The effect of the pineal body, thyroid gland, and thymus on the mast cell count in the thymus glands and lymph nodes of immunized rats has been studied as well as the correlations between the primary and secondary immune response and the above endocrine organs. The cndocrine glands were removed individually and in various combinations and the results obtained were subjected to detailed mathematical analysis. The extirpation of the pineal body lowered the thymic mast cell count, while thyroidectomy appeared to have no effect. Nevertheless, the inhibitory effect of the pineal body on the thyroid may be involved in the phenomenon. The increase in the mast cell count in the lymph nodes following immunization appeared to require the presence of all three endocrine glands. The thyroid appeared, however, to be of particular importance and its stimulatory effect seemed to be exerted indirectly through the pineal body and the thymus. The presence of the pineal body and thymus had a positive effect on the immune response in contrast to the thyroid's negative effect. In the secondary immune response, the thymus appeared to be inhibitory rather than stimulatory. Possible correlations of the phenomena observed are discussed and theoretical diagrams are presented concerning the correlations of the endocrine glands studied in respect of mast cell production.
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