When does the immune response begin?

The immune response begins when a pathogen invades the body. This initial encounter triggers the body's defense mechanisms to recognize and combat the foreign invaders. The immune system detects harmful microorganisms or pathogens, such as bacteria or viruses, and activates a series of responses involving various immune cells, antibodies, and signaling molecules to eliminate the threat and protect the body from illness.

The other choices provide relevant information about aspects of the immune system but do not pinpoint the exact moment the response begins. For example, vaccination is a proactive method to prepare the immune system for potential infections but does not mark the start of an immune response in the context of an actual invasion. A fever can occur as part of the immune response, indicating that infection is present, but it does not initiate the response itself. Exposure to antigens is related to the immune process, as antigens stimulate the response, but the action begins with the pathogen invading the body.