What does the term 'memory' of the immune system mean?

Explanation

Acquired immunity is pathogen specific and is characterised by memory. This means that when our body encounters a pathogen for first time our body produces a response called primary response which is of low intensity. At the same time memory B and T-cells are produced, which are stored in the lymph nodes and spleen throughout the life. If the same pathogen again enters memory B and T-cells will immediately produce effector B and T-cells. This is called as secondary response.