__________________________ circulating in the bloodstream give the body long term protection against many diseases.

Memory cells circulating in the bloodstream provide long-term protection against multiple diseases by retaining information from past infections. When the body encounters a pathogen, the immune system generates memory cells that specifically recognize that pathogen. If the same pathogen invades the body again in the future, these memory cells enable a rapid and efficient immune response, often neutralizing the threat before it can cause illness.

This phenomenon is foundational to the immune system's ability to provide lasting immunity, which is the basis for vaccination strategies. Vaccines work by introducing a harmless component of the pathogen or a modified version of the disease itself, stimulating the body's production of memory cells without causing the disease. Hence, memory cells play a critical role in how the immune system remembers and responds to previously encountered pathogens, ensuring robust protection against diseases over time.