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Avidity describes the binding strength of specific antibody to antigen. It was found to be low in the first phase after primary infection but then to increase over time. In addition to classic serodiagnosis of toxoplasmosis, measurement of avidity provides information making it possible to distinguish between acute and chronic infection and is of particular interest in pregnant women. In 229 women we found IgG and IgM antibodies suggestive of acute infection. A total of 188 (82%) had an avidity index0.3, indicating that the infection had occurred at least four months earlier. Based on this additional information, repeated testing and unnecessary anxiety can be avoided.

A monoclonal antibody with a high affinity for digoxin (KA = 5.5 x 10(10) M-1) and digitoxin KA = 5.0 x 10(10) M-1) was produced by somatic cell fusion. This antibody, designated 2A3, displayed little cross reactivity with other glucosides and no cross reactivity with endogenous steroids. It was shown that 2A3 is a suitable tool in an enzyme immunoassay for digoxin and digitoxin.

Alerted by a high incidence of positive autologous controls in pretransfusion compatibility testing for patients with acute leukemia, we retrospectively analysed 59 cases of severe hemoblastoses undergoing myelosuppressive treatment. Seventeen (29%) of these patients had a positive direct antiglobulin test (DAT) with the following characteristics: the test was of the IgG and/or C3d type in all but two cases, which reacted with polyvalent antiglobulin sera only; the reaction was very weak throughout; very high concentrations of monospecific sera were needed to elicit positive reactions; free antibodies were never detected in the patients' serum; the eluates of the patients' red cells were non-reactive against a very large panel of test erythrocytes. All these criteria suggested a non-immunologic absorption of proteins under the influence of drugs such as cephalosporins and clavulanic acid. Timenten (ticarcillin and clavulanic acid) was found to be the only common drug which had been administered to the majority of the patients with a positive DAT: 15 of the 17 patients with a positive DAT (88%) had received Timenten against 19 of 42 patients (45%) with negative DAT. 44% of all the patients receiving Timenten thus had a positive DAT, whereas only 8% of the patients without this drug reacted in this way. These data document the role of clavulanic acid in the development of a positive DAT. The literature and our experience show that this kind of unspecific binding of proteins to the red cell membrane is not associated with increased hemolysis. Knowledge of the phenomenon, however, is important for differential diagnosis of anemias and interpretation of difficulties arising during pretransfusional compatibility testing.

The importance of the receptor ligand system IL 2-receptor and IL 2 for the regulation of the T-cell proliferation was confirmed recently. In this review most important points are discussed as biochemical data, regulation of the expression, monoclonal antibodies, binding behaviour, affinity and activation events. Other activation antigens are also discussed in a short part.

A review is given about some aspects of the antigen-antibody interaction. Especially the specificity of antibodies to haptens and the binding forces are viewed. Some problems of the nature of antigenic determinants of globular proteins are discussed. Models of structure of antigenic determinants on such proteins are presented. The structural basis of antibody molecules, especially of the combining site are looked in review. Theories are presented, which are suitable to explain the high specificity and the great heterogeneity of antibodies. The concept of multispecificity of a single antibody molecule in relation to the high specificity of antisera is discussed.

In a LN-equivalence point titration system the modulation of immune complex formation of high avide antisera by specific and unspecific factors is measurable. Protamine-Cl is a potent enhancer of complex formation.

The dynamics of affinity of anti-DNP antibodies of carps immunized with T cell independent DNP-Ficoll was studied during the immune response. The intrinsic affinity (KO) of antibodies to monovalent epsilon-DNP-lysine is 10(5)--10(6)M-1 and does not change significantly in course of the immunisation. The affinity of tetrameric IgM antibodies to multivalent DNP-T4 conjugate is only 10(2)--10(3)-fold greater than to monovalent hapten, contrary to about 10(6)-fold higher values following immunisation with T cell-dependent antigens. The functional affinity (KF) increases during immune response slightly. KO and KF or antibodies are dependent on antigen dose. High antigen doses elicite antibodies with higher affinity.

Studies were performed to investigate the effect of chronic low level lead exposure on the regulatory functions of T cells in the humoral immune response to sheep red blood cells (SRBC) in mice. Female mice were exposed to lead (as lead acetate) in the diet at 545 (group 1) and 2180 ppm (group 2) for 10 weeks. Lead exposure resulting in blood lead levels (PbB) of about 50 micrograms/100 g (group 1) produced a substantial increase of the number of IgG antibodies secreting spleen cells on days 3 and 4 after challenge. At the higher exposure level (group 2; PbB 60-80 micrograms/100 g) a suppression of the number of IgG plaque forming cells was observed. The IgM response was much smaller than the IgG response. Although differences between the group means were small, the results indicate that there also is an enhancement of the IgM response in the lower dosage group on days 3 and 4. In a second experiment the effect of in vivo lead exposure on antigenic competition was examined. Lead substantially reduced the effect of antigenic competition. Results of both experiments suggest that suppressor T cells rather than helper T cells may represent the primary target for lead. Throughout this study serum complement C3 levels were determined. Complement C3 levels tended to be reduced in the lead exposed groups before as well as after inocculation with SRBC.

Carp anti-DNP antibodies were raised by various DNP-carrier conjugates. Their intrinsic affinity (K0) to monovalent E-DNP-L-lysine and functional affinity (DF) for binding to the multivalent DNP-T4 bacteriophages were determined. The functional affinity of antibodies elicited by T-cell-dependent DMPn-HSA (n = 3, 15, 33) is relatively high (KF):1010-1012 M-1). These KF values increase more than K0 during the immune response. The functional affinity is dependent on the molar KNP: HSA ratio. The mediate coupled DMP15-HSA elicits antibodies with the highest functional affinity. Carps immunized with T-cell-independent DNP-conjugates synthesize antibodies which have similar K0 as the antibodies elicited with DNP-HSA. However, the KF-values are in the range from 107 to 1010 M-1 only. The KF of antibodies raised with DNP-BA are 103-104 fold, those of DNP-S III and DNP-Ficoll elicited are only 4 . 101 to 4,7 . 102 fold higher than their corresponding K0-values. This means that these antibodies are not very effective in binding the multivalent DNP-T4. Specifically purified antibodies have also such low functional affinities. These strong differences in the functional affinity of carp DNP-antibodies elicited by T-cell dependent and independent DNP-conjugates are discussed with regard to stimulation of different B-cell subpopulations.

The application of hybridoma technology to the preparation of monoclonal antibodies has had a strong stimulatory effect on many areas of virological research. This development has expanded our knowledge with respect to antigenic variation among viruses and detailed analysis of complex antigenic structures of virus proteins and has also led to the elucidation of the molecular foundations of pathogenesis and virulence in model systems. The practical importance of these results spans many important areas of applied virology such as the development of new viral vaccines, passive immunization and diagnosis of viral infections.