Which of the following is a normal skin change associated with aging?

A. T-helper cells (CD4+ cells) play a crucial role in initiating and coordinating the immune response. They stimulate the activation of B-cells to produce antibodies and help activate cytotoxic T-cells to target infected cells.

B. The first line of defense against infection is provided by physical barriers like the skin, mucous membranes, and innate immune responses, not by T-helper cells.

C. While T-helper cells can activate macrophages indirectly by releasing cytokines, their primary role is in helping to activate other immune cells like B-cells and cytotoxic T-cells, rather than directly activating macrophages.

D. T-helper cells are not directly involved in minimizing cell damage caused by T-cells. That role is more closely related to regulatory T-cells (Tregs) that help maintain immune tolerance and prevent excessive immune responses.

A. Magnesium plays a role in many enzymatic reactions and muscle function, but it is not essential for thyroid hormone production.

B. Iodine is an essential element for normal thyroid function. The thyroid gland uses iodine to produce thyroid hormones (T3 and T4), which are critical for regulating metabolism. This is the correct element necessary for thyroid function.

C. Sodium is important for maintaining fluid balance and nerve function, but it is not directly involved in thyroid hormone production.

D. Calcium is important for bone health and cellular processes, but it does not directly influence thyroid hormone synthesis.