Immunofluorescent studies using live cells from antibody-forming organs and anti-immunoglobulin antibodies demonstrate two populations of small lymphocytes which are differentiated by the presence or absence of Ig on their surface membranes. Most of the lymphocytes with detectable surface Ig appear to derive from cells of the bone marrow, while most of the Ig-negative lymphocytes derive from the thymus. Thus, adult mice thymectomized, lethally irradiated, and transplanted with bone marrow cells showed a normal number of lymphocytes with surface Ig but were depleted of the Ig-negative lymphocytes. Injection of thymocytes into these mice did not result in an increase in the number of lymphocytes with surface Ig in spleen and lymph nodes. Most of the injected thymocytes could be identified by means of histocompatibility markers. Also, the spleen and lymph nodes of neonatally thymectomized mice contained lymphocytes with surface Ig but were depleted of the Ig-negative lymphocytes. Attempts were made to identify light chains on thymocytes by a sensitive radioimmunoassay. In some experiments no light chains were detected and in others a small amount, i.e. no more than 2.6% of the amount present on spleen lymphocytes, could be detected. Whether these low figures are significant or represent a small amount of serum contamination is not clear as yet.

Cooperation between two or more cell types is an essential feature of the immune response to some antigens (1). These cell types are commonly referred to as thymus derived (T) and bone marrow derived (B). Furthermore, in the immune response to a hapten bound to a carrier protein these two cell types perform quite separate and distinct cooperative functions (2-5). The T cell is responsible for recognition of the carrier and has been termed the helper cell (5, 6) ; the B cell synthesizes antibody to the hapten.In another series of observations lipopolysaccharide (LPS), endotoxin, from Gram-negative bacteria was shown to alter thymic-deficient mice so that they could respond with antibody formation to thymic-dependent antigens. Mice that were thymectomized, lethally irradiated, and bone marrow reconstituted (TxB) responded to heterologous red cells coated with LPS (7). Also, lethally irradiated, bone marrow reconstituted mice responded to injections of sheep red blood cells (SRBC) when accompanied by LPS (8). LPS alone acted as a mitogen on B cells (9, 10). Thus, when antigen is present LPS might substitute for T cell function by direct stimulation of B cells which are then able to form antibody to the antigen resulting in an immune response.If a h a p t e n such as trinitrophenyl ( T N P ) were coupled to a n o n i m m u n o g e n i c carrier such as isologous mouse red blood cells ( M R B C ) , no i m m u n e response would be expected from the injected host because the T, or helper, cell would not recognize M R B C as a foreign carrier. However, if L P S did stimulate B cells directly, it m i g h t be hypothesized that, in the presence of L P S , an i m m u n e response to a hapten on a n o n i m m u n o g e n i c carrier could be induced. I n this paper we report such an effect of L P S on the i m m u n e response in B A L B / c mice to T N P coupled to isologous M R B C . Materials and MethodsMice.--BALB/c Dub female mice were used at 10-12 wk of age and obtained from Dublin Farms, Dublin, Va.Reagents.