CD4+ T-cells are a type of white blood cell that play a crucial role in the immune system. They are often referred to as "helper T-cells" because they assist other immune cells in responding to infections. By recognizing specific antigens presented by antigen-presenting cells, CD4+ T-cells help activate B-cells to produce antibodies and stimulate CD8+ T-cells to kill infected cells. These cells are essential for maintaining a strong immune response and are particularly important in fighting off viral infections. A decrease in CD4+ T-cell count is a hallmark of HIV infection, which can lead to a weakened immune system and increased susceptibility to opportunistic infections.