What are Immunostimulants?
Immunostimulants are chemical compounds that stimulate the activity of white blood cells, thereby helping the host body develop better resistance to infections caused by viruses, bacteria, fungi, or parasites. In humans, immunostimulants can also help the body fight cancer by activating white blood cells, which are capable of recognizing and destroying cancer cells.
During the evolution of animals, their immune systems have developed mechanisms capable of recognizing and detecting chemical structures considered characteristic of potential microbial threats, and using these chemical structures as "alarm signals" to activate the body into a state of readiness to combat pathogen invasion.
In the presence of these chemical signals, the organism's immune system will respond as if it were being attacked by the pathogen itself.
Therefore, using immunostimulants before the body contracts a disease can help strengthen the body's protective barriers, thereby providing the organism with protection to avoid severe infection episodes or mortality due to pathogen invasion.
Chemical Nature of Immunostimulants
Most immunostimulants are chemical compounds that exist as structural components of bacteria, filamentous fungi, and yeasts. However, there are also some synthetic compounds in pure form, initially prepared for other purposes but incidentally found to possess immune-stimulating properties.
Immunostimulants are classified into the following groups:
- Structural components of bacteria: Lipopolysaccharides (LPS), Lipopeptides, capsular glycoprotein, and muramylpeptides;
- Various beta-1,3-glucan products from bacteria and filamentous fungi;
- Beta-1,3/1,6-glucan from yeast cell walls;
- Complex carbohydrate structures (glycans) from various biological sources, including seaweed;
- Nucleotides
- Synthetic products (Bestatin, muramylpeptides, FK-156, FK-565, Levamisole).
General Characteristics of Substances Capable of Activating and Modulating the Immune System
1. They are structural components of microorganisms.
2. They are detected by the surveillance components of the immune system present in all animal groups; – white blood cells present on tissue surfaces.
Beta-glucans
Found in filamentous fungi and yeasts, they differ from bacterial-derived immunostimulants in chemical structure and mode of action.
Beta-1,3/1,6-glucans are precisely defined chemically, and their mode of action on the immune system is highly specific, having been described in great detail, even at the molecular and cellular levels.
Furthermore, beta-1,3-glucan has been shown to improve the health, growth, and general characteristics of various animal groups, including farmed shrimp, fish, and terrestrial animals.
When to Use Immunostimulants?
In aquaculture, if used, it is recommended to apply them before:
Carrying out activities known to cause stress or have adverse effects on the health of aquatic animals. E.g.: seining/harvesting fish, shrimp; changes in environmental temperature; training shrimp/fish larvae to use artificial feed.
Known increased disease exposure levels (Transition from dry to rainy season; high-density fish stocking).
Developmental stages of shrimp/fish considered susceptible to pathogens (Larval stages of shrimp/fish; sexual maturation stage).
Immunostimulants can have synergistic effects with antibiotics, thereby enhancing the therapeutic efficacy of antibiotics. However, the primary characteristic of immunostimulants is disease prevention (not curative), and they are used to boost the body's general disease resistance, thereby reducing the risk of illness.
If used during the peak development stage of a disease, it might be perceived by the immune system as “another pathogen” alongside the existing one, and thus could exacerbate disease symptoms, at least for a short period.
Benefits of Using Immunostimulants
Today, immunostimulants are being used in both aquaculture and traditional animal husbandry with the aim of reducing mortality rates in farmed animals due to infectious diseases and improving their overall health.
a. Reducing mortality from opportunistic pathogens
Opportunistic pathogens only cause disease and mortality in hosts when the host is weakened or stressed, such as when environmental conditions deteriorate. These opportunistic pathogens can also reduce host health even when environmental conditions are good, feed is adequately supplied, and the host shows no signs of illness. Immunostimulants can enhance the host's defense mechanisms against opportunistic pathogens, thereby improving health, growth, and reducing host mortality throughout the culture cycle.
b. Preventing viral diseases
Developing vaccines for viral diseases is often time-consuming and costly. Furthermore, formulating vaccines capable of combating various viral pathogens across different animal species is not very practical. Therefore, one effective strategy to reduce the risk of viral diseases is a combination of good management and good feed, along with the use of immunostimulants to enhance the general disease resistance of farmed animals.
c. Enhancing disease resistance in farmed shrimp
Shrimp and other invertebrates have less developed immune systems compared to fish and warm-blooded animals; for example, they lack specialized white blood cells that, in higher animals, are involved in antibody production and immune memory (lymphocytes). Shrimp resistance to pathogens is primarily achieved through non-specific immune response mechanisms. Immunostimulants can stimulate these non-specific immune response mechanisms in shrimp, thereby helping them achieve better disease resistance. Therefore, the use of immunostimulants is an important tool in shrimp health management in commercial shrimp farms.
d. Reducing fry mortality
Fish, when in larval or juvenile stages, are highly susceptible to infectious diseases, leading to mass mortality of fry in hatcheries when attacked by opportunistic pathogens (Ellis, 1988). Young fish do not yet have a fully developed specific immune system, so to combat pathogens, they rely on non-specific cellular immune response mechanisms (Trust, 1986). Therefore, the use of immunostimulants to reduce the mortality rate of fingerlings and fry in fish nurseries is another important application of immunostimulants.
e. Enhancing vaccine efficacy
Immunostimulants are used as adjuvants in vaccines to activate antigen-presenting cells (e.g., macrophages) and stimulate these cells to produce more cytokines, which activate lymphocyte groups (B cells in warm-blooded animals) to produce specific antibodies.
Beta-1,3/1,6-glucan has been found to act as a true adjuvant, enhancing antibody production not only when co-injected with antigens in vaccines but also proving effective when administered independently by mixing with feed, with vaccine antigens provided via injection.
Source: academia.edu
