The T helper cells (Th cells), also known as CD4+ cells, are a type of T cell that play an important role in the immune system, particularly in the adaptive immune system. They help the activity of other immune cells by releasing T cell cytokines. These cells help suppress or regulate immune responses."
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But the precise physiological mechanisms which regulate the cells' role in immune responses is currently not understood.
Helper T cells are capable of influencing a variety of immune cells, and the T cell response generated (including the extracellular signals such as cytokines) can be essential for a successful outcome from infection. In order to be effective, helper T cells must determine which cytokines will allow the immune system to be most useful or beneficial for the host. Understanding exactly how helper T cells respond to immune challenges is currently of major interest in immunology, because such knowledge may be very useful in the treatment of disease and in increasing the effectiveness of vaccination."
Source : Wikipedia