19.4 Immunodeficiency

Immunodeficiencies can be primary (genetic) or secondary (acquired). They weaken the body's defense against infections, making people more susceptible to illnesses. Primary immunodeficiencies are present from birth, while secondary ones develop later due to factors like HIV or malnutrition.

Different types of immunodeficiencies affect various parts of the immune system. For example, chronic granulomatous disease impairs phagocyte function, while X-linked agammaglobulinemia affects antibody production. Understanding these conditions helps in diagnosing and treating patients with weakened immune systems.

Primary and Secondary Immunodeficiencies

Primary vs secondary immunodeficiencies

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• Primary immunodeficiencies are caused by genetic defects present from birth or early childhood lead to increased susceptibility to infections, recurrent and severe infections, and impaired immune response to specific pathogens
• Secondary immunodeficiencies are acquired later in life due to external factors or underlying conditions (HIV/AIDS, malnutrition) result in weakened immune response, increased susceptibility to opportunistic infections, and impaired ability to fight off infections effectively

Types of primary immunodeficiencies

• Chronic granulomatous disease (CGD) involves a defect in phagocyte function causing inability of phagocytes to produce reactive oxygen species (ROS) impairing killing of ingested microorganisms leads to recurrent bacterial and fungal infections and formation of granulomas in affected tissues
• X-linked agammaglobulinemia (XLA) is a deficiency in antibody production caused by a mutation in the BTK gene on the X chromosome affects B cell development and maturation resulting in absence or extremely low levels of immunoglobulins (antibodies) increases susceptibility to bacterial infections, particularly with encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae)

Causes of secondary immunodeficiencies

• HIV/AIDS causes destruction of CD4+ T cells impairing cell-mediated immunity increases susceptibility to opportunistic infections (Pneumocystis pneumonia, Candida albicans)
• Malnutrition leads to deficiencies in essential nutrients (protein, zinc, vitamin A) impairing immune cell function and antibody production increases susceptibility to infections, particularly in developing countries
• Immunosuppressive drugs used to prevent organ rejection in transplant patients or treat autoimmune diseases suppress immune cell function and antibody production increasing risk of opportunistic infections (cytomegalovirus CMV, Epstein-Barr virus EBV)
• Certain cancers like hematologic malignancies (leukemia, lymphoma) impair immune cell function due to the cancer itself or chemotherapy treatment increasing susceptibility to bacterial, fungal, and viral infections

Components of the immune system affected by immunodeficiencies

• Innate immunity: The first line of defense against pathogens, including physical barriers, phagocytes, and the complement system
• Adaptive immunity: The specific immune response involving T and B lymphocytes, which can be impaired in various immunodeficiencies
• Complement system: A group of proteins that enhance the ability of antibodies and phagocytes to clear pathogens, which can be deficient in some primary immunodeficiencies
• Antigen presentation: The process by which immune cells display foreign antigens to T cells, crucial for initiating an adaptive immune response
• Opportunistic infections: Infections caused by organisms that typically do not cause disease in individuals with healthy immune systems, but can be severe in immunocompromised patients