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An earlier report that there is a high incidence (50 to 60 per cent) of cytotoxic antilymphocyte antibody in the sera of parous ewes is confirmed. This antibody is directed against paternally inherited antigens of the fetus. Antibody could be detected pre partum in four out of 34 primiparous ewes. However, there was no strong association between parity and the incidence of antibody. Antibody was also found in three out of 10 older rams which implies that pregnancy is not the only stimulus for antibody production.

Bovine alloreactive cytotoxic lymphocyte (CTL) lines of known target specificity were infected in vitro with sporozoites of Theileria annulata and T parva and cultured in limiting dilution. The phenotypes of the CTL lines both pre- and post infection were assessed using a panel of monoclonal antibodies specific for defined bovine lymphocyte subpopulations. The effector function of the resultant infected cell lines was determined using a Cr51 release assay and compared to the uninfected control CTL line. The results indicated that T parva sporozoites consistently infected and transformed the CTL lines very efficiently even at the lowest cell doses. In contrast the T annulata sporozoites were largely unable to infect and transform the alloreactive CTL except at the very highest cell and sporozoite doses. A factor which appeared to influence susceptibility to T annulata infection was an increased level of class II expression on the CTL line. None of the cell lines showed cytotoxic effector function after infection with either T annulata or T parva sporozoites.

Using monospecific sheep antisera, the mean plasma immunoglobulin concentrations in 82 Ayrshire milking cows were found, by the single radial diffusion method, to be: IgG2 10-91 plus or minus 4-30, IgG1 9-76 plus or minus 2-59 and IgM 2-71 plus or minus 1-22 mg/ml (plus or minus SD). IgA levels in the same group of 82 cows were determined by a simple, sensitive coated tube radioimmunoassay and had a mean value of 0-32 plus or minus 0-16 mg/ml. The frequency distributions of IgG1, IgM and IgA were approximately log normal, whereas that of IgG2 was bimodal with two populations with means of approximately 7 and 14 mg/ml. An 18-fold difference was found between high and low IgG2 levels whereas only three- to four-fold differences were recorded for IgG1 levels in the same population. Eight-fold differences were found for IgM and IgA levels. The effect of age (2-9 years) on sampling could be disregarded for IgG1, IgG, IgM and IgA levels, but cows above 6 years of age had significantly higher IgG2 levels.

Purified populations of bovine antigen presenting cells (APCs) and T cells have been isolated from peripheral blood and characterised using various monoclonal antibodies (mAbs) for cell surface markers. Bovine APCs were found in an adherent cell fraction and were non-specific esterase positive, phagocytic and expressed bovine major histocompatibility complex (MHC) class II determinants, all of which are typical macrophage characteristics. T cells were rigorously depleted of accessory cell function before being used in an antigen presenting cell assay. The generation of T helper cells in response to the soluble antigen, ovalbumin, was entirely dependent upon a critical number of APCs. Further the proliferative response was inhibited by several mAbs to bovine MHC class II molecules. Thus the interaction between bovine APCs and helper/inducer T lymphocytes (TH/I) appears to be similar to that in other species.