A study was made of the cellular origin of human immunoglobulins (γ2, γ1M, γ1A). The results indicated that two closely related families of cells form immunoglobulins in human lymphoid tissue: germinal (reticular) centers and plasma cells. Thus their cellular origin in addition to their known antigenic relations further justifies placing the immunoglobulins in one family of proteins.

Immunoglobulins were also formed to a small extent in primitive reticular cells which resembled those of germinal centers but were separated from them. Possibly such cells were undergoing transition to the much more numerous plasma cells with which they were commonly associated.

The mantles of small lymphocytes which surrounded germinal centers did not contain detectable quantities of immunoglobulins.

While in general only one type of immunoglobulin was present in an individual cell or germinal center, γ2- and γ1M-globulin were identified on occasion in the same plasma cell and germinal center.

A peculiarity of the fetal thymus gland was the presence of immunoglobulin, mainly γ1M, in a small number of cells of small and intermediate size and primitive reticular appearance and in Hassall's corpuscles.

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