4.7 Chickenpox: adaptive immunity in action
Chickenpox infection demonstrates how effective the adaptive immune response can be in preventing reinfection with the chickenpox virus. Children who have suffered from chickenpox (Figure 8) and recovered are unlikely ever to develop it again, because they have circulating memory cells specifically programmed to recognise chickenpox viruses. These memory cells are unable to recognise any other pathogens, but they react swiftly and effectively if the chickenpox virus gets into the body again. These memory cells direct the more vigorous secondary adaptive immune response, which produces many new T cells and B cells programmed specifically to attack chickenpox viruses.
The secondary adaptive immune response is usually so effective that the person doesn’t become ill and may never know that they have been infected by chickenpox viruses for a second time. When the secondary response subsides, even more memory cells that recognise chickenpox viruses as their specific target are left in circulation, providing lifelong protection against this pathogen to almost everyone who suffered this disease as a child.
Chickenpox usually resolves without treatment and because it is extremely rare for someone to develop it a second time, many countries (including the UK) do not routinely vaccinate children against this virus unless they have a deficient immune system. But many other pathogens produce far more serious diseases during their first encounter because the infection develops faster than the primary adaptive immune response can react against it. In a later section, we explain how vaccination can enhance the secondary adaptive immune response and prevent an infection from developing.
But first we have to explain how leukocytes in the adaptive immune system recognise each type of pathogen so specifically.