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CD4 is a glycoprotein molecule found on the surface of certain white blood cells, primarily T lymphocytes, and is known as the T helper cell marker. CD4+ T cells play a crucial role in the immune system as they act as the "conductors" of the immune response, helping to coordinate the body's defense against pathogens such as bacteria, viruses, and other foreign invaders. CD4+ T cells are a type of T lymphocyte. When a pathogen enters the body, the immune response process begins with macrophages engulfing the pathogen and releasing antigen information, which is then passed on to CD4+ T cells. This antigen transfer leads to the production of antibodies that can easily attach to the pathogen, making it easier for macrophages to capture and engulf it. Therefore, CD4+ T cells are essential as they act as helpers and "eyes" of the immune system, transmitting pathogen information. CD4+ T cells are also a key indicator of the immune system's damage in HIV-infected patients, as HIV targets these cells. The count of CD4+ T cells can directly reflect the immune function and is the most explicit indicator of the immune system's damage in HIV patients.