Macrophages engulf and digest foreign pathogens and leave behind parts of the pathogen called antigens that stimulate the immune system to attack them. T lymphocytes are a type of white blood cell capable of recognizing antigens that are not part of the human body (non-self) and attacking human cells that have been invaded by the pathogen associated with that non-self antigen. self. B cells, another type of white blood cell, also recognize foreign antigens and produce antibodies that attack and neutralize the foreign antigens to fight infection. When a new pathogen is first detected by the immune system, it can take up to several days for the immune system to properly coordinate all of the different immune cells needed to fight the infection (Understanding How Vaccines Work, 2013). However, once the infection is cleared, the immune system has the ability to memorize any foreign antigens that have entered the body. Memory T cells and memory B cells will remember all the antigens they have encountered throughout the human being's life and are able to react quickly and initiate the immune response if the same antigen is present. new detected. The immune system maintains a huge reserve of “millions or even billions of different antibodies to prepare for any foreign invaders by constantly creating millions of new B cells” (How Do Vaccines Work?, 2011). This process of antigen memory and antibody production is also called acquired immunity and forms the basis for understanding how vaccines