Which of the following substances is necessary for the absorption of vitamin B12?

A. Peristalsis, the wave-like contraction of the digestive tract, is typically decreased during sympathetic stimulation. The sympathetic nervous system tends to slow digestive processes as the body prioritizes "fight-or-flight" responses.

B. Bladder tone is typically decreased by sympathetic stimulation. The sympathetic nervous system causes relaxation of the bladder, inhibiting urination during stressful situations.

C. Heart rate increases with sympathetic stimulation. The sympathetic nervous system prepares the body for action by increasing heart rate, enhancing blood flow to muscles, and increasing energy levels.

D. Saliva production decreases with sympathetic stimulation. The body reduces non-essential functions such as salivation during stress, focusing energy on immediate survival functions.

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.