Infections are one of the most significant and well-documented triggers for blood clots in individuals with antiphospholipid syndrome (APS). While the presence of antiphospholipid antibodies creates a lifelong state of “sticky blood,” a physical clot often requires a “second hit” to occur. Infections whether viral, bacterial, or fungal provide this secondary stimulus by causing widespread inflammation and activating the immune system. This activation can cause the antibodies to become more aggressive, leading to the formation of a thrombus (clot). In the UK, healthcare professionals advise those with APS to seek medical advice promptly when they develop symptoms of an infection to ensure their clotting risk is managed appropriately.
What We’ll Discuss in This Article
- The “double hit” hypothesis and the role of infection
- How the immune response to infection interacts with APS antibodies
- The role of inflammation and the “sticky” blood vessel lining
- Specific types of infections that pose a higher risk
- Preventative measures during periods of illness
- When to contact your GP or specialist in the UK
The “Double Hit” Hypothesis and Infection
The “double hit” hypothesis is a clinical theory used to explain why people with APS do not have clots all the time. The “first hit” is the genetic or autoimmune predisposition—the existence of the antibodies themselves. The “second hit” is an environmental trigger that pushes the system over the edge. Infection is a classic second hit. During an infection, the body produces chemical signals called cytokines that alert the immune system. These same signals can also “switch on” the blood vessel walls, making them more receptive to the binding of APS antibodies and the subsequent formation of a clot.
How the Immune System Triggers Clotting
When you have an infection, your body increases the production of white blood cells and inflammatory proteins. One specific type of white blood cell, the neutrophil, can release “Neutrophil Extracellular Traps” (NETs). While these traps are designed to catch and kill bacteria, they also act as a scaffold that traps platelets and activates the clotting cascade. In individuals with APS, this process is often exaggerated. The antibodies can bind to these traps, making them even more potent at forming clots. This interaction between the immune response and the clotting system is a primary reason why even a common infection can lead to a vascular event in APS patients.
Inflammation and the Endothelium
The endothelium is the delicate inner lining of your blood vessels that usually prevents clotting. However, systemic inflammation caused by an infection can damage this lining. When the endothelium is “activated” by inflammation, it stops producing natural anticoagulants and starts producing pro-clotting factors. For someone with APS, their antibodies are already primed to attack these activated cells. This “perfect storm” of damaged vessel walls, active antibodies, and inflammatory cells creates the ideal conditions for a deep vein thrombosis or stroke. The NHS notes that because APS is an immune system disorder, its activity is closely linked to how the body responds to illness.
High-Risk Infections and APS
While any significant infection can be a trigger, certain types are more frequently associated with clotting episodes in APS. Respiratory infections, such as influenza or pneumonia, and urinary tract infections (UTIs) are common catalysts. More severe systemic infections, such as sepsis, carry an extremely high risk. Additionally, certain viral infections are known to temporarily increase the levels of antiphospholipid antibodies in the blood, which can exacerbate the underlying condition. NICE guidelines recommend that patients with APS who are acutely unwell with an infection should be assessed for their venous thromboembolism (VTE) risk and may require temporary preventative treatment.
Managing Risk During Illness
If you have APS and develop an infection, there are several steps you can take to manage your risk. Staying well-hydrated is essential, as dehydration makes the blood thicker and slower moving. It is also important to remain as mobile as possible, even if you are feeling unwell, to keep the blood circulating in your legs. Your doctor may review your current medication; in some cases, they might temporarily increase your dose of anticoagulants or prescribe a short course of heparin injections to protect you until the infection has cleared and the inflammation has subsided.
When to Seek Medical Advice
Early intervention is key to preventing an infection from turning into a clotting crisis. You should contact your GP or specialist if you have symptoms of a significant infection, such as a high fever, persistent cough, or localized pain and redness that could indicate a skin infection. It is vital to inform any medical professional treating you for an infection that you have APS, as this may change how they manage your care. If you experience sudden symptoms like leg swelling, chest pain, or shortness of breath while you are ill, you should seek emergency medical attention immediately.
Conclusion
Infections act as a powerful trigger for blood clots in people with APS by causing systemic inflammation and activating the immune response. By understanding the “double hit” mechanism, patients can better appreciate why managing an illness promptly is so important for their vascular health. With proper hydration, mobility, and medical oversight, the risks associated with infections can be effectively mitigated. If you experience severe, sudden, or worsening symptoms, call 999 immediately.
Can infections trigger clotting episodes in APS?
Yes, infections cause inflammation that can activate the blood vessel lining and immune cells, triggering the “sticky” antibodies to form a physical clot.
Does a common cold always cause a clot in APS?
No, most minor colds do not lead to a clot, but more significant illnesses that cause a fever or widespread inflammation carry a much higher risk.
Should I take extra blood thinners when I have the flu?
You should never change your medication dose without consulting your doctor; they will decide if temporary prophylaxis is needed based on your symptoms.
Can vaccinations trigger a clot in APS?
While vaccines stimulate the immune system, the risk of a clot from a vaccine is generally much lower than the risk of a clot from the actual infection they prevent.
How does dehydration during illness affect APS?
Dehydration reduces blood volume and makes the blood more concentrated, which can slow down circulation and make a clot more likely to form.
Can a UTI be a trigger for a stroke in APS?
Any significant infection, including a UTI, can cause the systemic inflammation that triggers an arterial or venous clot in predisposed individuals.
Will my antibody levels go up when I am sick?
Infection can cause a temporary spike in antibody levels or activity, which is why doctors often wait until you are healthy to perform diagnostic blood tests.
Authority Snapshot (E-E-A-T)
This article examines the clinical relationship between infection and blood clotting in APS, aligned with UK medical standards. The content is reviewed by Dr. Stefan Petrov, a UK-trained physician with experience in internal medicine and intensive care where the management of “thrombo-inflammatory” triggers is a priority. All information provided is strictly aligned with the patient safety standards and investigative pathways established by the NHS and NICE.
