6.2 Different stages of adaptive immune response

Steps of adaptive immune response follow a cascade orchestrated by the antibodies and the cells of the adaptive immune system.

Step 1 Capture and display of antigens

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Step 2 Recognition of antigen by lymphocytes

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Step 3 Activation of T lymphocytes

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Step 4 Activation of B lymphocytes

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Step 5 Production of memory cells

6.2.1 Capture and display of antigens

Dendritic cells present in the epithelial body surfaces and connective tissues are the major antigen presenting cells. They display the antigens to the CD4+ T cells to activate the antibody mediated (humoral) immune response and CD8+ T cells to activate cell mediated immune response. The antigen presenting cells contain a specialized structure over their surface called the MHC molecules that helps in the display of antigenic peptides to the cells of immune system (discussed in later chapters). Pathogens entering into lymph nodes and spleen are displayed by the antigen presenting cells to the B and T lymphocytes.

6.2.2 Recognition of antigen by lymphocytes

Lymphocyte specific for an antigen is activated upon encountering with antigen presenting cells loaded with an antigenic peptide. The concept of activation of lymphocyte is called clonal selection theory . The theory was put forth first by Neils Jerne and explained further by Burnet . This theory says that antigen specific clones of lymphocytes exist even before the exposure of antigens and a large number of clones are generated during lymphocytic maturation to diversify the recognition of microbial antigens.