Secondary Immunodeficiency: HIV/AIDS and Infection-Related

Secondary immunodeficiency is acquired failure of a previously competent immune system, and the prototype is infection with the human immunodeficiency virus (HIV). By infecting and progressively depleting CD4+ T helper cells, HIV undermines the coordinating cell of adaptive immunity, eventually producing the acquired immunodeficiency syndrome (AIDS) defined by characteristic opportunistic infections and malignancies.

Definition

Secondary (acquired) immunodeficiency is impaired immune function in a person who was previously immunocompetent, caused by an external factor such as infection, malignancy, malnutrition, or therapy. HIV/AIDS is the leading infection-related form: HIV depletes CD4+ T lymphocytes, and AIDS is the advanced stage defined by severe CD4 depletion and the appearance of opportunistic infections or AIDS-defining cancers.

Scope

The entry frames acquired immunodeficiency caused by infection, centred on HIV/AIDS, and situates it against other infection-related and acquired causes of immune failure. It explains the mechanism of CD4 depletion, the concept of an opportunistic-infection threshold, and the reframing of treated HIV as a chronic condition, as a reference rather than diagnostic or treatment guidance.

Core questions

• How does HIV produce immunodeficiency at the cellular level?
• What distinguishes HIV infection from the syndrome of AIDS?
• Why does antiretroviral therapy reframe HIV as a chronic disease rather than a terminal one?

Key concepts

• CD4+ T-cell depletion
• Opportunistic infection
• AIDS-defining illness
• Viral set point and chronic immune activation
• Antiretroviral therapy (ART) and immune reconstitution
• Primary vs. secondary immunodeficiency
• Infection-related immune suppression

Mechanisms

HIV is a retrovirus that enters CD4+ T cells (and other cells bearing CD4 and a chemokine co-receptor), integrates into the host genome, and over years drives progressive loss of CD4+ T cells through direct infection, immune-mediated killing, and chronic immune activation. Because CD4+ helper cells coordinate both cellular and humoral responses, their depletion degrades adaptive immunity broadly; once the CD4 count falls below a threshold, the host can no longer contain organisms that an intact immune system controls, and opportunistic infections and AIDS-defining cancers appear (Deeks, 2015). This contrasts with primary immunodeficiency, where the defect is intrinsic and present from birth (Notarangelo, 2010). Effective antiretroviral therapy suppresses viral replication and permits partial immune reconstitution, converting a once-fatal infection into a manageable chronic condition, though residual immune activation persists (Deeks, 2013).

Clinical relevance

The pattern of opportunistic infection at low CD4 counts is the clearest illustration of how loss of a single coordinating cell type unravels host defence, and it anchors how acquired immunodeficiency is conceptualised. As a reference topic this entry explains the mechanism and natural history; it is educational and not a basis for individual diagnosis, testing, or treatment.

Epidemiology

HIV infection is the largest single cause of acquired immunodeficiency worldwide, with tens of millions of people living with the virus globally; the burden is greatest in sub-Saharan Africa. The introduction and scale-up of antiretroviral therapy has dramatically reduced progression to AIDS and AIDS-related mortality where treatment is available, reshaping HIV into a long-term chronic disease (Deeks, 2015; Deeks, 2013).

Evidence & guidelines

Comprehensive disease-primer and review syntheses describe HIV pathogenesis, natural history, and management at a reference level (Deeks, 2015), and the reframing of treated HIV as a chronic disease is articulated in the broader literature (Deeks, 2013). The contrast with intrinsic immune defects is drawn from primary-immunodeficiency reviews (Notarangelo, 2010).

History

AIDS was recognised in 1981 as a cluster of opportunistic infections and Kaposi sarcoma in previously healthy young adults, and HIV was identified as its cause within a few years. The 1990s introduction of combination antiretroviral therapy transformed the prognosis, shifting HIV from an almost uniformly fatal infection to a chronic, treatable condition and establishing it as the defining model of acquired immunodeficiency (Deeks, 2015; Deeks, 2013).

Key figures

• Steven Deeks
• Sharon Lewin
• Diane Havlir
• Robert Gallo
• Luc Montagnier

Seminal works

• deeks-2015
• deeks-2013
• notarangelo-2010

Frequently asked questions

What is the difference between HIV infection and AIDS?

HIV infection is the presence of the virus and its ongoing depletion of CD4+ T cells, which may be asymptomatic for years; AIDS is the advanced stage, defined by severe CD4 depletion together with the occurrence of specific opportunistic infections or AIDS-defining cancers.

Why is HIV considered a secondary rather than a primary immunodeficiency?

Because the immune system is normal until the virus is acquired; HIV is an external cause that degrades a previously competent immune system, which is the defining feature of secondary (acquired) immunodeficiency, in contrast to the intrinsic, usually genetic defects of primary immunodeficiency.