The acceleration of population aging and the improved management of comorbidities lead to an increase in the number of multi-pathological elderly patients admitted to geriatric services. This is an extremely specific population in terms of immunology due to the presence of immunosenescence (physiological aging of the immune system) and the potential additional effect of multiple chronic pathologies (comorbidities) presented by these patients.

The specific effect of multiple chronic pathologies on the immune status of elderly patients is currently underexplored because most studies on immune system aging only include elderly patients who do not have comorbidities or treatments that could impact the immune system (e.g. SENIEUR protocol). Therefore, their results are not applicable to patients in geriatric services.

The decrease in monocyte expression of HLA-DR (mHLA-DR) has been studied as a marker of immune alteration in intensive care units in the context of sepsis, but also in chronic pathologies (e.g. cancer, diabetes, renal insufficiency). These chronic pathologies are frequently found in elderly subjects.

The objective of the study would therefore be to investigate the relative contribution of age (immunosenescence) and comorbidities to the alteration of the immune status of geriatric patients through the measurement of mHLA-DR.

To date, no study has explored mHLA-DR in a population of multi-pathological geriatric patients. Our work aims to generate initial data through a pilot study in hospitalized multi-pathological subjects over 75 years old in geriatric services.

Inclusion Criteria:

For all patients:

• Patients aged over 75 hospitalised in one of the geriatric units defined as the research sites;
• Patient affiliated to a social security scheme;
• Patients or relatives who have been informed and have no objections to the research.

To be eligible in the acute situation sub-population:

- Patients presenting with an acute episode on admission or following a stay in the research sites; The acute episode is defined, for example, by an infection, cardiac decompensation, a complicated fall, acute renal failure, any acute or decompensated cardiovascular pathology, any discovery of cancer that has not been stabilised.

To be eligible in the stable situation sub-population:

- A patient who has not had an acute episode for at least 7 days.

Exclusion Criteria:

For all patients:

• Patients under legal protection: curatorship, guardianship, safeguard of justice ;
• Patients under exclusive comfort care for whom it has been decided not to take a blood sample;
• Patients undergoing long-term antibiotic treatment, i.e. with no defined stopping date;
• An immunocompromised patient or a patient undergoing immunosuppressive treatment (for example: HIV infection, a patient awaiting solid organ transplantation, a patient undergoing haematopoietic stem cell transplantation, a patient undergoing chemotherapy for a solid tumour or haematological malignancy, a patient suffering from an auto-immune disease and being treated with immunotherapy and/or immunosuppressive therapy and/or biotherapy, an asplenic or hyposplenic patient, a patient undergoing experimental potentially immunosuppressive treatment, a patient undergoing Eculizumab treatment, a patient presenting a hereditary immune deficiency).

For patients in an acute clinical situation:

- Patient previously included in the same sub-population.

This study will focus on polypathological patients aged over 75 hospitalised in a geriatric ward.