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Having identified the discretely defined lymph nodes (#38) and the more ‘embedded’ mucosal associated lymphoid tissue (MALT: e.g. adenoids, tonsils) as specialised anatomical niches where ‘primary’ immune responses to a novel pathogen or vaccine develop, the next step is to look more closely at the key players, the immunologically naïve (or precursor) B cells and T cells (#18) that traffic selectively to those sites. The discussion focuses on the lymph nodes, as they are (for an experimentalist or pathologist) easy to remove and measure as single entities. The naive T cells that find their way to the nodes are the survivors of a selection process in the thymus that, though not perfect, functions to get rid of most autoreactive (#36) ‘entry candidates’ while at the same time ‘educating’ those that survive and progress to be responsive to peptide-induced changes in our MHC class I (CD8+ killers) or class II (CD4+ helpers) cell-surface glycoproteins (#29). A later essay will deal in more detail with how the thymus works but, for the moment, that can be set aside as we concentrate on understanding the complexities of a virus-specific immune response.