What might happen to a person born without a thymus gland?
The thymus gland is a small organ in the neck which produces thymosin, which stimulates the production of T-cells for the immune system. The thymus is only active until puberty, when it begins to shrink and become replaced by fat. Though the thymus gland isn't active for a whole lifetime, it has a very important job while it is active. If someone were born without a thymus gland, they would not have the capability to produce thymosin or T-cells. This would leave someone at much greater risk of infection, as there would be none of these specialized white blood cells helping to defend the body. This is described as an immunodeficiency, and means that someone may be more likely to catch infections and have a harder time fighting off and surviving the infection.
A thymus transplant is possible, granted there is a tissue match between donor and recipient.