The fields of ecological immunology and disease ecology have rapidly converged in the past few years to produce studies investigating both the underlying causes for heterogeneity in host response (eco-immunology) and the mechanisms of disease spread in populations and communities (disease ecology). One cause of heterogeneity in host response is variation in underlying immune mechanisms. Recent studies have defined genetically determined patterns in life history response to infection by broad characterizations such as resistance and tolerance (e.g., Raberg & Sim, 2007; Vale & Little, 2012), but most do not directly measure immune response. Furthermore, studies measuring immune response to a particular pathogen take place in controlled laboratory settings, ignoring important environmental factors, including co-infections. No studies have characterized overall “immune phenotype” as an animal trait, though many studies have quantified genetic variation for immune response at certain loci. Immune phenotype consists of an animal’s relative investment in basic functional divisions within the immune system. Variation in immune phenotype can manifest in emphasis on constitutive versus inducible and fast versus lasting immunity (innate versus adaptive), as well as micro- versus macroparasite defenses (TH1 versus TH2). In addition, an immune phenotype allows the animal’s tendency toward one pole of these axes to be predictable over time, therefore allowing us to predict infection probability. Here I propose to use a long-term dataset in free ranging African buffalo (Syncerus caffer) to define immune phenotype as an animal trait, as opposed to a specific response to a particular infection.