GC, leading to a constant ratio of GC lymphocytes. But the model also points to a problem: the high mutation rate of GC B cells can lead to “backsliding” of affinity. The proposed solution – a fitness-dependent mutation rate – has been experimentally verified, and has implications beyond B cell affinity maturation.

Many energy, environmental, industrial, and microfluidic processes rely on the viscous flow of polymer solutions through porous media. These fluids are typically shear-thinning; however, these solutions can unexpectedly flow thicken when forced through confined, tortuous spaces such as in porous media. The reason why has been a puzzle for over half a century. In this talk, I will describe how by directly visualizing the flow in a transparent 3D porous medium, we have found that this anomalous flow thickening reflects the onset of an elastic instability in which the fluid exhibits chaotic velocity fluctuations reminiscent of inertial turbulence, despite the vanishingly small Reynolds number. In addition to characterizing this fascinating flow state, we have found that this phenomenon can be harnessed for improving mixing and the efficiency of flow-mediated chemical reactions — with implications for a broad range of processes that are typically limited by poor mixing.

Replaced by Center for Living Systems student-organized special lecture.