After an antigenic stimulus, antibody is first demonstrable in the cytoplasm, and often in a spot in the nucleus, of large, immature cells in the medullary areas of the lymph node draining the site of injection. Morphologically, these cells have basophilic cytoplasm and a large nucleus, and are typical hematogenous stem cells. As these cells multiply and differentiate, the concentration of antibody in their cytoplasm increases, until colonies of typical mature plasma cells containing antibody have developed.
There is a marked difference between the primary and the secondary responses, the former characterized by the development of very few antibody-containing cells while in the latter there are hundreds in a similar area. The morphology of the cells involved in both responses is identical.
Occasionally, antibody was also found in low concentration in association with the lymphoid follicles.
The implications of these findings for an understanding of antibody synthesis are discussed.