Lymphocytes
Lymphocytes enable the body to remember antigens and to distinguish self from harmful nonself (including viruses and bacteria). Lymphocytes circulate in the bloodstream and lymphatic system and move into tissues as needed.
The immune system can remember every antigen encountered because after an encounter, some lymphocytes develop into memory cells. These cells live a long time—for years or even decades. When these cells encounter an antigen for the second time, they recognize it immediately and respond quickly, vigorously, and specifically to that particular antigen. This specific immune response is the reason that people do not contract chickenpox or measles more than once and that vaccination can prevent certain disorders.