Final answer:
Memory cells are antigen-specific B or T lymphocytes that rapidly respond to a pathogen upon re-exposure, providing faster immune responses due to the body's 'memory' of the pathogen. They are a key component of the immune system and essential for long-term immunity and the effectiveness of vaccinations.
Step-by-step explanation:
The subject of this question is related to the concept of memory cells within the immune system. A memory cell is an antigen-specific B or T lymphocyte that does not differentiate into effector cells during the primary immune response. However, upon re-exposure to the same pathogen, these cells can immediately become effector cells. This allows for a quicker and more efficient response to an infection the body has encountered previously. Essentially, memory cells provide long-term immunity by 'remembering' the pathogen.
The activation and proliferation of specific lymphocyte clones through a polyclonal response during an initial exposure to an antigen ensure the body is prepared with a large number of specific lymphocytes to combat an infection. These memory cells are crucial for faster and more potent responses during subsequent exposure to the same antigen, forming the basis of how vaccinations protect the body against diseases.