Immunodeficiency and Immune Dysregulation

Immunodeficiency is the state in which one or more components of the immune system are absent, reduced, or functionally impaired, leaving the host unable to mount effective defence against infection and, in many cases, prone to autoimmunity and malignancy. This area organises the conditions in which immune defence fails, ranging from inherited single-gene defects present from birth to acquired failures caused by infection, disease, or therapy.

Definition

Immunodeficiency denotes impaired or absent function of one or more arms of innate or adaptive immunity, classified as primary when caused by intrinsic (usually genetic) defects of the immune system and secondary when an external factor such as infection, malnutrition, malignancy, or immunosuppressive therapy degrades a previously competent immune system.

Scope

The area surveys the major categories of immune failure as a reference framework: primary (inborn) immunodeficiencies affecting antibody, combined cellular, complement, and phagocyte function; secondary immunodeficiency arising from infection such as HIV; and therapeutic or drug-induced immunosuppression. It frames how defects are classified, what infection patterns they produce, and how they are conceptually distinguished, rather than offering diagnostic or treatment instructions.

Sub-topics

• Therapeutic Immunosuppression and Drug-Induced Immunodeficiency
• Primary Immunodeficiencies: Complement and Phagocytic Cell Disorders
• Primary Immunodeficiencies: Combined and B-Cell Disorders
• Secondary Immunodeficiency: HIV/AIDS and Infection-Related

Core questions

• Which component of the immune system is defective, and what infection pattern does that predict?
• Is the immune failure primary (intrinsic) or secondary (acquired)?
• How does the same defect produce both susceptibility to infection and immune dysregulation such as autoimmunity?

Key concepts

• Primary vs. secondary immunodeficiency
• Inborn errors of immunity
• Antibody (humoral) deficiency
• Combined cellular and humoral deficiency
• Complement deficiency
• Phagocyte defects
• Immune dysregulation (autoimmunity and autoinflammation)
• Opportunistic infection
• Therapeutic immunosuppression

Mechanisms

Immune failure can arise at any point in host defence. Defects in B-cell development or antibody production impair humoral immunity and predispose to recurrent pyogenic bacterial infection; defects in T-cell development or function impair cell-mediated immunity and, when combined with antibody failure, produce the severe combined phenotype with susceptibility to viral, fungal, and opportunistic organisms. Loss of complement components removes a key opsonic and lytic effector arm, and phagocyte defects impair the killing of ingested microbes. Beyond susceptibility to infection, the modern classification recognises that many of these defects also disturb immune regulation, producing autoimmunity, lymphoproliferation, and autoinflammation; this is why the contemporary term inborn errors of immunity is preferred over the older immunodeficiency alone (Tangye, 2022; Notarangelo, 2010). Secondary immunodeficiency reflects the same effector arms being degraded by an external cause, the prototype being HIV-induced depletion of CD4+ T cells (Deeks, 2015).

Clinical relevance

The categories of immune failure underpin how clinicians reason about recurrent, severe, or unusual infections and about the opportunistic organisms that signal specific defects. As a reference area it explains how immune defects are framed and classified; it describes patterns of disease association and is not a basis for individual diagnosis or treatment.

Epidemiology

Inborn errors of immunity are individually rare but collectively affect an appreciable minority of the population, and more than 480 single-gene conditions are now recognised in the international classification (Tangye, 2022). Secondary immunodeficiency is far more common worldwide, with HIV infection the single largest contributor to acquired immune failure globally, alongside immunosuppressive therapy, malignancy, and malnutrition (Deeks, 2015).

Evidence & guidelines

The reference classification for primary immunodeficiency is maintained and periodically updated by the International Union of Immunological Societies (IUIS) Expert Committee, which groups inborn errors of immunity by the affected component and phenotype (Tangye, 2022). Narrative syntheses summarise the clinical categories (Notarangelo, 2010), while the HIV literature provides the framework for the leading acquired form (Deeks, 2015).

History

Recognition of immunodeficiency as a defined field followed mid-twentieth-century descriptions of inherited antibody deficiency and combined immune failure in infants. Over subsequent decades the molecular bases of many single-gene defects were identified, and the IUIS established and repeatedly revised a unifying classification. The 1980s emergence of HIV/AIDS reframed acquired immune failure as a major global health problem and sharpened the conceptual distinction between primary and secondary immunodeficiency (Tangye, 2022; Deeks, 2015).

Key figures

• Luigi Notarangelo
• Stuart Tangye
• Steven Deeks
• Capucine Picard
• Alain Fischer

Seminal works

• tangye-2022
• notarangelo-2010
• deeks-2015

Frequently asked questions

What is the difference between primary and secondary immunodeficiency?

Primary immunodeficiency results from an intrinsic, usually genetic, defect of the immune system itself, whereas secondary immunodeficiency is an acquired failure of a previously competent immune system caused by an external factor such as HIV infection, malignancy, malnutrition, or immunosuppressive therapy.

Why is the term 'inborn errors of immunity' now used instead of 'primary immunodeficiency'?

Because many of the same genetic defects cause not only susceptibility to infection but also immune dysregulation such as autoimmunity, autoinflammation, and lymphoproliferation; the broader term captures this fuller range of consequences.