During any sort of immune response, the B and T cells in our bodies begin to produce memory cells–these cells are clones. They hold information on the threats that the body has been exposed to, and if the body faces the same threat again in the future, the immune response is much faster, and stronger this time around considering the memory of the past incident.
A memory cell is specifically a B or a T cell that didn’t differentiate into an effector cell during the first exposure to an outside threat or pathogen. Because of this quality, when the memory cell is exposed to the same threat or pathogen again, they’re able to react quickly and become effector cells–the cells that are responsible for the immune response (plasma B cells or cytotoxic T cells). What usually occurs after an immune response is that B and T cells go through apoptosis–planned cellular death–but memory cells remain in circulation in the body.If the body doesn’t ever encounter the pathogen or threat again in its lifetime, the memory cells continue to circulate for long periods of time, and they will eventually die off.
The production of effector cells by the memory cells is known as the secondary immune response. The primary immune response is much slower because it takes time for the B and T cells to recognize the intruding antigen and to react properly as well. On the other hand, the memory cells hold these appropriate responses and react incredibly quickly. So quickly in fact that individuals such as you and I, wouldn’t even know that we had become infected or exposed to pathogens.
Vaccines actually create the memory cell response to much more harmful, or even rare pathogens such as hepatitis or the human papilloma virus. The vaccination that we receive is not seen as an infection, but it’s still able to produce memory cells. Since exposure to these pathogens can create resistances, repeated vaccinations are sometimes needed.
Boundless (2016). Boundless Biology. Retrieved from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-immune-system-42/adaptive-immune-response-234/immunological-memory-878-12128/