CD5 is a protein found on the surface of certain immune cells, acting as a “cellular marker” that helps scientists identify different cell types. This protein plays a part in the complex communication network of the immune system, helping regulate immune responses.
Understanding CD5: A Key Molecule
CD5 is a transmembrane glycoprotein, spanning the cell membrane. It belongs to the scavenger receptor cysteine-rich (SRCR) family of proteins. Upon engagement, CD5 interacts with intracellular signaling molecules, influencing cell survival, proliferation, and differentiation.
Cells That Carry CD5
CD5 is prominently expressed on T lymphocytes, often called T cells. Its presence is particularly notable during their development in the thymus, where these cells are referred to as thymocytes. CD5 also appears on mature T cells after they leave the thymus. A distinct subset of B lymphocytes, known as B-1 B cells, also express CD5. T cells are responsible for cell-mediated immunity, directly attacking infected cells or regulating other immune cells. B-1 B cells, on the other hand, are involved in producing natural antibodies that provide an early defense against pathogens.
How CD5 Influences Immune Responses
CD5 acts as a co-receptor on T cells, fine-tuning their activation thresholds by dampening excessive T cell receptor (TCR) signaling to prevent overactivation. This regulatory function helps maintain immune homeostasis and prevents the development of autoimmune responses. On B-1 B cells, CD5 helps regulate B cell activation and promotes self-tolerance, preventing the immune system from attacking the body’s own tissues. It serves to mitigate activating signals from the B cell receptor (BCR), ensuring that B-1 cells are only activated by strong stimuli like bacterial antigens, rather than normal tissue antigens. This helps prevent autoimmune reactions and maintains a balanced antibody response.
When CD5 Expression Changes
Altered CD5 expression can indicate certain conditions. For instance, chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), both B cell cancers, commonly show strong CD5 expression, acting as a marker for these cancerous populations. CD5 is generally expressed in T cell lymphomas. However, its absence in cutaneous T-cell lymphoma can be an indicator of malignancy in this condition. Its potential involvement in some autoimmune conditions suggests that its regulatory role might be disrupted in these diseases.