🛡️Immunobiology Unit 12 – Immunodeficiency Disorders

Overview of Immunodeficiency Disorders

• Immunodeficiency disorders impair the immune system's ability to defend against pathogens and foreign substances
• Can be classified as primary (inherited) or secondary (acquired) immunodeficiencies
• Primary immunodeficiencies (PIDs) result from genetic defects affecting immune system development or function
  • Estimated to affect 1 in 10,000 to 1 in 50,000 individuals worldwide
  • Examples include severe combined immunodeficiency (SCID) and X-linked agammaglobulinemia (XLA)
• Secondary immunodeficiencies develop due to external factors such as infections, malnutrition, or immunosuppressive treatments
  • HIV/AIDS is a well-known example of a secondary immunodeficiency
• Immunodeficiencies increase susceptibility to recurrent and severe infections, autoimmune disorders, and certain cancers
• Early diagnosis and appropriate treatment are crucial for improving patient outcomes and quality of life

Types of Immunodeficiencies

• Primary immunodeficiencies can affect different components of the immune system
  • Humoral immunodeficiencies impact B-cell function and antibody production (XLA, common variable immunodeficiency)
  • Cellular immunodeficiencies affect T-cell development and function (SCID, DiGeorge syndrome)
  • Combined immunodeficiencies involve defects in both B-cell and T-cell function (SCID, Wiskott-Aldrich syndrome)
• Phagocyte defects impair the function of neutrophils, macrophages, or monocytes (chronic granulomatous disease)
• Complement deficiencies affect the complement system, a group of proteins involved in immune responses (C3 deficiency)
• Secondary immunodeficiencies can be caused by various factors
  • Infections (HIV/AIDS, measles)
  • Malnutrition (protein-energy malnutrition)
  • Immunosuppressive treatments (chemotherapy, corticosteroids)
  • Chronic diseases (diabetes, kidney failure)

Key Immune System Components

• T-cells, a type of lymphocyte, play a central role in cell-mediated immunity
  • CD4+ T-cells (helper T-cells) coordinate immune responses and activate other immune cells
  • CD8+ T-cells (cytotoxic T-cells) directly kill infected or abnormal cells
• B-cells, another type of lymphocyte, are responsible for humoral immunity through antibody production
  • Antibodies neutralize pathogens, facilitate phagocytosis, and activate complement
• Natural killer (NK) cells are innate lymphocytes that recognize and destroy virus-infected or tumor cells
• Phagocytes, including neutrophils and macrophages, engulf and destroy pathogens and debris
• Complement system consists of plasma proteins that enhance immune responses and promote pathogen clearance
• Cytokines are signaling molecules that regulate immune cell communication and function
  • Examples include interleukins, interferons, and tumor necrosis factors

Genetic Factors and Inheritance Patterns

• Primary immunodeficiencies are caused by genetic mutations affecting immune system development or function
• Inheritance patterns vary depending on the specific disorder and the gene(s) involved
  • X-linked recessive inheritance (XLA, Wiskott-Aldrich syndrome)
    • Affects males more frequently than females
  • Autosomal recessive inheritance (SCID, chronic granulomatous disease)
    • Requires two copies of the mutated gene, one from each parent
  • Autosomal dominant inheritance (Hyper-IgE syndrome)
    • One copy of the mutated gene is sufficient to cause the disorder
• Genetic testing can identify specific mutations and guide diagnosis and treatment
• Genetic counseling helps families understand inheritance patterns and recurrence risks

Diagnostic Techniques and Procedures

• Thorough medical history and physical examination are essential for initial assessment
• Immunoglobulin levels (IgG, IgA, IgM, IgE) are measured to evaluate humoral immunity
  • Low levels may indicate antibody deficiencies (XLA, common variable immunodeficiency)
• Flow cytometry quantifies and characterizes immune cell populations (T-cells, B-cells, NK cells)
  • Abnormal cell counts or ratios can suggest specific immunodeficiencies (SCID, DiGeorge syndrome)
• Lymphocyte proliferation assays assess T-cell function in response to mitogens or antigens
• Neutrophil function tests (nitroblue tetrazolium test, dihydrorhodamine assay) evaluate phagocyte activity
• Complement assays measure individual complement protein levels and overall pathway function
• Genetic testing can confirm the diagnosis and identify the specific mutation(s) involved

Common Symptoms and Clinical Presentations

• Recurrent, severe, or unusual infections are a hallmark of immunodeficiency disorders
  • Sinopulmonary infections (pneumonia, sinusitis, otitis media)
  • Gastrointestinal infections (diarrhea, malabsorption)
  • Skin and soft tissue infections (abscesses, cellulitis)
• Failure to thrive in infants and children due to chronic infections and malnutrition
• Autoimmune manifestations, such as autoimmune hemolytic anemia or thrombocytopenia
• Chronic inflammation and granuloma formation in certain disorders (chronic granulomatous disease)
• Increased risk of malignancies, particularly lymphomas and leukemias
• Specific clinical features may suggest particular immunodeficiencies
  • Absence of lymph nodes and tonsils (XLA)
  • Eczema and thrombocytopenia (Wiskott-Aldrich syndrome)
  • Coarse facial features and congenital heart defects (DiGeorge syndrome)

Treatment Approaches and Management

• Immunoglobulin replacement therapy (IVIG or SCIG) for antibody deficiencies
  • Provides passive immunity and reduces infection risk
• Antimicrobial prophylaxis to prevent recurrent infections
  • Antibiotics, antifungals, or antivirals depending on the patient's susceptibility
• Hematopoietic stem cell transplantation (HSCT) for severe immunodeficiencies (SCID)
  • Replaces defective immune system with healthy donor stem cells
• Gene therapy as an emerging treatment for specific disorders (ADA-SCID)
  • Corrects the underlying genetic defect using modified viral vectors
• Supportive care measures
  • Nutritional support to address malnutrition and promote growth
  • Vaccinations in select patients to boost immunity against specific pathogens
• Regular monitoring and follow-up to assess treatment response and detect complications

Research Frontiers and Emerging Therapies

• Advancements in gene editing technologies (CRISPR-Cas9) for targeted correction of genetic defects
• Development of improved gene therapy vectors with enhanced safety and efficacy profiles
• Exploration of novel immunomodulatory agents to fine-tune immune responses
  • Targeted cytokine therapies
  • Small molecule drugs targeting specific signaling pathways
• Personalized medicine approaches based on individual genetic and immunological profiles
• Expansion of newborn screening programs to enable early detection and intervention
• International collaborations and registries to better understand the epidemiology and natural history of rare immunodeficiencies
• Ongoing clinical trials investigating new treatment strategies and long-term outcomes
  • Ex vivo gene therapy for SCID
  • Hematopoietic stem cell gene editing for Wiskott-Aldrich syndrome