Rabbit antisera against normal guinea pig lymph node, when injected into guinea pigs, produced transient depression of the level of blood lymphocytes. It had no effect on other circulating cellular elements. Repeated injection over several days produced lymphopenia, which became progressively less marked with continued treatment, and clear-cut depletion of small lymphocytes in lymph nodes, whether draining an inoculation site or remote. In guinea pigs treated with lymphocyte antiserum, there was marked suppression of the tuberculin and contact allergic reactions and the "delayed" skin reaction to purified diphtheria toxoid, and a relative suppression of allergic encephalomyelitis and the rejection of first set skin homografts. There was a slight effect on second set graft rejection and no effect on PCA or the reversed passive Arthus reaction. Non-specific reactions to intradermal turpentine or to concentrated dinitrochlorobenzene placed on the skin were moderately reduced. The suppression of these reactions (except allergic encephalomyelitis) was closely correlated with the degree of lymphopenia. Lymphocyte antiserum absorbed with normal blood white cells lost both its lymphopenic effect and its ability to suppress the tuberculin reaction. It is tentatively concluded that a circulating mononuclear cell, probably the small lymphocyte, is the primary reactant in the various types of delayed hypersensitive reactions.

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