congenital athymia congenital athymia congenital athymia is a rare immune disorder in which a child is born without a thymus – an organ that plays a critical role in helping the body learn to fight infections. children impacted by this disease typically die within the first two years of life and may have repeated, often life-threatening infections because they lack adequate working t cells (a kind of infection-fighting white blood cell). treatment in october 2021, the thymus tissue product rethymic was approved by u.s. food and drug administration (fda) as a medical therapy for the treatment of children with congenital athymia. it takes six months or longer to reconstitute the immune function in treated children.