28 research outputs found
Population mechanics: A mathematical framework to study T cell homeostasis
Unlike other cell types, T cells do not form spatially arranged tissues, but move independently throughout the body. Accordingly, the number of T cells in the organism does not depend on physical constraints imposed by the shape or size of specific organs. Instead, it is determined by competition for interleukins. From the perspective of classical population dynamics, competition for resources seems to be at odds with the observed high clone diversity, leading to the so-called diversity paradox. In this work we make use of population mechanics, a non-standard theoretical approach to T cell homeostasis that accounts for clone diversity as arising from competition for interleukins. The proposed models show that carrying capacities of T cell populations naturally emerge from the balance between interleukins production and consumption. These models also suggest remarkable functional differences in the maintenance of diversity in naïve and memory pools. In particular, the distribution of memory clones would be biased towards clones activated more recently, or responding to more aggressive pathogenic threats. In contrast, permanence of naïve T cell clones would be determined by their affinity for cognate antigens. From this viewpoint, positive and negative selection can be understood as mechanisms to maximize naïve T cell diversity
Conductivity/activation energy relationships for cement-based materials undergoing cyclic thermal excursions
The electrical conductivity of a range of concrete
mixes, with and without supplementary cementitious
materials (SCM), is studied through multiple cycles of
heating and cooling over the extended temperature range
-30/?70 C. When presented in an Arrhenius format, the
experimental results display hysteresis effects at the lowtemperature
end of the thermal cycle and, in those concretes
containing supplementary cementitious materials at
higher water/binder ratios, hysteresis effects were evident
over the entire temperature range becoming more discernible
with increasing number of thermal cycles. The
depression in both the freezing and thawing point could be
clearly identified and was used to estimate pore-neck and
pore-cavity radii. A simplified approach is presented to
evaluate the volumetric ratio of frozen pore water in terms
of conductivity measurements. The results also show that
the conductivity and activation energy of the concrete
specimens were related to the water/binder ratio, type of
SCM, physical state of the pore water and the thermal
cycling regime