The immune system distinguishes between self and non-self but also between different types of non-self such as viruses and worms. T helper (Th) cells (CD4+) have a fundamental role in that challenge; following their first interaction with a pathogen, Th cells can differentiate into regulatory or effector lineages that differentially express cytokine genes. The effector lineages Th1, Th2, and Th17 are characterized by the expression of the signature cytokines Interferon g (IFNg), Interleukin-4 (IL-4), and Interleukin-17 (IL-17), respectively. IFNg exerts protective functions in microbial infections and is observed clinically in cases of autoimmune diseases. IL-4 is strongly apparent in parasitic infections, and is associated with allergic reactions. IL-17 plays a role in the eradication of extracellular pathogens, but inappropriate responses can lead to autoimmunity.