Spontaneous macrophage-erythrocyte rosette formation. A species-specific phenomenon.

Alveolar macrophages of some species have been found to react specifically with several xenogeneic red cells. The reaction, which is detected by spontaneous rosette formation, has two characteristic features. (a) The spontaneous rosettes do not produce, in the absence of exogenous serum proteins, any demonstrable cell-bound immunoglobulins, or divalent ions. These properties distinguish this newly described phenomenon from those known macrophage reactions mediated by cytophilic antibodies or by complement. (b) In contrast to the non-specific reactions of macrophages with inert particles, the reaction described here has distinct specificity and appears to utilize a membrane-associated `receptor'-type mechanism. This receptor is not blocked by anti-globulin but is suppressed by anti-macrophage antibodies. Except for having a wider range of specificity, in some of its characteristics the receptor on macrophages appears to be analogous to the T-cell receptor for xenogeneic red cells. It is suggested that these receptors may play a role in macrophage—T cell co-operation, and in the immune response in general.