HIV Basics

Human immunodeficiency virus (HIV) is a retrovirus that targets CD4-positive T lymphocytes, cells that coordinate cellular immune responses. Viral entry begins when the envelope protein binds the CD4 receptor and a chemokine co-receptor, usually CCR5 or CXCR4. Reverse transcriptase converts viral RNA into DNA; integrase then inserts that DNA into the host cell’s genome. This allows infected cells to produce new virions and creates an integrated reservoir that current antiretroviral therapy (ART) does not eradicate. The clinical consequence is progressive immune dysfunction rather than an isolated fall in one laboratory value. Early in infection, viral replication is intense and CD4 cells may fall sharply, particularly in lymphoid tissue. The immune response partially reduces the viral burden, but without treatment, ongoing replication and immune activation gradually deplete CD4 cells. Opportunistic infections, certain malignancies, and other AIDS-defining illnesses become more likely as immune function deteriorates. AIDS may be diagnosed when the CD4 count is below 200 cells/µL or when an AIDS-defining condition occurs, even if the current CD4 count is higher. ART suppresses viral replication, permits immune recovery, and reduces the risk...