2 Discussion Focus

1.2

Lymphocytes are a crucial component of the human immune system. Based on their origin, morphological characteristics, and immune functions, they can be classified into three main types: T cells, B cells, and NK cells. T cells can further be divided into three subgroups: cytotoxic T cells (referred to as Tc cells), helper T cells (referred to as Th cells), and suppressor T cells (referred to as Ts cells). Since effector T cells can directly kill target cells, immune responses mediated by T cells are known as cellular immunity. Both CD4 and CD8 molecules belong to the immunoglobulin superfamily. In peripheral blood and lymphoid organs, CD4-positive T cells are predominantly Th cells; in the thymus, CD4-positive cells include single-positive CD4 cells and immature T cells that are double-positive for both CD4 and CD8. The CD4 molecule serves as a co-receptor for Th cells’ TCR-mediated antigen recognition and is also the receptor for the human immunodeficiency virus HIV, participating in signal transduction during TCR recognition of antigen peptide-MHC class II complex. The CD8 molecule is primarily expressed on the surface of peripheral blood CD8-positive cytotoxic T cells, including single-positive CD8 cells and double-positive CD4/CD8 T cells. The CD8 molecule participates in signal transduction during TCR recognition of antigen peptide-MHC class I complex.

2 Discussion Focus