Medical Laboratory Scientist (MLS) ASCP Practice Exam

The T cell that expresses the CD8 marker is the T cytotoxic cell. CD8 is a glycoprotein that serves as a co-receptor for the T cell receptor (TCR) and plays a key role in the immune response by recognizing and binding to antigens presented by major histocompatibility complex (MHC) class I molecules on the surface of infected or cancerous cells. This interaction is crucial as it enables cytotoxic T cells to identify and kill cells presenting foreign antigens, thus playing an essential role in the adaptive immune response.

In contrast, T helper cells, which are characterized by the expression of CD4, primarily assist other immune cells by releasing cytokines. T regulatory cells, also identified by CD4 (along with the CD25 marker), are involved in maintaining immune tolerance and suppressing excessive immune responses. Natural killer (NK) cells, while they play a role in the innate immune response and can also express CD8 in some cases, are distinct from T cells and do not develop from the same precursors or pathways in the thymus. Therefore, the T cytotoxic cell is specifically marked by CD8, enabling its unique function in targeting and destroying infected or malignant cells.