Th2 cells, or T helper 2 cells, are a subset of CD4+ T cells that play a crucial role in the immune system by promoting the humoral immune response and facilitating the activation of B cells, eosinophils, and mast cells. They are primarily involved in the defense against extracellular parasites and are implicated in allergic reactions and asthma due to their cytokine profile, which includes IL-4, IL-5, IL-9, and IL-13.