Patient description: The patient is a 23-year-old man who came to the hospital with high blood pressure (175/95 mmHg) and signs of septicemia. He developed respiratory failure requiring mechanical ventilation and renal failure requiring hemodialysis. His blood smear showed microangiopathic hemolytic anemia and thrombocytopenia. His blood tests revealed elevated lactate dehydrogenase and reduced human complement C3 levels with a normal coagulation profile. He was diagnosed with atypical hemolytic uremic syndrome. He was treated with plasma exchange and corticosteroids. He has been previously vaccinated with meningococcal group ACWY conjugate vaccine and meningococcal group B vaccine. The genetic survey revealed ADAMTS13 >10%.

Clinical trial eligibility criteria: eligible ages (years): 13.0-999.0,  Preeclampsia Severe Preeclampsia Eculizumab HELLP HELLP Syndrome HELLP Syndrome Second Trimester Pregnancy Related AHUS PNH Complement Regulatory Factor Defect Complement Abnormality HELLP Syndrome Third Trimester Provision of signed and dated informed consent form 2. Stated willingness to comply with all study procedures & availability for study duration 3. Biologically female, aged ≥13, body weight ≥40kg 4. Diagnosed with preeclampsia between 23-29+6/7 weeks gestation, by following 1. Blood pressure ≥160 mmHg systolic or ≥110 mmHg diastolic OR 2. Blood pressure ≥140 mmHg systolic or ≥90 mmHg diastolic and at least one of the following i. Proteinuria (spot protein/creatinine ≥0.3mg/mg or 24Hr protein ≥300 mg) ii. Platelet count <100,000/μl iii. Aspartate or alanine transaminase >2x upper limit of normal iv. Creatinine >1.1 mg/dl or oliguria v. Pulmonary edema 5. Ability to take intravenous medication and be willing to adhere to the eculizumab regimen 6. Ability to receive meningococcal vaccine and be willing to adhere to antibiotic regimen  An individual who meets any of the following prior to enrollment will be excluded from participation in this study: 1. Known allergic reactions eculizumab or meningococcal vaccine 2. Febrile illness within prior 2 weeks 3. Treatment with another investigational drug within previous 6 months 4. Inpatient expectant management for preeclampsia >72 hours prior to enrollment 5. Fetal contraindication to expectant management of pregnancy 6. Platelet count <50,000/μl 7. Diagnosis of hemolysis, elevated liver enzymes, low platelet count (HELLP) syndrome Must meet all of the following to be excluded: LDH >600 U/L, platelet count < 100,000/μl, AST >2x upper limit of normal, ALT >2x upper limit of normal 8. Diagnosis of Eclampsia 9. Diagnosis of Placental abruption 10. Intrauterine fetal demise 11. Coagulopathy (INR ≥ 1.5) 12. Fibrinogen <200 mg/dl 13. Persistent, severe headache unresponsive to medications 14. Persistent, severe visual disturbances 15. Persistent, severe epigastric or RUQ pain unresponsive to medications 16. Diagnosis of Systemic lupus erythematosus 17. Diagnosis of Anti-phospholipid antibody syndrome 18. Diagnosis of Atypical hemolytic uremic syndrome 19. Diagnosis of Paroxysmal nocturnal hemoglobinuria 20. Known complement deficiency 21. Diagnosis of Venous thromboembolism active or within 6 months of enrollment 22. Diagnosis of Human immunodeficiency virus (HIV) 23. Diagnosis of Hepatitis C virus (active viremia) 24. Diagnosis of Cancer (not in remission) 25. History of Solid organ transplant 26. Systemic viral or bacterial infection (active, untreated) 27. Active use of eculizumab at time of enrollment 28. Contraindication to eculizumab treatment or complement system blockade 29. Contraindication to meningococcal vaccine 30. Body weight <40kg 31. Age <13 32. Neutropenia (<1500/mm3) 33. Gonorrhea, chlamydia, or syphilis in current pregnancy 34. Illicit substance use in current pregnancy 35. Currently homeless or incarcerated 36. Alcoholism 37. Liver cirrhosis 38. Insulin dependent diabetes 39. Active use of immunosuppressive therapies, other than use of corticosteroids for fetal lung maturity 40. Use of prophylactic or therapeutic heparin, or low molecular weight heparin, in pregnancy for hypercoagulable condition