Explain vasculitis, immune-complex disease, antiphospholipid pathology and their O&G relevance.

Vasculitis, Immune-Complex Disease and Antiphospholipid Pathology

Start with one organ: the blood vessel. Everything in this chapter is the story of what happens when the inside surface of a vessel stops behaving normally. A healthy vessel keeps blood liquid, keeps it inside, and keeps inflammation out. When that surface is injured or attacked, the same wall can bleed, clot, inflame, leak, spasm, scar or rupture. Hold that single idea and the whole chapter follows from it.

O&G tests this basic pathology constantly, because pregnancy stresses every vessel in the body. Pre-eclampsia is fundamentally endothelial disease. Postpartum haemorrhage is a test of how fast a wound can clot. Antiphospholipid syndrome (APS) injures the placenta through thrombo-inflammation. Lupus nephritis deposits immune complexes in glomeruli. Vasculitis inflames the vessel wall itself and starves organs of blood. Massive transfusion is what happens when the whole haemostatic system is overwhelmed at once.

So this chapter builds upward in a deliberate order. We first describe the vessel wall as a living organ and what "endothelial activation" actually means. From that single hinge we derive why pregnancy clots more easily (Virchow's triad), how clotting is really organised on cell surfaces, how the immune system can weaponise clotting (immunothrombosis), and only then the named diseases — vasculitis, immune-complex disease and APS — followed by the obstetric microangiopathies and disseminated intravascular coagulation. Every later section presupposes only what came before.

The repeating mechanism, in one line, is:

immune or physical trigger -> endothelial activation -> complement/neutrophil/platelet recruitment -> coagulation or vessel-wall injury -> tissue ischaemia, bleeding, placental dysfunction or organ failure

The Vessel Wall as an Organ

Begin at the simplest level: what is a blood vessel actually made of, and which part does the work?

A blood vessel is not a pipe. It is a living organ with endothelium, smooth muscle, extracellular matrix, nerves, inflammatory cells and haemostatic regulators.

Endothelial injury is the hinge. It changes a vessel from anti-adhesive and anticoagulant to sticky, leaky, vasoconstricted and procoagulant.

A single signalling pair captures much of this balance. Healthy endothelium makes prostacyclin (PGI2), a powerful vasodilator and inhibitor of platelet aggregation, which is poised against platelet-derived thromboxane (TXA2), a vasoconstrictor and platelet aggregator. In health the balance favours an open, non-clotting vessel; in injury or pre-eclampsia the balance shifts towards thromboxane, vasoconstriction and platelet thrombosis. This same imbalance is the rationale for low-dose aspirin prophylaxis in women at high risk of early-onset pre-eclampsia — aspirin preferentially suppresses platelet thromboxane while sparing endothelial prostacyclin. The drug is discussed in detail in the linked Final chapter; here the point is purely the mechanism.

A second protective layer worth naming is the glycocalyx, the gel-like sugar coat lining the endothelium. It keeps cells and proteins from sticking, senses flow, and holds heparan sulphate and other anticoagulant molecules at the surface. Sepsis and pre-eclampsia shed the glycocalyx, which is one reason these states leak fluid (oedema) and clot at the same time.

Virchow's Triad in O&G

Virchow's triad explains why thrombi form:

1. Endothelial injury
2. Abnormal blood flow
3. Hypercoagulability

Pregnancy contains all three.

The physiology is protective: placental separation would be lethal without rapid haemostasis. At delivery a blood flow of several hundred millilitres per minute through the placental bed has to be staunched within seconds, so pregnancy biases the entire system towards clotting. The pathology appears when that protective bias is amplified by obesity, sepsis, pre-eclampsia, caesarean birth, dehydration, thrombophilia, APS, malignancy or immobility.

The consequence is a measurable rise in venous thromboembolism (VTE) risk: pregnancy roughly increases the risk of VTE several-fold compared with the non-pregnant state, and the puerperium — the weeks after delivery — is the single highest-risk window of a woman's reproductive life. This is why thromboprophylaxis decisions cluster around delivery and the postnatal period, and why a breathless or leg-swollen postpartum woman is never reassured casually. The specific risk-scoring and prophylaxis regimens belong to the linked clinical chapters; the Primary point is that the biology of pregnancy is a prothrombotic state.

The Cell-Based Model of Coagulation

We now have endothelium that can flip from anticoagulant to procoagulant, and a pregnancy that biases the whole system towards clotting. The next question is mechanistic: when a vessel is breached, how does a clot actually assemble? The classic intrinsic/extrinsic cascade is useful for interpreting the laboratory tests — prothrombin time (PT) and activated partial thromboplastin time (aPTT) — but it is not how clotting behaves in a real wound. In vivo, coagulation is organised on cell surfaces.