An immune complex is a structure created when antibodies, produced by the immune system, bind to specific molecules known as antigens. Antigens are often parts of foreign invaders like bacteria or viruses, but they can also be substances from within the body. This binding process is highly specific, where a particular antibody will only recognize and attach to its matching antigen. The resulting immune complex is a larger molecule formed from these joined components.
The Role of Immune Complexes in a Healthy Body
In a healthy immune system, the formation of immune complexes is a normal and beneficial process. Their primary function is to neutralize and eliminate threats. When antibodies bind to antigens on pathogens, they can prevent them from attaching to and infecting the body’s cells, a process called neutralization. This action disarms the invader, stopping its harmful activity.
These complexes also act as signals, or tags, for other components of the immune system. This tagging process, known as opsonization, marks the pathogen for destruction. Specialized immune cells called phagocytes recognize these tagged complexes. Guided by these signals, phagocytes engulf and digest the entire complex, safely removing the pathogen from circulation.
How Immune Complexes Cause Disease
Disease can arise when the system for clearing immune complexes becomes overwhelmed. If complexes are produced faster than they can be removed, they continue to circulate in the bloodstream. The size and ratio of antigen to antibody in these complexes can influence their tendency to be deposited in tissues. Small and intermediate-sized complexes are often more difficult for the immune system to clear, increasing their chances of lodging in various locations.
Once deposited, these complexes can trigger a damaging inflammatory response. They activate a part of the immune system called the complement system, a cascade of proteins that generates inflammation. This process attracts white blood cells to the site of deposition, which release enzymes and reactive molecules to destroy the complexes. These substances also damage the surrounding healthy tissue, leading to injury in the organs where the complexes have settled, which is the direct cause of symptoms.
Conditions Associated with Immune Complex Deposition
Several diseases are driven by the deposition of immune complexes in various tissues. Systemic Lupus Erythematosus (SLE) is an example of an autoimmune disease where the body produces antibodies against its own cellular components, such as DNA. The resulting immune complexes can deposit in numerous organs, including the skin, joints, and kidneys, leading to symptoms like rashes, arthritis, and kidney failure.
Post-streptococcal glomerulonephritis is a condition that can develop after a streptococcal infection. In this case, antibodies form complexes with lingering bacterial antigens. These complexes are filtered by the kidneys and become trapped in the small blood-filtering units called glomeruli, causing inflammation that impairs kidney function. Vasculitis, or inflammation of blood vessels, can also be caused by immune complexes depositing in the vessel walls, leading to their weakening.
Diagnosis and Management of Immune Complex Disorders
Diagnosing conditions caused by immune complexes involves a combination of clinical evaluation and laboratory tests. Blood tests can measure levels of complement proteins C3 and C4, as low levels suggest they are being consumed by ongoing immune complex activation. Other blood tests can identify autoantibodies, such as those targeting DNA in lupus. A urinalysis may check for blood and protein in the urine, which are signs of kidney inflammation. A tissue biopsy is sometimes necessary, where a small sample of an affected organ is examined to confirm the presence of deposited immune complexes.
Management of these disorders focuses on reducing inflammatory damage and addressing the underlying cause of antigen production. To control inflammation, doctors often prescribe immunosuppressive medications, such as corticosteroids, which dampen the immune response. For autoimmune conditions, other drugs may target the cells producing the harmful antibodies. If the condition is triggered by a persistent infection, treatment will focus on eradicating the pathogen with antimicrobial drugs to stop the source of the antigens.