Newer knowledge of the immunology of bone and cartilage.

By the use of modern techniques, the nature of the immunological response to bone and cartilage grafts is becoming clear. Fresh bone, whether cancellous or cortical, will elicit a cell-mediated immunological response; removal of the bone marrow has little effect in reducing immunogenicity. Antibodies against cellular components of the graft are detectable in the recipient only when host and donor have a disparity for the major histocompatibility (H) antigen. Treatment of bone grafts, for the bone bank, by freezing removes their immunogenicity with regard to antibody production but leaves them capable of stimulating the cellmediated immune response. Freeze drying, on the other hand, impairs immunogenicity for both types of responses. Cartilage, grafted alone, is probably non-antigenic as far as both immune responses are concerned and, although there have been a few reports of stimulation of CMI and antibody production by cartilage, these have not been confirmed. Cartilage cells do, however, possess antigens of the major H-antigen system. The cartilage graft is therefore antigenic but only feebly immunogenic, as the matrix proteoglycans protect the cells from the afferent arm of the immune response. Osteoarticular allografts, consisting of both bone and cartilage, sensitize the host due to their bone components. The effect of the immune response upon the bone allograft is to destroy the graft-derived first phase of osteogenesis which, in turn, leads to a poor or non-existent host phase of new bone formation in most allografts. The exact effector mechanism by means of which this destruction is brought about is not known. Bone grafts may be protected from the immune response by use of immunosuppressive measures. Cartilage enjoys a considerable measure of protection from immunological effectors by virtue of its matrix. If this breaks down then the cartilage can become permeable to antibodies. It is suggested that "lymphokines," produced by sensitized lymphocytes, may play some role in destroying the cartilage graft.