Regulatory t cells
a. may function in preventing autoimmune reactions
b. decrease their activity as antigenic stimulus decreases
c. trigger antibody production and stimulate macrophages
d. release cytokines that increase the activity of cytotoxic t cells and activated b cells
e. secrete interferons

The correct answer is option A, that is, may function in preventing autoimmune reactions.  

Explanation:

The Tregs or the regulatory T cells refers to the T cells that exhibit a function in suppressing or regulating other cells in the immune system. They monitor the immune response to foreign and self particles and assist in preventing autoimmune disease. The Tregs generated by a normal thymus is known as natural. They are produced due to the proliferation of naive T cells external to the thymus.  

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when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.