Cytokines play crucial roles in all aspects of health and diseases. They influence immune expression, the development of immunologic memory, and regulation of antigen-specific and nonspecific immune activation. Based on their production by T helper (T_H) cells, cytokines are categorized into T_H1 and T_H2 types. T_H1 type includes inflammatory cytokines and growth factors. The former induces signal three that bridge innate and adaptive immunity, whilst the latter sustains proliferation and survival of T cells. Therefore, T_H1 cytokines profoundly mediate generation of preventive immune responses toward infectious diseases and cancer. By contrast, T_H2 cytokines are anti-inflammatory through suppression of T_Hl-mediated immune responses, but they also accelerate T_H2-mediated diseases. This article describes the impact of the T_Hl/T_H2 cytokines on shaping the nature of the immune cell responses to inflammation, infection, and cancer. We predict that T_Hl/T_H2 could be considered as a fingerprint for probing the identity of diseases. Fundamental knowledge of the design of cytokine-based therapeutic strategies is now reaching the phase of application and a number of cytokines are already on the market and many others are under clinical investigation.