Complex sentence: Memory T cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure; they live weeks to several years to a whole lifetime, which is very long compared to other leukocytes.
Split into simpler sentences: Memory T cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure. They live weeks to several years to a whole lifetime, which is very long compared to other leukocytes.