Both these categories involve the loss of the immune response. In the case of the primary type, the immunodeficiency is brought about by a genetic defect while acquired form results from infection with HIV. Being genetic, primary immunodeficiencies present at birth and are characterized by persistent infections in young children. They can affect any part of the immune response. HIV is mainly transmitted by sexual intercourse and intravenous drug use so it usually presents in young adults. HIV infects CD4 helper T cells and results in a loss of the adaptive immune response, which leads to AIDS.
HIV infection has three phases, the acute, the asymptomatic and the symptomatic phase. During the acute phase large numbers of virus are produced and lymphadenopathy results. During this phase cytotoxic T cells will kill the HIV infected CD4 helper T cells. The helper T cell population initially decreases but then rebounds to near normal numbers as the virus becomes latent. In the asymptomatic phase, the population of helper T cells declines at steady rate. It is thought that the infection is controlled by cytotoxic T cells during this phase but this population also declines at the end of the asymptomatic phase. When the helper T cell population drops to below 500 per cubic millilitre of blood, the patient is said to be in the symptomatic phase and the cytotoxic T cell level also continues to decline. AIDS is defined by a helper T cell population below 200.