Overall, we show that variation in both innate and adaptive ITs is under substantial genetic control (Figure 1 ; Tables 1 and 2) . Similar heritability estimates for innate and adaptive ITs and also between cell number and cell response parameters were observed. Further, heritability estimates do not differ consistently between in vivo and ex vivo measures with no apparent bias due to phenotyping methods. These data also suggest candidate ITs for QTL mapping. Indeed, mapping studies have already started for total leukocyte count ( [20, 21] ; AnimalQTLdb, http://www.animalgenome.org/cgi-bin/QTLdb/index), mitogen-induced proliferation [20] , antibody response [20, 22] , cytokine production (IL2, IL10 and IFNc) [23, 35] , complement activity [22] , and acute phase protein serum concentration [22] .