T cells, or T lymphocytes, are white blood cell subtypes playing crucial roles in immunity and disease. T cell types include effector T cells, T helper cells, cytotoxic T cells, memory T cells, regulatory T cells, and natural killer T cells. One method of T cell isolation is through binding of key cell surface markers, such as CD4 or CD3, to magnetic microbeads. In positive separation, the desired cell populations are bound to beads while unwanted cells are washed away. Before purchasing a T cell positive isolation product, consider the applicable markers as well as sample volumes and formats.