BACKGROUND AND OBJECTIVES: The human immune system is an ever-changing composition of innumerable cells and proteins, continually ready to respond to pathogens or insults. The cost of maintaining this state of immunological readiness is rarely considered. In this paper we aim to discern a cost to non-acute immune function by investigating how low levels of C-reactive protein (CRP) relate to other energetic demands and resources in adolescent Gambian girls. METHODOLOGY: Data from a longitudinal study of 66 adolescent girls was used to test hypotheses around investment in immune function. Non-acute (under 2 mg/L) CRP was used as an index of immune function. Predictor variables include linear height velocity, adiposity, leptin, and measures of energy balance. RESULTS: Non-acute log CRP was positively associated with adiposity (β = 0.16, p < 0.001, R2 = 0.17) and levels of the adipokine leptin (β = 1.17, p = 0.006, R2 = 0.09). CRP was also negatively associated with increased investment in growth, as measured by height velocity (β = -0.58, p < 0.001, R2 = 0.13) and lean mass deposition β = -0.42, p = 0.005, R2 = 0.08). Relationships between adiposity and growth explained some, but not all, of this association. We do not find that CRP was related to energy balance. CONCLUSIONS AND IMPLICATIONS: These data support a hypothesis that investment in non-acute immune function is facultative, and sensitive to energetic resources and demands. We also find support for an adaptive association between the immune system and adipose tissue.