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An example of auto-anti-Kpb in a Kp(a+b-) patient is described. The antibody present in the patient's serum and in eluates from her red cells was IgG. It did not bind complement, and did not cause in vivo hemolysis. 9 months after recognition of the autoimmune state the direct antiglobulin test had become negative and anti-Kpb was no longer detectable. It is postulated that autoimmunity involving the Kell blood group may be precipitated by antigens or enzymes of microbial origin.

Specificity of IgG autoantibodies that react with random red cell samples but not with cells treated with 2-aminoethylisothiouronium bromide (AET)-previously was related to the Kell blood group system. We now describe an IgG autoantibody, from a patient who died with autoimmune hemolytic anemia, that reacted with random red cell samples, Ko cells, and with cells treated with a dithiothreitol-papain solution, but did not react with AET-treated cells or with Rhnull cells. Serological reactivity of the autoantibody is unlike that of IgG autoantibodies that do not react only with Rhnull cells. It appears that the reactive antigen is not part of the Kell or Rh systems, but is probably modified by the membrane anomaly present in Rhnull red cells.

Anti-Sdx is an IgM complement-binding autoantibody that defines a red cell antigen which is independent of I, i, Sp1 (Pr) and Gd. Hemagglutination by the antibody is unusually sensitive to variation in pH, salt, or other charged molecular species. The antibody is inhibited by urine from Sd(a+) persons, but inhibition is a nonspecific effect caused by charged molecules. No specific Sdx substance could be demonstrated, and Sdx antigen does not appear to be directly associated with the Sid blood group. In view of these findings we propose that this antibody should be renamed anti-Rx.

A 19-year-old patient with a long history of idiopathic thrombocytopenic purpura developed a potent antibody against a high-incidence antigen in the Kell blood group system. The direct antiglobulin test on his red cells was negative. His cells exhibited profound depression of Kell blood group antigens, but antigens of other blood groups were normal. Transfusion of incompatible blood was well tolerated and differential agglutination tests, using selected Rh antisera, showed in vivo survival of the transfused red cells for more than 8 weeks. However, the transfused red cells also showed acquired loss of Kell antigens. Five months after the initial findings, Kell-related antibody disappeared and Kell antigens reappeared on his red cells. The patient's serum stored from the initial investigation now reacted with his freshly collected red cells. These data suggest that an environmental agent in the patient's plasma was responsible for the temporary loss of Kell antigens from red cells in his circulation.

An IgG autoantibody, named anti-Wj, reacts with all random and most selected red cell samples. It does not agglutinate cells of Lu(a-b-) type caused by the In(Lu) dominant inhibitor gene, but cells of recessive Lu(a-b-) type are reactive. These data establish that synthesis of the Wj antigen is suppressed by the In(Lu) gene, but it is not a part of the Lutheran blood group system. The Wj antigen is not well developed on fetal red cells. This example of auto anti-Wj is a non-complement-binding IgGl protein that did not appear to cause accelerated in vivo destruction of the patient's red cells.

A case of auto-immune hemolytic anemia caused by auto anti-K13 is described. Direct antiglobulin tests using monospecific reagents showed that IgG and the C3 component of complement were present on the patient's red cells. Eluted auto-antibody did not react with Ko or with K:--13 cells. Kell blood group antigenicity of the patient's red cells did not appear to be reduced.

Two examples of a "new" IgM saline-agglutinating auto-antibody are described. The antibodies bind complement, have the ability to cause in vivo hemolysis, and are most active at room temperature at a pH of about 6.5. Despite tests on more than 5,000 people, no nonreactive cell sample has been found. The reactive antigen is not denatured by neuraminidase, papain, or ficin, and is present on i adult red blood cells. The antibodies appear to be slightly inhibited by human saliva and milk, and more convincingly inhibited by urine from Sd(a+) persons. They are not inhibited by urine from Sd(a-) persons, but are strongly inhibited by guinea pig urine. The serologic characteristics indicate a relationship to the Sda blood group and the auto-antibody has been named antiSdx. Sdx antigen is present on red blood cells from some higher primates and is absent from rabbit, rhesus monkey, dog and sheep cells.

Approximately one in 250 people with autoimmunity involving their red cells have IgG autoantibodies with specificity in the Kell blood groups. Red cells of these individuals have an acquired temporary weakening of their Kell antigens. Some of the patients also have allo-anti-K in their serum. This report presents a case in which an IgG autoantibody may define a new high-incidence red cell antigen related to the Kell blood groups. The patient's Kell blood group antigens are depressed, and his serum contains allo-anti-K. It is postulated that reduced red cell Kell antigenicity is caused by enzymatic degradation, possibly of bacterial origin, and that the acquired loss of Kell antigens, the Kell-specific autoimmune state, and the serum all0-anti-K, are all related aspects of one phenomenon.

An 84-year-old woman with intestinal bleeding had marked reduction of red blood cell antigenicity in the Kell system, and a positive direct antiglobulin test caused by auto-anti-Kpb. KX antigen activity of her cell was increased, an observation which supports the belief that KX marks a precursor structure utilized in the normal Kell biosynthetic pathway. It is postulated that reduced Kell antigenicity was an acquired change that resulted from enzymatic degradation, possibly of bacterial origin.