MCs localize in the paracortical and medullary area of lymph nodes in close contact with B cells. Double immunohistochemistry for mast cell tryptase and either CD20 (A-B) or CD3 (C) on lymph node samples with reactive follicular hyperplasia. (A-B) Mast cells (blue) are localized mainly in the parafollicular zone extending into the medullary area and showing tight spatial interactions with B cells (red). (C) In lymph nodes, MCs (blue) are likely to interact also with T cells (red) as the parafollicular area is rich in T lymphocytes. These pictures are illustrative of the interactions occurring between MCs and B cells in the context of T cell–rich areas of lymph nodes (STREPT-ABC and APAAP methods; original magnifications: panels A,C: ×100; panel B, ×200).