No abstract

Natural 'antibodies' are substances found in the blood of animals that have not been immunised against infective agents. However, exposure to these agents or to cross-reacting antigens may well have taken place. Fish contain naturally-occurring, relatively nonspecific, lectin-like proteins or glycoproteins, which are distinct from immunoglobulins, and which react with a wide variety ofantigens and may confer some degree of immunity against natural infection. In most cases the cause of the antigenic stimulus is not obvious although the formation of these 'antibodies' may have been brought about by exposure to various micro-organisms. Many of these antibody-like molecules behave in a similar manner to immune antibodies or immunoglobulins and cross-react with specific carbohydrate moieties on the cell walls of bacteria, erythrocytes and certain other cellular antigens, due to the presence of similar antigenic determinants.It is difficult to ascribe an appropriatc dcfinition to the term 'natural antibody'. In fish, thesc 'antibodies' have been so designated on the basis of functional rather than structural criteria. Such naturally-occurring, low grade, antibody-like ' immune ' substances include 'acute phase' proteins, lysozyme and chitinase, interferon, agglutinins, lysins, complement and properdin, precipitins, and non-immunoglobulin, lectin-like molecules. In addition to the above non-immunoglobulin materials, natural immunoglobulins identifiable as IgM have also been reported in fish. Furthermore, mucus contains many biochemical agents capable of reaction against infective organisms and thus providing the host with an immediate or a first line of defence mechanism.This review compiles some of the relevant information in the literature concerned with natural 'immune' substances, present in the serum and mucus of fish, involved in protection against pathogens. Wherever possible the basic physicochemical properties of these substances are indicated and their potential immunobiological functions discussed.

The primary immune response of brown trout (Salmo trutta L.) was studied after injections of two cellular antigens-Salmonella typhi H, flagellar antigen d, and human group ' 0 ' Rh+ red blood cells. Both intraperitoneal and intramuscular injections were employed. Agglutinins and complement-fixing antibodies were produced to S. typhi and haemagglutinins to human ' 0 ' red blood cells. Maximum titres to S. typhi were reached after 49 days in the case of both agglutinins and complement-fixing antibody. Haemagglutinins reached a maximum value of 1 : 512 between 35 and 42 days. Haemagglutinins to human ' 0 ' red blood cells were detected as early as 7 days after injection. Antibodies against S. typhi were found after 14days. Natural haemolysins were present against horse, sheep and human groups ' A ', ' B ' and ' AB ' but not with group ' 0 '. No natural haemagglutinins were present to the six types of red blood cells tested. No precipitins were detected to either S. typhi or human ' 0 ' red blood cells by immunodiffusion.

Brown trout produced high molecular weight, thermostable, dithiothreitol sensitive, nonprecipitating, complement-fixing antibodies and agglutinins to lipopolysaccharides after intramuscular injection with adjuvant. Antibodies were first detected on Day 14 and reached maximum titres after 56 to 63 days when a single injection was given. When either a second or a third injection was administered maximum titres occurred 34 to 40 days after the injection. After each injection the titres increased significantly, and the protein concentration of the sera was significantly decreased. In cellulose acetate electrophoresis experiments those bands which migrated in thep-to y-globulin regions were increased.Antibody-secreting and antigen-binding cells were detected on Days 8 and 4 respectively and maxima were reached between Day 16 and Day 18. The number of cells per lo6 lymphoid cells was higher in the spleen than in the kidney.

The concentration of protein in the sera of rainbow trout, Salmo gairdneri, brown trout S. trutta and Atlantic salmon S. salar has been measured by six standard techniques viz refractometry, copper sulphate specific gravity, automated and manual biuret, optical density and Lowry et al. phenol reagent and the results compared. Good correlation was obtained in most cases and interconversion formulae are given between each method in the three salmonid species. The concentrations obtained with the refractometer and optical density methods were approximately one and a half times those obtained with the others.