Jimmy has always been told that if you have chicken pox once, you will not get the disease again. He had chicken pox when he was eight, but he only had five to ten little pox blisters on his skin. As an adult, Jimmy is again showing symptoms of chicken pox. Using your knowledge of the human immune system, explain how this is possible.

In immunology two types of immune response are described depending on whether there is prior contact with the antigen.

• At this first contact, only lymphocytes with receptors specific to the antigen in question are stimulated and lead to the production of antibodies capable of neutralizing it. The lymphocytes capable of recognizing a specific antigen are very few and the production of antibodies - of low intensity and limited duration - is detectable only after a period of latency of several days. This is called the primary response. The specificity of the resulting seropositivity, however, makes it possible to detect contamination (seroconversion).

• Upon re-contact with an antigen that has already triggered a primary response, specific antibody production is found to increase rapidly after a short latency period. The secretion of IgG then reaches levels much higher than those observed during the primary response, while that of IgM is of the same order. This early and intense response, called secondary response, most often results in the destruction of infectious agents before any clinical sign.

• Many centuries before the discovery of microbes, it was noted that people who developed a contagious disease without death were then specifically immunized against the disease, without being protected against others.

• The ability to react appropriately to an already received stimulus supposes the existence of a memory. The immune memory is based on the existence of memory B and T lymphocytes, resulting from the clonal expansion due to the first contact with the antigen. Their lifespan is significantly higher than that of other lymphocytes and their high reactivity gives its rapid and intense character to the secondary response.

The first time Jimmy was exposed to the chicken pox he did not get enough of the virus into his body for the immune system to know what it was. when he was exposed again as an adult his body was unable to fight it off and he got the virus again. the other possibility is that his body is unable to fight the virus and he may never be immune to it due to an auto immune disease.