T cells, or T lymphocytes, are white blood cell subtypes defined by the presence of the T-cell receptor (TCR) on their cell surface. T cells are further classified into different types, each with varying functions. These include effector T cells, T helper cells, cytotoxic T cells, memory T cells, regulatory T cells, and natural killer T cells. Presence or absence of key cellular markers, such as CD4, CD8, or CD45, are used in distinguishing the subtypes. In choosing a T cell isolation product, consider the applicable separation methods, cell markers and sample formats.