The immune system can be looked at as a cognitive system. This is often done inanalogy to the neuro-psychological system. Here, it is demonstrated that the cognitivefunctions of the immune system can be properly described within a new theory ofcognitive science. Gärdenfors’ geometrical framework of conceptual spaces is appliedto immune cognition. Basic notions, like quality dimensions, natural properties andconcepts, similarities, prototypes, saliences etc., are related to cognitive phenomena ofthe immune system. Constraints derived from treating the immune system within acognitive theory, like Gärdenfors’ conceptual spaces, might well prove to beinstrumental for the design of vaccines, immunological diagnostic tests, andimmunotherapy.