Stroke represents a major cause of morbidity and mortality despite significant progress in recent decades. In individuals under the age of 65, the etiologies of ischemic stroke (IS) are diverse, and management is well-established. Antiphospholipid syndrome (APS) accounts for 10 to 20% of the causes of stroke in this population. In elderly individuals, APS is not systematically investigated due to the predominance of embolic, atherosclerotic, and small vessel disease causes. However, delayed discovery of APS is not uncommon and is more frequently associated with the occurrence of arterial thrombosis. Moreover, the management of APS involves several challenges given the risk of recurrence of thrombosis and the potential association with conventional cardiovascular risk factors. The antithrombotic treatment consists of lifelong anticoagulation, excluding direct oral anticoagulants (DOACs) due to the risk of thrombotic recurrence. The main objective of the study will be to assess the prevalence of antibodies useful for the diagnosis of APS (Sapporo criteria) in individuals aged 65 or older hospitalized for an ischemic stroke (IS) or transient ischemic attack (TIA). Furthermore, the classification of APS is likely to evolve in the coming years with the inclusion of new clinically relevant antibodies (anti-phosphatidylserine and anti-phosphatidylethanolamine) because of their strong association with the occurrence of thrombosis. Even though they are often associated with circulating anticoagulants, they are also found in 10% of APS cases negative for other antibodies. Patient inclusion in the study should occur during the acute phase of the stroke, before the initiation of anticoagulant treatment. Thus, after verifying the inclusion and exclusion criteria, patients will be informed and must sign the informed consent form if they agree to participate. After inclusion, the research procedure will be as follows: * Conduct a unique immunological biological assessment with: * Part performed as part of standard care: circulating anticoagulants, anti-cardiolipin antibodies, and anti-β2-glycoprotein type 1. * Part performed specifically for the study (3.5 mL of additional blood): anti-phosphatidylserine and anti-phosphatidylethanolamine antibodies. The search for these antibodies will be performed using the 7mL dry tube collected for anti-cardiolipin and anti-β2-glycoprotein type 1 antibody testing. * If the diagnostic sample is positive for any of these antibodies, a follow-up at 3 months is recommended and will be performed as part of standard care to confirm the APS diagnosis. * Data collection will include patient details, stroke/TIA details, biological data, and follow-up. As part of routine follow-up, patients will be seen in a neurological consultation at 6 months. Clinical and biological data will be reviewed at the end of the study by two doctors (a neurologist and an internist) to confirm or exclude the APS diagnosis and its contribution to the neurological condition. An internal medicine follow-up will be initiated for patients with confirmed APS, and an appropriate treatment will be proposed.