T4 cells, also known as CD4 cells, are a type of white blood cell that plays a crucial role in the functioning of the immune system. They are a subset of T lymphocytes, a type of immune cell that helps in protecting the body against various infectious agents such as bacteria, viruses, and fungi. T4 cells are referred to as "helper" cells because they coordinate and regulate the immune response.

The main function of T4 cells is to recognize specific antigens presented by infected cells or foreign invaders. These antigens are small molecules present on the surface of pathogens that activate the immune response. Upon identifying the antigens, T4 cells release chemical messengers, known as cytokines, which stimulate the activation and proliferation of other immune cells, such as B cells and T8 cells. B cells produce antibodies, while T8 cells destroy infected cells directly.

T4 cells also play a critical role in memory immune response, allowing the immune system to remember previous encounters with specific pathogens and mount a more rapid and effective response upon reinfection. This is particularly important in the prevention and control of recurrent infections.

A decrease in the number of T4 cells, known as T4 cell depletion, is a hallmark of human immunodeficiency virus (HIV) infection. HIV specifically targets and destroys T4 cells, resulting in a weakened immune system and increased susceptibility to various opportunistic infections and diseases. Monitoring T4 cell counts is essential in managing and assessing the progression of HIV infection and determining the need for antiretroviral therapy.

In summary, T4 cells are a specific subset of T lymphocytes that play a crucial role in regulating and coordinating the immune response, and their depletion is a key aspect of HIV infection.