This could be due to a malfunction of her thymus.
The thymus is a soft triangular shaped organ that is situated in the mediastinum of the thoracic cavity. The thymus is the predominant donor of cells for the lymphatic system. The thymus performs an important role in the training and growth of T cells (thymus-derived cells) that are produced in the red bone marrow. The T cells defend the body by destroying infected or cancerous cells and harmful pathogens such as bacteria, viruses, and fungi. The thymus also helps in the proper growth of other organs within the immune system.