Final answer:
Anti-glutamic acid decarboxylase antibodies are found in patients with autoimmune diseases such as type 1 diabetes and stiff person syndrome, leading to insulin deficiency and muscle stiffness and spasms, respectively.
Step-by-step explanation:
Anti-glutamic acid decarboxylase (anti-GAD) antibodies are found in patients with autoimmune diseases such as type 1 diabetes and stiff person syndrome.
These antibodies target the enzyme glutamic acid decarboxylase, leading to a decrease in its activity.
In type 1 diabetes, the destruction of pancreatic beta cells by the immune system results in insulin deficiency, while in stiff person syndrome, it causes muscle stiffness and spasms.