Th1, Th2, and Beyond: What We Know About Adaptive Immunity for Fungal Infections

Fungi are ubiquitously present in the environment and some species can survive on the external and mucosal surfaces of human hosts; however, both the innate and the adaptive arms of the immune response effectively protect a host organism against the development of fungal diseases. The fungal pathogen finds a window of opportunity for uncontrolled growth and tissue invasion when the balance of the immune system is altered by different sets of conditions. Thus, the majority of invasive fungal infections (IFI) are associated with defects of the innate and/or the adaptive immune system. These deficiencies can result from the use of immunosuppressive therapies, HIV infection, or hematopoietic malignancies. Other severe diseases, extreme stress, prolonged antibiotic treatment, and subsequent changes in the host's microbiome may also contribute to development of fungal diseases owing to their effects on the immune system. In few circumstances, the highly virulent fungi can evade or modulate host responses, leading to invasive mycoses in hosts without any apparent defect in the immune system. Extensive research into many of these fungal mycoses has greatly expanded our understanding of T-cell biology and its importance in host defenses against various fungi. In this review, we will highlight the role of T-cell-mediated immune polarization during IFI.