T-helper cells are a type of white blood cell that play a crucial role in the immune system. They help coordinate the body's response to infections by signaling other immune cells, such as B cells and cytotoxic T cells, to take action against pathogens. These cells are characterized by the presence of a protein called CD4 on their surface. When T-helper cells recognize foreign invaders, they release signaling molecules called cytokines, which enhance the immune response and help the body fight off infections effectively.