*3.2.3. Cellular immunity*

The thyroid gland in Graves' disease is characterized by non-homogenous lymphocytic infiltration. Majority of the intrathyroidal lymphocytes are T lymphocytes. B-lymphocytes are much less common than Hashimoto's thyroiditis. Cytokine profile produced by intrathyroidal T lymphocytes suggested that both TH1 & TH2 cells are present in thyroid. Majority of the T-lymphocytes are of TH1 subtype.26,27

TH1 cells are mainly involved in delayed type of hypersensitivity reactions, and it produces the cytokines like TNF-B, IFN-Y, IL-2, IL-10 and IL-17. TH1 cells are implicated in the pathogenesis of organ specific autoimmune diseases that is mediated mainly by TNF-β subtype, which uniquely produces IL-17. TH 2 cells are mainly responsible for humoral immune responses and they produce cytokines like IL-4, IL-5, IL-6 and IL-13.TH1 cells may also induce antibody formation through secretion of IL-10 28. IgG1 subclass of antibody are selectively induced by TH1 cells. Most of the intrathyroidal T cells are of memory (CD4+, CD29+) subtype. Concentration of cytotoxic T cells (CD8+) are much less in Graves' disease patients than patients of Hashimoto's thyroiditis. So the functional role of T cells in Graves' disease is primarily a helper than a suppressor or cytotoxic role.
