A Tale of Immune Fragility: Severe Mycoplasma pneumoniae Infection With Mixed Autoimmune Hemolytic Anemia in Selective IgM Deficiency Disorder.

Mycoplasma pneumoniae is widely recognized as a leading cause of community-acquired atypical pneumonia and can trigger a wide spectrum of extra-pulmonary manifestations. This report discusses a rare clinical presentation of a young female patient who was admitted with an acute-onset, severe febrile illness. Her initial symptoms included a persistent cough with minimal purulent expectoration and mild difficulty in breathing, which rapidly progressed to severe dyspnea at rest. Her condition deteriorated swiftly, resulting in acute hypoxemic respiratory failure that required emergent endotracheal intubation and mechanical ventilation. Initial diagnostic imaging revealed diffuse bilateral interstitial and alveolar infiltrates, suggestive of severe pneumonia. The initial comprehensive workup, which included routine blood analyses, blood and sputum cultures, and a viral respiratory panel, was negative for any specific pathogen. During the hospital stay, the patient developed new-onset moderate anemia. A detailed hematological investigation showed mixed autoimmune hemolytic anemia (AIHA). The presence of AIHA in a patient presenting with acute respiratory infection guided the diagnostic workup, ultimately identifying M. pneumoniae as the causative agent. Further immunological evaluation was done to explore this unusual presentation and revealed an underlying selective IgM deficiency with the presence of anti-La (SS-B) antibodies, suggesting a pre-existing state of immune dysregulation that may have predisposed her to these severe pulmonary and systemic manifestations. The management of this case was multifaceted. The patient received aggressive respiratory support along with targeted antimicrobial therapy, an extended course of azithromycin. After a prolonged hospitalization, she was successfully weaned from mechanical ventilation and eventually made a full clinical recovery. This report provides insight into the diverse clinical spectrum of M. pneumoniae infection and emphasizes the importance of maintaining a high index of suspicion in cases of unexplained AIHA. A clinical presentation of this severity in response to a common pathogen should prompt a thorough investigation for an underlying immunodeficiency state. This report is, to our knowledge, a rare clinical presentation that depicts the unique clinical triad of mixed AIHA, acute M. pneumoniae infection, and an underlying selective IgM deficiency.