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A lymphocyte is a type of white blood cell (leukocyte) in the vertebrate immune system. Lymphocytes play an important and integral role in the body's defenses, and are part of a system of extraordinary complexity and coordination. There are two main types of lymphocytes, T cells and B cells, which function as part of the adaptive immune system. Other lymphocyte-like cells are commonly known as natural killer cells, or NK cells, and are part of the innate immune system. The NK cells are sometimes labeled "large granular lymphocytes," while the T cells and B cells are labeled as "small lymphocytes."

Pop music is a common type of contemporary popular music (distinguished from classical or art music and from folk music). The term does not refer specifically to a single genre or sound, and its meaning is different depending on the time and place. Within popular music, "pop music" is often distinguished from other subgenres by stylistic traits such as a danceable rhythm or beat, simple melodies and a repeating structure. Pop song lyrics are often emotional, relating to love or dancing.