Immunological memory is the term used to describe how the adaptive immune system appears to remember all pathogens it has encountered previously, and changes behaviour as a consequence of earlier experience. It relates only to epitopes that have been encountered previously.
Small lymphocytes are T and B cells found circulating in the blood and lymphatic system and stationary in the lymph nodes, spleen and other lymphoid organs. Each has receptor molecules on its surface with a unique structure and specificity that binds to just one epitope shape. B cells mature in the bone marrow. Contact with antigen → clonal expansion → differentiation into memory B cells or plasma cells. T cells mature in thymus. Some have regulatory role, some involved in prolonging inflammation, and others kill infected host cells. All carry antigen receptors and undergo clonal selection, expansion, differentiation into memory cells and defensive cells.
Clonal selection. Each small lymphocyte has specific receptor molecules. Which bind to one particular epitope shape. Newborn animals have several billion small lymphocytes divided up into groups known as lymphocyte clones. Each clone consists of a few identical cells with identical antigen specificity. A particular antigen can only be bound to small lymphocytes with receptors for its epitopes. This is clonal selection. In effect the antigen selects the appropriate clone of small lymphocytes which has receptors that can bind to its epitopes.
An antigen is an organism, cell or macromolecule which the body can recognize as not being part of the body ie as non-self. The antigen has small clusters of molecules on its surface known as an epitope. In the blood there are several types of cell. One type are the leukocytes or white blood cells which can be further grouped into three lineages; the granulocytes, the monocytes and the lymphoid cells. The lymphoid cells circulate between the blood system and the lymphatic system and can be further broken down into large granular lymphocytes and small lymphocytes. The small lymphocytes are T cells and B cells. These cells have receptor molecules on their surface which bind to specific epitopes on specific antigens (antigen receptors). The ability of the small lymphocytes to recognize specific antigens by their epitopes is known as antigen specificity.
Newborns are born with several billion small lymphocytes divided into small groups with identical antigen specificity. These groups are known as lymphocyte clones. Each antigen can only be bound by the lymphocyte clones with the right antigen specificity, and this is termed clonal selection.
When an antigen is first encountered the body mounts a primary adaptive response. The specific lymphocyte clones bind to the antigen and an immune response is seen in the host. The response is slow (7-10 days) and takes two or three weeks to reach its peak. The response is slow as the few cells with the correct antigen specificity must divide repeatedly in order to provide enough cells with the correct receptor molecules to bind all of the antigen (clonal expansion). Some of the cells formed in this way remain after the antigen has been destroyed and differentiate into memory cells. These cells have the same epitopes as the original small lymphocytes and increase the amount of a particular clone in the body.
When an epitope which has been encountered before enters the body as part of an antigen, the higher number of specific clonal cells for that epitope are able to mount a faster response and the host may be able to prevent the ifection reaching the stage of visable symptoms. This is known as the secondary adaptive response.
The virus which causes cowpox is very similar in antigenic structure to the smallpox virus. When pus from the milkmaid was introduced into the blood stream of James Phipps, the virus causing cowpox was also introduced. James’ small lymphocytes recognized epitopes on the antigen as being non-self and a primary adaptive response occurred. As the response was slow it tool some time for his body to remove the antigen, hence symptoms of the illness were seen. On subsequently being infected with the smallpox virus a secondary response took place and the virus was removed from the body before allowing time for symptoms to develop.
vaccination induces a primary adaptive response and allows memory cells to develop, increasing the number of clones to a particular antigen. This means that subsequent infections can be controlled rapidly by the body without illness being seen.
