Immune Reconstitution

Normal age-related T-cell alterations include a decline in CD4⁺ cells, loss of naive (antigenic virgin) cells, increase in memory (antigen experienced) cells, decline in T-cell proliferative responses, and narrowing of the T-cell receptor repertoire. The evolutionarily more primitive innate immune system consists of cells using germline genes to express receptors that recognize specific bacterial, viral, or otherwise foreign antigens. Some hagfish and lampreys have been reported to have lymphocytes and plasma cells despite absence of an identifiable thymus, spleen, or lymph nodes. These arise within lymphoid clusters in the pronephros (kidney), and intestinal lamina propria that may be phylogenetic precursors of a thymus and spleen, respectively. Cartilaginous fish (chondrichthyans), such as sharks are the first vertebrates to have adaptive immunity. They have demonstrable lymphocytes with TCR, MHC class I and II molecules, and Igs. Although lymph nodes are absent, the primary lymphoid organ or thymus appears for the first time in evolution with chondrichthyans. TRECs are extrachromosomal excision DNA segments, generated within the thymus during rearrangement of V, D, and J genes to generate TCR and TCR chains, or V and J genes to generate TCR and TCR chains.