A model of non-specific immunity.

Though the importance of the non-specific immune response is well known, it has often been neglected in theoretical studies. Whereas adaptive or antigen-specific immune responses arise from the proliferation of clones of antigenic-specific cells to form populations sufficiently large to control the parasite, the non-specific response involves the activation of cells such as macrophages from a reservoir consisting of a fixed number of cells. In this paper, we use simple mathematical models to investigate the dynamics of the non-specific immune response to parasites. In particular we describe the conditions under which the non-specific immune response can clear a parasite, control a parasite, or merely reduce the growth rate of a parasite. We also show that non-specific response to concurrent infections of hosts with two parasites can lead to competitive exclusion of one of the parasites. The model incorporating non-specific immunity is then expanded to include specific immune responses. This more complex model, is used to investigate the relative roles of non-specific and specific immunity in dealing with parasites and shows that the non-specific immune system may control the density of parasites prior to the generation of specific immune responses which are capable of clearing them. Finally we show that the predictions of the models conform with results from published experiments on listeria infections.