An immunostimulant is a substance that enhances the immune system's response to infections and diseases. These agents can be derived from natural sources, such as plants or microorganisms, or can be synthetically produced. By boosting the activity of immune cells, immunostimulants help the body recognize and fight off pathogens more effectively. Common examples of immunostimulants include certain vaccines and compounds like interferons and cytokines. They are often used in medical treatments to improve the immune response in patients, especially those with weakened immune systems, such as individuals undergoing chemotherapy or those with HIV/AIDS.