On the evolutionary ecology of specific immune defence

Evolutionary ecology has developed along two major conceptual avenues, starting from the observation that hosts vary in their immune defence against parasites The first avenue, rooted in life-history theory, assumes fitness costs of immune defence and tradeoffs in the face of limited resources, rather than specific host–parasite interactions. The second avenue focuses on specific responses, especially those generated by genotype–genotype interaction between and within host and parasite species. Specificity in the interactions between hosts and parasites play a crucial role in the field but analysis is difficult. Here, we classify concepts about host–parasite interactions into these two families and discuss their reconciliation with the help of a defence component model and two-dimensional classification scheme of the individual components. This helps to clarify some of the confusing terminology and might guide further research in the field.