What body systems are affected by down syndrome?

Multi-system. Heart, thyroid, eyes, iq are the most common to be affected. The child may have a slight problem or a significant impairment in one or several of those areas.
Noticed vs actual. With down syndrome, every cell in the body has an extra chromosome that can alter the cells ability to act/grow/develop normal. In complex areas like the heart, intestines, and brain this often results in major problems. (defects) in other complex areas like hands, the face and feet, this alters appearance or growth.In simple areas like skin it seems to have little effect, .
Most. Brain, spine, function, heart, motor skills, .
Down Syndrome. People with down syndrome may live a healthy life but they are at higher risk of a number of conditions., which include diabetes, thyroid problems, mood disorders, spinal deformities, eye problems, alzheimer's, heart problems among others. Doesn't mean every person with down syndrome will develop complications but it is definitely important that they get checked regularly.