**Core Concept**
Immunosuppressants are a class of medications that modulate the immune system to prevent organ rejection in transplant patients or to treat autoimmune diseases. They work by inhibiting the activity of immune cells, such as T cells and B cells, or by reducing the production of cytokines.

**Why the Correct Answer is Right**
Immunosuppressants can be broadly categorized into several classes, including calcineurin inhibitors (e.g., cyclosporine, tacrolimus), mammalian target of rapamycin (mTOR) inhibitors (e.g., sirolimus), and antiproliferative agents (e.g., azathioprine). These medications are used to prevent rejection in transplant patients and to treat autoimmune diseases such as rheumatoid arthritis and lupus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it contradicts the established classification of immunosuppressants.
**Option B:** This option is incorrect because it is a known side effect of certain immunosuppressants, such as cyclosporine and tacrolimus.
**Option C:** This option is incorrect because it is a mechanism of action for some immunosuppressants, such as sirolimus.

**Clinical Pearl / High-Yield Fact**
When using immunosuppressants, it is essential to monitor patients for signs of infection, as these medications can increase the risk of opportunistic infections.

**Correct Answer: A.**