12.5 Immunological disorders and hypersensitivities
6 min read•august 14, 2024
Immunological disorders and hypersensitivities can wreak havoc on our bodies. From allergies to , these conditions occur when our immune system goes haywire, attacking harmless substances or our own tissues.
Understanding these disorders is crucial for managing health. We'll explore different types of , autoimmune disorders, immunodeficiencies, and how can help treat these conditions.
Hypersensitivity Reactions: Types and Mechanisms
IgE-Mediated Type I Hypersensitivity
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- (immediate hypersensitivity) is mediated by IgE
- Involves the release of histamine and other inflammatory mediators from mast cells and basophils
- Triggered by the cross-linking of IgE antibodies bound to high-affinity FcεRI receptors on mast cells and basophils
- Leads to the of mast cells and basophils, releasing preformed mediators (histamine, tryptase) and newly synthesized mediators (leukotrienes, prostaglandins)
- Clinical manifestations include allergic reactions (urticaria, angioedema), anaphylaxis, and asthma
Antibody-Mediated Type II and III Hypersensitivity
- (antibody-mediated cytotoxicity) is caused by IgG or IgM antibodies that bind to cell surface antigens
- Leads to cell destruction through or antibody-dependent cell-mediated cytotoxicity (ADCC)
- Examples include autoimmune hemolytic anemia (antibodies against red blood cells), Graves' disease (antibodies against thyroid-stimulating hormone receptor), and myasthenia gravis (antibodies against acetylcholine receptor)
- (immune complex-mediated) occurs when antigen-antibody complexes deposit in tissues
- Triggers complement activation and inflammation, leading to tissue damage
- Examples include serum sickness (reaction to foreign serum proteins), systemic lupus erythematosus (SLE), and
T Cell-Mediated Type IV Hypersensitivity
- (delayed-type hypersensitivity) is mediated by
- Involves the release of cytokines that attract and activate macrophages, leading to tissue damage
- Divided into three subtypes based on the type of T cell involved and the cytokine profile:
- Type IVa: Th1 cells secrete IFN-γ, activating macrophages (tuberculin reaction, granulomatous diseases)
- Type IVb: Th2 cells secrete IL-4, IL-5, and IL-13, recruiting eosinophils (chronic asthma, allergic contact dermatitis)
- Type IVc: Cytotoxic T cells directly kill target cells (contact dermatitis, graft rejection)
- Examples include contact dermatitis (reaction to skin allergens), tuberculosis (granuloma formation), and graft rejection (T cell-mediated rejection of transplanted organs)
Autoimmune Disorders: Pathogenesis and Manifestations
Pathogenesis of Autoimmune Disorders
- Autoimmune disorders occur when the immune system mistakenly attacks the body's own tissues
- Involves a complex interplay of genetic, environmental, and immunological factors
- Genetic factors: certain HLA alleles, polymorphisms in genes encoding cytokines, and immune regulators increase susceptibility
- Environmental factors: infections, drugs, and toxins can trigger autoimmunity through molecular mimicry, bystander activation, or epitope spreading
- Immunological factors: defects in central and peripheral tolerance mechanisms, impaired regulatory T cell function, and abnormal B cell activation contribute to the development of autoimmunity
- Chronic inflammation and tissue damage result from the persistent activation of autoreactive T cells and the production of autoantibodies
Clinical Manifestations of Common Autoimmune Disorders
- Rheumatoid arthritis (RA):
- Chronic inflammatory disorder primarily affecting the joints
- Synovial inflammation, cartilage destruction, and bone erosion
- Symmetric polyarthritis, morning stiffness, fatigue, and extra-articular involvement (rheumatoid nodules, interstitial lung disease, cardiovascular disease)
- Systemic lupus erythematosus (SLE):
- Multisystem autoimmune disorder characterized by the production of various autoantibodies, particularly antinuclear antibodies (ANA)
- Formation of immune complexes that deposit in tissues, leading to inflammation and organ damage
- Diverse clinical manifestations: malar rash, photosensitivity, arthritis, serositis, nephritis, neuropsychiatric symptoms, and hematologic abnormalities
- Multiple sclerosis (MS):
- Chronic inflammatory demyelinating disorder of the central nervous system (CNS)
- Activation of autoreactive T cells that cross the blood-brain barrier and attack the myelin sheath
- Demyelination, axonal damage, and the formation of sclerotic plaques
- Variable clinical manifestations depending on the location and extent of CNS lesions: visual disturbances, sensory and motor deficits, coordination problems, bladder and bowel dysfunction, and cognitive impairment
- Course can be relapsing-remitting, secondary progressive, or primary progressive
Immunodeficiency: Concepts and Causes
Primary Immunodeficiencies (PIDs)
- Primary immunodeficiencies (PIDs) are caused by genetic defects that affect the development or function of various components of the immune system
- X-linked agammaglobulinemia (XLA):
- Caused by mutations in the BTK gene, which encodes Bruton's tyrosine kinase
- Leads to defective B cell development and antibody deficiency
- Characterized by recurrent bacterial infections, particularly with encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae)
- Severe combined (SCID):
- Caused by various genetic defects that impair the development of both T and
- Results in a lack of adaptive immunity and increased susceptibility to opportunistic infections
- Examples include X-linked SCID (mutations in the IL2RG gene) and adenosine deaminase (ADA) deficiency
- Chronic granulomatous disease (CGD):
- Caused by mutations in genes encoding components of the NADPH oxidase complex
- Leads to defective phagocyte function, particularly in the generation of reactive oxygen species
- Characterized by recurrent bacterial and fungal infections, granuloma formation, and inflammatory complications
Secondary Immunodeficiencies
- Secondary immunodeficiencies are acquired as a result of external factors, such as infections, malnutrition, or immunosuppressive treatments
- AIDS (acquired immunodeficiency syndrome):
- Caused by HIV infection, which targets and depletes CD4+ T cells
- Impairs cellular immunity, leading to increased susceptibility to opportunistic infections and malignancies
- Progression marked by declining CD4+ T cell counts and the development of AIDS-defining illnesses
- Immunodeficiency associated with malnutrition:
- Micronutrient deficiencies (zinc, vitamin A, iron) can impair various aspects of immune function
- Protein-energy malnutrition leads to atrophy of lymphoid tissues, reduced antibody production, and impaired cell-mediated immunity
- Increases susceptibility to infections, particularly in developing countries
- Iatrogenic immunodeficiency:
- Caused by immunosuppressive medications (corticosteroids, chemotherapy, biologics) or radiation therapy
- Intentional suppression of the immune system to treat autoimmune diseases, prevent graft rejection, or manage certain malignancies
- Increases the risk of opportunistic infections and secondary malignancies
Immunotherapy: Principles and Applications
Immunotherapy in Cancer Treatment
- Immune checkpoint inhibitors:
- Monoclonal antibodies that block inhibitory signals (CTLA-4, PD-1/PD-L1) that dampen T cell responses
- Enhance anti-tumor immunity by releasing the brakes on T cell activation
- Examples include ipilimumab (anti-CTLA-4), nivolumab, and pembrolizumab (anti-PD-1)
- Chimeric antigen receptor (CAR) T cell therapy:
- Patient-derived T cells are genetically engineered to express a receptor specific for a tumor antigen
- Expanded ex vivo and reinfused to target and destroy cancer cells
- Approved for the treatment of certain hematologic malignancies (B-cell acute lymphoblastic leukemia, diffuse large B-cell lymphoma)
- Cancer vaccines:
- Designed to stimulate an immune response against tumor-associated antigens
- Can be prophylactic (preventing cancer development) or therapeutic (treating existing cancer)
- Examples include HPV vaccines (prophylactic) and sipuleucel-T (therapeutic vaccine for prostate cancer)
Immunotherapy in Autoimmune Diseases
- Biologic agents:
- Monoclonal antibodies that target specific pro-inflammatory cytokines or their receptors
- Reduce inflammation and tissue damage by neutralizing the effects of cytokines
- Examples include TNF-α inhibitors (infliximab, adalimumab) for rheumatoid arthritis and Crohn's disease, and IL-6 receptor antagonists (tocilizumab) for rheumatoid arthritis
- B cell depletion therapy:
- Monoclonal antibodies that target CD20 on B cells, leading to their depletion
- Reduces the production of autoantibodies and modulates B cell-mediated immune responses
- Example: rituximab for the treatment of rheumatoid arthritis, systemic lupus erythematosus, and ANCA-associated vasculitis
- Co-stimulation blockade:
- Fusion proteins that block the interaction between co-stimulatory molecules on T cells and antigen-presenting cells
- Inhibits T cell activation and proliferation, reducing the inflammatory response
- Example: abatacept for the treatment of rheumatoid arthritis and juvenile idiopathic arthritis
Other Applications of Immunotherapy
- Allergen-specific immunotherapy (allergy shots):
- Gradual exposure to increasing doses of allergens to induce tolerance
- Reduces allergic symptoms and the need for rescue medications
- Commonly used for the treatment of , allergic asthma, and venom allergies
- Monoclonal antibodies for inflammatory diseases and transplantation:
- Infliximab and adalimumab for the treatment of Crohn's disease and ulcerative colitis
- Basiliximab (anti-IL-2 receptor) for the prevention of acute graft rejection in kidney transplantation
- Eculizumab (anti-C5) for the treatment of paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome
Key Terms to Review (21)
Allergic rhinitis: Allergic rhinitis is an inflammatory condition of the nasal passages caused by an allergic response to airborne allergens. Common triggers include pollen, dust mites, mold spores, and pet dander, which can lead to symptoms like sneezing, runny nose, and nasal congestion. This condition is closely linked to hypersensitivity reactions, particularly Type I hypersensitivity, where the immune system overreacts to harmless substances.
Antibodies: Antibodies are specialized proteins produced by the immune system that help identify and neutralize foreign invaders like bacteria and viruses. They play a crucial role in the immune response by binding to specific antigens, which are molecules found on the surface of pathogens, marking them for destruction. Antibodies also facilitate communication between different immune cells and can provide long-term immunity against previously encountered pathogens.
Antihistamines: Antihistamines are a class of drugs that block the action of histamine, a chemical released during allergic reactions that causes symptoms like itching, swelling, and mucus production. By inhibiting histamine's effects on the body, these medications help alleviate symptoms associated with allergies and certain immunological disorders. Antihistamines can be classified into two main categories: first-generation antihistamines, which can cause sedation, and second-generation antihistamines, which are less likely to cause drowsiness.
Autoimmune diseases: Autoimmune diseases are conditions where the immune system mistakenly attacks the body’s own healthy cells, tissues, or organs, viewing them as foreign invaders. This misdirected immune response can lead to inflammation, tissue damage, and impaired function of the affected areas. Understanding how this process occurs involves knowledge about the immune system's cells and tissues, as well as recognizing the various disorders and hypersensitivity reactions that can arise from such failures in immune regulation.
B cells: B cells are a type of white blood cell that plays a crucial role in the adaptive immune response by producing antibodies. They originate from bone marrow and are essential for recognizing specific antigens, leading to the activation of other immune cells and the formation of memory cells, which helps the body remember past infections.
C. V. Raman: C. V. Raman was an Indian physicist known for his groundbreaking work in the field of light scattering, specifically for the discovery of Raman Effect, which describes how light interacts with molecules. This phenomenon has important applications in various scientific disciplines, including chemistry and biology, and contributes to the understanding of immunological processes.
Complement activation: Complement activation refers to a crucial part of the immune response where a series of proteins, known as complement proteins, are triggered to help eliminate pathogens. This process enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells, promote inflammation, and attack the pathogen's cell membrane. Complement activation can lead to various immunological disorders when it is overactive or misdirected, contributing to hypersensitivities and autoimmune diseases.
Degranulation: Degranulation is the process by which immune cells, particularly mast cells and basophils, release granules containing histamine and other inflammatory mediators into the surrounding tissue. This process plays a crucial role in the body's immune response, especially during allergic reactions and inflammation, contributing to the symptoms associated with these conditions.
Hypersensitivity reactions: Hypersensitivity reactions are exaggerated immune responses that occur when the immune system overreacts to a harmless substance, causing tissue damage and a variety of clinical symptoms. These reactions can be classified into four types: immediate (type I), cytotoxic (type II), immune complex-mediated (type III), and delayed-type (type IV), each involving different immune mechanisms and cell types. Understanding these reactions is essential for recognizing immunological disorders and managing allergic conditions effectively.
Ige-mediated response: The IgE-mediated response is an immune reaction characterized by the production of Immunoglobulin E (IgE) antibodies, which play a key role in allergic reactions and protection against parasitic infections. This response involves the activation of mast cells and basophils, leading to the release of histamines and other inflammatory mediators that can cause symptoms like itching, swelling, and difficulty breathing. It is primarily associated with hypersensitivity reactions, particularly Type I hypersensitivity, where the immune system overreacts to harmless substances.
Immunodeficiency: Immunodeficiency refers to a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. This condition can result from genetic factors, infections, malnutrition, or medical treatments, leading to increased susceptibility to infections and a reduced ability to mount effective immune responses. Understanding immunodeficiency is crucial as it connects to both innate and adaptive immunity, as well as various immunological disorders.
Immunoexcitability: Immunoexcitability refers to the heightened responsiveness of the immune system, particularly how certain immune cells can become overactive or overly sensitive to stimuli. This concept is crucial in understanding various immunological disorders and hypersensitivities, as it explains why some individuals may experience exaggerated immune reactions to typically harmless substances, leading to allergic reactions and autoimmune diseases.
Immunotherapy: Immunotherapy is a type of medical treatment that utilizes the body's immune system to fight diseases, particularly cancer. By enhancing or restoring the immune system's ability to recognize and destroy cancer cells, immunotherapy aims to improve patient outcomes and reduce side effects compared to traditional treatments like chemotherapy and radiation. It can also be used to treat various immunological disorders and hypersensitivities by modulating immune responses.
Paul Ehrlich: Paul Ehrlich was a pioneering German physician and scientist known for his significant contributions to immunology and the development of the concept of antibodies. He is particularly renowned for his work on the humoral immune response, where he postulated that antibodies are produced in response to foreign substances, helping to protect the body from infections. His discoveries laid the groundwork for understanding immunological disorders and hypersensitivities by highlighting how the immune system can sometimes overreact or respond inappropriately.
Rheumatoid arthritis: Rheumatoid arthritis is a chronic autoimmune disorder that primarily affects the joints, causing inflammation, pain, and stiffness. It occurs when the immune system mistakenly attacks the synovium, the lining of the membranes that surround the joints, leading to joint damage and reduced mobility over time. This condition not only impacts the musculoskeletal system but is also linked to other systemic effects, highlighting its complex nature.
Sensitization: Sensitization is the process by which the immune system becomes increasingly reactive to a specific substance, often leading to exaggerated responses upon subsequent exposure. This phenomenon is crucial in understanding how certain individuals develop allergies or hypersensitivity reactions, as the immune system mistakenly identifies harmless substances as threats and mounts an inappropriate defense.
T cells: T cells are a type of white blood cell that play a crucial role in the adaptive immune response, specifically in recognizing and eliminating infected or cancerous cells. They are vital for orchestrating the immune system's response and are derived from stem cells in the bone marrow, maturing in the thymus. T cells help to distinguish between self and non-self, making them essential for targeted immunity and overall immune system regulation.
Type I Hypersensitivity: Type I hypersensitivity, also known as an immediate hypersensitivity reaction, is an exaggerated immune response to an allergen that occurs within minutes upon exposure. This reaction is primarily mediated by immunoglobulin E (IgE) antibodies, which bind to allergens and trigger the release of histamines and other chemicals from mast cells and basophils, leading to symptoms like itching, swelling, and difficulty breathing. Understanding this condition is crucial for recognizing common allergic reactions and developing effective treatment strategies.
Type II Hypersensitivity: Type II hypersensitivity is an immune response characterized by the binding of antibodies to antigens on the surface of cells, leading to cell damage and inflammation. This type of hypersensitivity reaction can occur in various medical conditions, such as autoimmune diseases and transfusion reactions, highlighting its importance in understanding immunological disorders and hypersensitivities.
Type iii hypersensitivity: Type III hypersensitivity is an immune response characterized by the formation of immune complexes that lead to tissue damage and inflammation. This occurs when antibodies bind to soluble antigens, forming complexes that can deposit in various tissues, triggering an inflammatory response and potentially resulting in diseases like lupus and rheumatoid arthritis.
Type IV hypersensitivity: Type IV hypersensitivity, also known as delayed-type hypersensitivity, is an immune response mediated by T cells rather than antibodies, leading to inflammation and tissue damage several hours to days after exposure to an antigen. This type of reaction is crucial in the defense against intracellular pathogens but can also contribute to chronic inflammatory conditions and autoimmune diseases.