Fig. 3.

Classical conditioning of the immune system. This figure is a graphical representation of the classic taste aversion experiment by Ader and Cohen (1991). Here, the unfilled circles at the top represent the hidden states of the generative model (the saccharine-context for the nervous system and the presence of infection for the immune system). The filled blue circles represent sensory states or ‘observations’ (e.g. MHC-I presentation of sheep red blood cell antigens). The ε and μ symbols represent prediction errors and expectations (of a categorical sort) of the nervous (right) and immune (left) systems, which encode probabilistic beliefs (Q) about the hidden states. Cyclophosphamide treatment (unconditioned stimulus) suppresses the immune response, which precludes an ‘infected’ inference in the presence of antigen. We could interpret this as attenuating the precision with which antigens are predicted (allowing for some probability of detection in the non-infected condition, and for some probability of non-detection in the infected condition). If this happens in the presence of saccharin only, this attenuated precision may be learned in a context specific way. Eventually, the presence of saccharin (conditioned stimulus), leading to an inference of ‘context 1’ implies low precision in the immune modality, and an attenuated immune response even in the presence of antigen