13.1 Types of hypersensitivity reactions

Ige-mediated refers to a type of immune response that involves immunoglobulin E (IgE), a class of antibodies produced by the immune system, which plays a crucial role in allergic reactions. This response typically occurs when an individual is exposed to allergens, leading to the activation of mast cells and basophils that release histamines and other chemicals, resulting in various allergic symptoms. Ige-mediated responses are fundamental to understanding allergic diseases and hypersensitivity reactions.

Allergen: A substance that triggers an allergic reaction, often proteins found in pollen, dust mites, pet dander, and certain foods.

Mast Cells: A type of white blood cell that is part of the immune system and plays a key role in the allergic response by releasing histamine upon activation.

Anaphylaxis: A severe, potentially life-threatening allergic reaction that can occur rapidly after exposure to an allergen, often characterized by difficulty breathing, swelling, and a drop in blood pressure.

Inflammation is a biological response of the immune system to harmful stimuli, such as pathogens, damaged cells, or irritants. It serves to protect the body by initiating healing processes, recruiting immune cells to the site of injury or infection, and facilitating the removal of harmful agents. This complex response can be acute or chronic and is crucial for maintaining homeostasis and defense mechanisms.

Cytokines: Small proteins released by cells that have a specific effect on the interactions and communications between cells during immune responses.

Vasodilation: The widening of blood vessels, which increases blood flow to tissues and is a key feature of the inflammatory response.

Acute Phase Response: A rapid systemic response to inflammation characterized by increased production of certain proteins by the liver, changes in white blood cell count, and alterations in metabolism.

Mast cells are a type of white blood cell that plays a crucial role in the immune system, particularly in allergic reactions and defense against parasites. They are found in various tissues, especially in the skin, lungs, and digestive tract, where they release histamine and other chemicals that contribute to inflammation and immune responses.

Histamine: A chemical released by mast cells during an immune response that causes blood vessels to dilate and contributes to inflammation and allergy symptoms.

IgE: Immunoglobulin E (IgE) is an antibody produced by the immune system that binds to allergens and triggers mast cell activation, leading to allergic reactions.

Anaphylaxis: A severe, life-threatening allergic reaction characterized by rapid onset of symptoms, including difficulty breathing, swelling, and a drop in blood pressure, often involving mast cell degranulation.

Basophils are a type of white blood cell that plays a crucial role in the immune response, particularly in allergic reactions and inflammation. These cells are characterized by their large granules filled with histamine and other chemicals that are released during immune responses. Basophils are derived from hematopoietic stem cells in the bone marrow and are part of the innate immune system, making them essential in mediating hypersensitivity reactions and contributing to the body's defense mechanisms.

Mast Cells: Mast cells are immune cells similar to basophils, found in tissues rather than circulating in the blood, that release histamine and play a key role in allergic responses.

Histamine: Histamine is a chemical released by basophils and mast cells during allergic reactions that contributes to inflammation and symptoms like itching, swelling, and bronchoconstriction.

IgE Antibodies: IgE antibodies are a class of immunoglobulins that bind to allergens and trigger basophils and mast cells to release histamine, initiating an allergic response.

An allergen is a substance that triggers an allergic reaction in sensitive individuals, often leading to an immune response characterized by the production of specific antibodies, particularly Immunoglobulin E (IgE). Allergens can be found in various forms, such as proteins, pollen, dust mites, certain foods, and insect venom, and they play a central role in hypersensitivity reactions that can vary in severity from mild to life-threatening.

IgE: Immunoglobulin E (IgE) is a type of antibody produced by the immune system in response to allergens and is primarily involved in allergic reactions and protection against parasitic infections.

hypersensitivity: Hypersensitivity refers to an exaggerated immune response to a harmless substance, which can lead to tissue damage and various clinical symptoms.

anaphylaxis: Anaphylaxis is a severe, potentially life-threatening allergic reaction that occurs rapidly after exposure to an allergen, characterized by symptoms such as difficulty breathing, swelling, and shock.

Complement activation refers to a complex series of biochemical reactions that enhance the ability of antibodies and phagocytic cells to clear pathogens from an organism. This process is critical for the immune response, involving the complement system, which consists of proteins that work together to promote opsonization, cell lysis, and inflammation. Understanding complement activation is essential, as it plays significant roles in antibody function, autoimmune disorders, and various hypersensitivity reactions.

Opsonization: A process where pathogens are marked for ingestion and destruction by phagocytes through the binding of antibodies or complement proteins.

Anaphylaxis: A severe allergic reaction that can occur rapidly and may involve multiple organ systems, often mediated by IgE antibodies and complement activation.

C3 Convertase: An enzyme complex that plays a crucial role in the complement system, responsible for cleaving the complement protein C3 into C3a and C3b, initiating downstream effects of complement activation.

Complement-dependent cytotoxicity refers to a mechanism of immune response where the complement system is activated and leads to the destruction of target cells, usually through opsonization and formation of the membrane attack complex. This process plays a significant role in mediating antibody-dependent cell-mediated cytotoxicity and is crucial in the context of hypersensitivity reactions, particularly those classified as type II hypersensitivity.

Complement System: A group of proteins that play an essential role in the immune response by promoting inflammation, opsonization, and cell lysis.

Opsonization: The process by which pathogens are marked for destruction by immune cells, enhancing their recognition and ingestion by phagocytes.

Type II Hypersensitivity: An allergic reaction that involves the production of antibodies against specific antigens on cell surfaces, leading to complement activation and cell damage.

Antibody-dependent cell-mediated cytotoxicity (ADCC) is an immune response mechanism where antibodies bound to a target cell, such as an infected or tumor cell, facilitate the destruction of that cell by immune effector cells like natural killer (NK) cells or macrophages. This process relies on the specific recognition of the antibody-bound target by Fc receptors on effector cells, leading to the activation of these immune cells and the release of cytotoxic substances to kill the target cell.

Natural Killer (NK) Cells: A type of lymphocyte that plays a critical role in the innate immune system, specifically targeting and destroying virus-infected cells and tumor cells.

Monoclonal Antibodies: Antibodies produced from a single clone of B cells, designed to bind to specific antigens; they are used in therapies to target specific cells for destruction.

Fc Receptors: Proteins found on the surface of certain immune cells that bind to the Fc region of antibodies, crucial for mediating various immune responses including ADCC.

T cell-mediated refers to immune responses that are primarily orchestrated by T lymphocytes, a type of white blood cell. These responses are crucial in the defense against intracellular pathogens, such as viruses and some bacteria, and play a key role in the regulation of immune activity. T cell-mediated immunity is essential in the context of hypersensitivity reactions, where T cells can contribute to tissue damage and inflammation.

Cytotoxic T cells: A subset of T cells that directly kill infected or cancerous cells by recognizing specific antigens presented on their surface.

Helper T cells: A type of T cell that assists other immune cells by releasing cytokines, helping to activate B cells and cytotoxic T cells.

Delayed-type hypersensitivity: A form of hypersensitivity reaction mediated by T cells, characterized by a delayed response to an antigen, often leading to inflammation and tissue damage.

Pro-inflammatory cytokines are signaling molecules produced by immune cells that promote inflammation and are critical in the body’s immune response to pathogens and tissue injury. They play a key role in activating and recruiting immune cells to sites of infection or injury, and their production is often triggered by pattern recognition receptors that detect pathogen-associated molecular patterns, linking them to inflammation and the overall immune response.

Cytokines: Small proteins released by cells that have a specific effect on the interactions and communications between cells, particularly in the immune system.

Inflammation: A biological response of body tissues to harmful stimuli, such as pathogens or damaged cells, characterized by redness, swelling, heat, and pain.

Chemokines: A subset of cytokines that specifically induce chemotaxis in nearby responsive cells, guiding immune cells to sites of inflammation.

Type I hypersensitivity, also known as immediate hypersensitivity, is an exaggerated immune response mediated by Immunoglobulin E (IgE) antibodies upon exposure to allergens. This reaction occurs rapidly after re-exposure to the allergen, resulting in symptoms such as itching, swelling, and respiratory issues. The role of IgE in this process links it closely to allergic responses and reveals its connection to different types of hypersensitivity reactions, along with the diagnostic and management strategies used in dealing with allergies.

Allergen: A substance that causes an allergic reaction by triggering an immune response, often leading to the production of IgE.

Mast Cells: Cells that play a crucial role in type I hypersensitivity by releasing histamine and other mediators when activated by IgE.

Anaphylaxis: A severe, potentially life-threatening allergic reaction that can occur rapidly after exposure to an allergen, often associated with type I hypersensitivity.

Anaphylaxis is a severe, potentially life-threatening allergic reaction that occurs rapidly after exposure to an allergen. It involves a sudden release of mediators from mast cells and basophils, primarily driven by immunoglobulin E (IgE) antibodies. Understanding anaphylaxis is crucial as it represents a critical form of hypersensitivity reaction and requires immediate medical intervention.

Allergen: A substance that can cause an allergic reaction, such as food proteins, insect venom, or certain medications.

Epinephrine: A hormone and neurotransmitter used as a first-line treatment for anaphylaxis, which helps to reverse the symptoms by constricting blood vessels and relaxing airway muscles.

Mast Cells: A type of white blood cell that plays a key role in allergic responses by releasing histamines and other inflammatory mediators during an allergic reaction.

Type II hypersensitivity is an immune response that occurs when antibodies target and bind to specific antigens on the surface of cells, leading to cell damage or destruction. This type of hypersensitivity involves IgG or IgM antibodies and can result in various clinical manifestations depending on the affected cells and tissues.

Autoimmune Hemolytic Anemia: A condition in which the immune system produces antibodies against its own red blood cells, leading to their destruction and resulting in anemia.

Blood Transfusion Reactions: Adverse reactions that occur when a patient receives incompatible blood, causing the recipient's immune system to attack the transfused red blood cells.

Cytotoxic T Cells: A type of immune cell that can kill infected or damaged cells, but is not directly involved in Type II hypersensitivity reactions, which primarily involve antibodies.

Type III hypersensitivity is an immune response characterized by the formation of antigen-antibody complexes that deposit in tissues, leading to inflammation and tissue damage. This type of hypersensitivity is primarily mediated by the immune system's interaction with soluble antigens, resulting in complement activation and recruitment of inflammatory cells, which can cause local tissue injury and systemic effects.

immune complex: A complex formed when antibodies bind to soluble antigens, which can lead to various immunological reactions if not effectively cleared from the body.

complement system: A group of proteins in the blood that enhance the ability of antibodies and phagocytic cells to clear pathogens and promote inflammation.

serum sickness: A type of type III hypersensitivity reaction that occurs when antibodies form against foreign proteins introduced into the body, often following the administration of antiserum or certain medications.

Type IV hypersensitivity, also known as delayed-type hypersensitivity, is an immune response mediated by T cells that occurs 24 to 72 hours after exposure to an antigen. This form of hypersensitivity does not involve antibodies but rather relies on the activation of CD4+ T helper cells and CD8+ cytotoxic T cells that recognize specific antigens, leading to inflammation and tissue damage.

T Cells: A type of white blood cell that plays a central role in cell-mediated immunity, including helper T cells (CD4+) and cytotoxic T cells (CD8+).

Antigen-Presenting Cells (APCs): Cells such as dendritic cells, macrophages, and B cells that process and present antigens to T cells, crucial for initiating immune responses.

Contact Dermatitis: An inflammatory skin condition resulting from exposure to allergens or irritants, often associated with type IV hypersensitivity reactions.

Contact dermatitis is a type of skin inflammation caused by direct contact with an irritant or allergen. This condition can lead to red, itchy, and inflamed skin, often resulting in discomfort and sometimes blisters. Understanding contact dermatitis is important as it falls under the category of hypersensitivity reactions, specifically as a manifestation of delayed-type hypersensitivity, which involves the immune system's response to certain substances after repeated exposure.

Allergen: A substance that can trigger an allergic reaction, leading to symptoms such as redness, itching, and swelling.

Irritant: A non-allergic substance that causes damage to the skin or mucous membranes upon contact.

Delayed-type hypersensitivity: A type of immune response that takes hours to days to develop, typically mediated by T cells rather than antibodies.

Granulomatous inflammation is a specific type of chronic inflammatory response characterized by the formation of granulomas, which are small aggregates of macrophages that transform into epithelioid cells. This type of inflammation often arises in response to persistent irritants, such as pathogens or foreign substances, and is a hallmark of certain diseases, reflecting an adaptive immune response that aims to contain and isolate these irritants.

Macrophages: A type of white blood cell that engulfs and digests cellular debris and pathogens, playing a key role in the immune response.

Epithelioid Cells: Modified macrophages that resemble epithelial cells, found in granulomas and involved in chronic inflammatory responses.

Chronic Inflammation: A prolonged inflammatory response that can last for months or years, often resulting from an unresolved acute inflammatory process.

Autoimmune hemolytic anemia is a condition in which the immune system mistakenly targets and destroys the body’s own red blood cells, leading to anemia. This disorder arises from the production of antibodies that bind to red blood cells, causing their premature destruction in the spleen or other parts of the reticuloendothelial system. Understanding this condition highlights the role of the immune system in regulating blood cell health and illustrates a specific type of immune response where self-tolerance fails.

Hemolysis: The breakdown of red blood cells, which can lead to anemia when it occurs at an accelerated rate.

Antibodies: Proteins produced by the immune system that can specifically target and bind to foreign substances, or in this case, self-antigens on red blood cells.

Spleen: An organ that plays a significant role in filtering blood and recycling iron from hemolyzed red blood cells; it is often involved in the destruction of antibody-coated red blood cells.

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease where the body's immune system mistakenly attacks healthy tissues, leading to widespread inflammation and damage to various organs. This condition highlights the breakdown of immune tolerance, as the body fails to distinguish between self and non-self antigens, resulting in the production of autoantibodies that target its own cells. SLE can manifest in a variety of ways, making it a complex disease that encompasses multiple symptoms and organ systems.

Autoimmunity: A condition where the immune system mistakenly attacks its own body tissues, often leading to autoimmune diseases like systemic lupus erythematosus.

Antinuclear antibodies (ANAs): Autoantibodies that target substances within the nucleus of cells, commonly found in individuals with systemic lupus erythematosus and used as a diagnostic marker.

Immune tolerance: The state in which the immune system does not mount an attack against the body's own tissues, crucial for preventing autoimmune diseases.

Rheumatoid arthritis is a chronic autoimmune disorder that primarily affects the joints, leading to inflammation, pain, and eventual joint damage. It is characterized by the body's immune system mistakenly attacking its own joint tissues, which reflects a breakdown of the mechanisms that normally prevent such reactions. The disorder exemplifies how loss of tolerance can lead to significant health issues and demonstrates the mechanisms involved in various autoimmune disorders.

autoimmunity: A condition where the immune system mistakenly attacks the body's own cells and tissues, leading to various autoimmune diseases.

synovial fluid: A viscous fluid found in the cavities of synovial joints that lubricates and nourishes the cartilage and joint structures.

inflammation: A biological response of body tissues to harmful stimuli, characterized by redness, swelling, pain, and heat, which is a key feature in rheumatoid arthritis.

Multiple sclerosis (MS) is a chronic autoimmune disease that affects the central nervous system, leading to the destruction of the protective myelin sheath surrounding nerve fibers. This demyelination disrupts communication between the brain and other parts of the body, causing a wide range of neurological symptoms. The disease is characterized by episodes of exacerbation and remission, which can vary in severity and duration among individuals.

autoimmunity: A condition where the immune system mistakenly attacks the body's own tissues, leading to various autoimmune diseases, including multiple sclerosis.

myelin sheath: A fatty layer that surrounds and insulates nerve fibers, facilitating the rapid transmission of electrical signals in the nervous system.

neurological symptoms: Physical or cognitive issues that result from damage to the nervous system, including those seen in multiple sclerosis such as weakness, numbness, and vision problems.

Degranulation is the process by which immune cells, particularly mast cells and basophils, release their pre-stored granules containing various biologically active substances, including histamines and cytokines. This process plays a crucial role in the body's immediate response to pathogens and allergens, leading to inflammation and recruitment of other immune cells to the site of infection or injury. Degranulation is central to both innate immunity and allergic responses, highlighting its importance in various hypersensitivity reactions.

Mast Cells: A type of immune cell that is involved in allergic reactions and the defense against pathogens, known for storing and releasing histamines during degranulation.

Histamine: A chemical released during degranulation that plays a key role in inflammatory responses, causing blood vessel dilation and increased permeability.

Cytokines: Signaling proteins released by immune cells during degranulation that help regulate immunity, inflammation, and hematopoiesis.