Define Type III hypersensitivity reaction?
Type III hypersensitivity reaction, also called immune complex-mediated hypersensitivity, occurs when clusters of antigens and antibodies, known as immune complexes, form in the blood and get deposited in tissues. This leads to inflammation and damage. Normally, the body clears these complexes, but when they persist, they can cause harmful immune responses, resulting in autoimmune and inflammatory conditions.
Table of Contents
Causes of Immune Complex Formation
Immune complexes form when antibodies bind to antigens. Usually, these complexes are quickly removed by the immune system, but certain factors can lead to their buildup and persistence:
Chronic Antigen Exposure
Prolonged exposure to antigens, such as in chronic infections or autoimmune disorders, results in ongoing formation of immune complexes. If the body fails to eliminate the antigen, complexes continue forming and lead to inflammation.
Poor Immune Complex Clearance
Immune complexes are typically cleared by cells like macrophages in organs such as the liver and spleen. When this process is impaired, the immune complexes accumulate in tissues.
Excessive Antibody Production
When the body produces too many antibodies, especially IgG or IgM, large numbers of immune complexes form. These can overwhelm the system’s ability to clear them, causing deposits in tissues.
Size of Immune Complexes
Larger complexes are generally removed more easily by the immune system, while smaller complexes can circulate in the bloodstream longer and are more likely to be deposited in tissues, causing inflammation.
Mechanism of Type III Hypersensitivity
The process of Type III hypersensitivity occurs in several steps:
Immune Complex Formation
When the immune system encounters antigens, it produces antibodies (usually IgG or IgM) that bind to the antigens, forming immune complexes. These complexes circulate in the bloodstream.
Immune Complex Deposition
If not cleared, these immune complexes deposit in tissues like the walls of blood vessels, joints, kidneys, or lungs. The size, quantity, and location of these deposits determine the severity and site of the reaction.
Complement System Activation
Once immune complexes are deposited, they activate the complement system, a group of proteins that boosts immune responses. This activation draws immune cells like neutrophils and macrophages to the deposition sites, increasing inflammation.
Inflammatory Response
These immune cells release enzymes and chemicals that attempt to eliminate the immune complexes. However, this process also causes damage to the surrounding tissues, leading to disease symptoms.
Tissue Damage and Disease
Chronic inflammation and tissue damage result from the prolonged presence of immune complexes, leading to conditions such as vasculitis (inflammation of blood vessels), arthritis, or kidney inflammation.
Types of Type III Hypersensitivity Reactions
Type III hypersensitivity reactions can occur in localized or systemic forms. These are some common examples:
Localized Reactions
In these cases, immune complexes form where the antigen enters the body, leading to localized inflammation.
Arthus Reaction: A well-known example of a localized reaction, the Arthus reaction occurs when an antigen is injected into the skin of someone who already has antibodies against that antigen. This causes immune complexes to form at the injection site, leading to swelling, redness, and tissue damage.
Systemic Reactions
In systemic reactions, immune complexes circulate through the bloodstream and deposit in various tissues, causing widespread damage.
Serum Sickness: This condition arises when a person receives foreign proteins, such as those from animal-derived treatments. The immune system forms antibodies against these proteins, and the resulting immune complexes circulate and deposit in tissues, causing fever, joint pain, skin rashes, and kidney inflammation.
Systemic Lupus Erythematosus (SLE): SLE is an autoimmune disorder in which the immune system creates antibodies against the body’s own tissues, leading to the formation of immune complexes that deposit in multiple organs. This results in chronic inflammation and damage, with common symptoms including joint pain, skin rashes, and kidney problems.
Post-Streptococcal Glomerulonephritis: After a streptococcal infection, some people develop kidney inflammation due to immune complexes depositing in the kidneys, causing symptoms like blood in the urine, swelling, and high blood pressure.
Rheumatoid Arthritis (RA): In RA, immune complexes settle in the joints, leading to chronic inflammation and joint damage, which causes pain, stiffness, and swelling.
Conclusion
Type III hypersensitivity reactions result from the formation and deposition of immune complexes in tissues, which lead to inflammation and tissue damage. Type III hypersensitivity reactions can be triggered by ongoing antigen exposure, excess antibody production, or poor immune complex clearance. The resulting inflammation may cause localized reactions, like the Arthus reaction, or systemic diseases, such as serum sickness, lupus, and rheumatoid arthritis. Understanding these mechanisms is essential for diagnosing and treating immune complex-mediated diseases.
Frequently Asked Questions (FAQ)
What do you mean by Tissue Damage ?
Tissue damage pertains to the impairment or annihilation of bodily tissues, frequently resulting from physical trauma, infection, inflammation, or an immunological reaction, which compromises structural integrity or function.
Define Systemic Lupus Erythematosus ?
Systemic Lupus Erythematosus (SLE) is a chronic autoimmune disease where the immune system attacks the body’s own tissues, causing inflammation and damage to various organs, including the skin, joints, kidneys, and heart.
Define Type III hypersensitivity reactions?
Type III hypersensitivity reaction, also called immune complex-mediated hypersensitivity, occurs when clusters of antigens and antibodies, known as immune complexes, form in the blood and get deposited in tissues.
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