There are 25 million to a billion different T-cells in your body. Each cell has a unique T-cell receptor that can fit with only one kind of antigen, like a lock that can fit with only one shape of key. Antigens and receptors work a lot like a lock and key. Each of the 25 million to a billion different T-cells fit with a different shape of antigen.
Motivating operation. Motivating operation (MO) is a behavioristic concept introduced by Jack Michael in 1982. It is used to explain variations in the effects in the consequences of behavior. Most importantly, a MO affects how strongly the person is reinforced or punished by the consequences of their behavior.
In music, the subdominant is the technical name for the fourth tonal degree of the diatonic scale. It is so called because it is the same distance  below  the tonic as the dominant is above the tonic-in other words, the tonic is the dominant of the subdominant. It also happens to be the note immediately below the dominant. It is sung as fa in solfege.