A protein subunit vaccine is a type of vaccine that uses harmless pieces of the target pathogen, specifically proteins, to stimulate an immune response. Unlike traditional vaccines that may use live or inactivated viruses, these vaccines focus on specific proteins that are essential for the pathogen's ability to cause disease. This approach helps the immune system recognize and remember the pathogen without the risk of infection. These vaccines are designed to trigger the body’s immune system to produce antibodies against the targeted proteins. When the actual pathogen enters the body, the immune system can quickly respond, providing protection against the disease. Examples of protein subunit vaccines include those developed for hepatitis B and human papillomavirus (HPV).