1. In rabbits receiving a standard dose of bovine tubercle bacilli, a high pre-infection monocyte-lymphocyte ratio in the circulating blood has been correlated with the development of an acute fulminating tuberculous infection; but with an average or low index, the course of the disease has varied. Susceptibility has been further marked by the promptness with which the M/L index rose after infection and with its maintenance at a high level; resistance, on the other hand, has been evidenced by a continued low index. The M/L index is one measure of resistance to tuberculosis.
2. Some evidence has been presented to show that when monocytes, the forerunners of the epithelioid cell, are decreased through the action of an antiserum, a greater proportion of the animals survive into the chronic stage of the disease than of the controls.
3. The production of tubercular tissue in considerable quantity by injection into the tissues of the phosphatide or liquid saturated fatty acid from the tubercle bacillus does not render the animal allergic, and seems definitely to lower resistance to the disease upon subsequent infection with tuberculosis. The degree to which the tissues react specifically with the formation of new epithelioid cells is indicated by the amount of the change in the M/L index in the blood.
4. The differences observed in individual animals in the amount of tissue reaction to a given amount of phosphatide derived from the tubercle bacillus are definite and are similar to those long noted in connection with humoral antigen-antibody responses with proteins.
5. Both in tuberculosis itself and after intraperitoneal injections of the phosphatide, the relationship of monocyte to lymphocyte in the blood before death has been a measure of the extent of the epithelioid and lymphoid proliferation found at autopsy. Hence, the M/L ratio can be taken as an index of the relative abundance of these cells in the tissues.
6. Antigenic intravenous doses of the phosphatide itself, or of antiphosphatide serum, given either before or after infection, may give a slight protection to an animal if the dose of infecting organisms be not too great.
7. Taken together, the observations of the present paper implicate the monocyte and its derivative, the epithelioid cell, when harboring living bacilli, as factors in the spread of tuberculosis in the animal. The type of reaction of an animal to the lipoids of the tubercle bacillus, whether predominately cellular or humoral, may be a decisive factor in determining resistance on the one hand and susceptibility on the other.
