Figure 1.

Phase portraits for the dimensionless immune model (equation 2.1) as immune stimulation increases across the columns. Across the top row, ${\displaystyle {\beta }_1={\beta }_2=0.05\left(\mathit{a}\right),0.08\left(\mathit{b}\right),0.11\left(\mathit{c}\right)\mathrm{a}\mathrm{n}\mathrm{d}0.14\left(\mathit{d}\right)}$ ; across the bottom row, ${\beta }_2=2{\beta }_1$ , with ${\displaystyle {\beta }_1=0.04\left(\mathit{e}\right),0.06\left(\mathit{f}\right),0.07\left(\mathit{g}\right)\mathrm{a}\mathrm{n}\mathrm{d}0.075\left(\mathit{h}\right)}$ . Nullclines are shown by black and red lines. Nullcline intersections represent equilibria. Stable nodes are indicated by filled-in triangles, with the colour indicating the immunological state (as in figure 2); unstable nodes are indicated by hollow triangles; unstable saddles are indicated by hollow diamonds.