CD4 is a type of protein found on the surface of certain immune cells, particularly T-helper cells. It plays a crucial role in the immune system by helping these cells recognize and respond to infections. When a T-helper cell encounters an antigen, the CD4 protein binds to it, facilitating communication between immune cells and enhancing the body's ability to fight off pathogens. In addition to its role in immune response, CD4 is also significant in the context of HIV infection. The virus specifically targets CD4 cells, leading to a weakened immune system over time. Monitoring CD4 cell counts is essential for assessing the health of individuals living with HIV.