Opsonins are antibodies found in normal and immune sera which alter microorganisms and prepare them for more intensive phagocytosis. Under the influence of opsonins, a change takes place in the surface properties of microbial and other antigens in particular, in their electrical potential rendering them more easily susceptible to phagocytosis.
The phagocytic activity of leukocytes reveals itself better in the presence of immune serum than in normal serum. Early researchers explained this phenomenon by the presence of specific substances (stimulins) in immune sera. A. Wright, S. Douglas, and others confirmed the presence of substances stimulating phagocytosis in immune sera. These substances were named opsonins (from the Greek opsonein, meaning “to provide with food”) by A. Wright. Opsonins are inactivated upon heating at 56°C for 30 minutes, as they contain complement. F. Neufeld and V. Rimpau discovered thermostable substances in immune sera which do not contain complement and named them tropins (bacteriotropins).
Opsonins and bacteriotropins bring about specific sensitization and increased sensitivity of bacteria to phagocytosis. The thermostable fraction, which carries the function of antibodies reinforcing phagocytosis, exerts the main opsonizing action. Complement strengthens the action of bacteriotropins. The degree of opsonin activity is expressed as the opsonic index, which is the ratio of the phagocytic index of the immune serum to the phagocytic index of normal serum.
The phagocytic index is determined by dividing the number of microbes engulfed by 100 phagocytes by the number of phagocytes. This test consists of counting the phagocytized bacteria in 25 segmented neutrophils. The results of the reaction vary from 0 to 75. In a healthy individual, the index of phagocytic activity is 0 –5. Values of 10 –24 are considered weakly positive; values of 25–49 are clearly marked; and values of 50–75 are distinctly positive.
THE IMMOBILIZATION REACTION In diseases caused by pathogenic Treponema organisms, antibodies form in the blood of the sick patient, which arrest the movement of the microorganisms and cause their death. The immobilizing effect of the antibodies is accomplished in the presence of complement, although its fixation does not occur at all or is limited. The reaction of immobilization has been demonstrated not only in relation to Treponema organisms, but also in relation to other microorganisms where the participation of complement is not obligatory and does not always lead to the death of the microbes. THE
REACTION OF IMMUNE ADHESION There are antibodies in the presence of which microorganisms become adhesive. After treatment with such antibodies, microbes stick not only to one another but also to leukocytes, erythrocytes, blood platelets, yeasts, and other microorganisms. The reaction of immune adhesion is highly sensitive; it occurs with the participation of complement and promotes the phagocytic activity of leukocytes. THE
REACTION OF VIRUS NEUTRALIZATION The reaction of virus neutralization by means of viral antibodies is used to demonstrate the presence of a virus or antivirus antibodies in the blood of infected humans and animals. It is usually performed on animals or in tissue cultures. If the serum is specific, it will protect the animal against viral infection or protect the cells of the tissue culture against the cytopathic effect of the virus. THE
REACTION WITH FLUORESCENT ANTIBODIES A more effective method comprising the binding of antibodies (immunoglobulins) with a fluorescent dye fluorescein isothiocyanate (FITC), has been used recently in the diagnosis of infectious diseases. On illumination with ultraviolet rays, the labeled antibodies produce a bright apple-green/yellow fluorescence. The method of luminescent serodiagnosis may be direct, or indirect.The antigen interacts directly with antibodies previously labeled with fluorescent dyes and is first bound to an unlabeled primary antibody, and then this antigen-antibody complex is treated with a fluorescent anti-gamma-globulin.
THE AUTHOR IS AN ASSOCIATE PROFESSOR (RETD.) AND FORMER HEAD, DEPARTMENT OF BOTANY, ANANDA MOHAN COLLEGE.