Fish Immune System: Mechanism, Types & Lymphoid Organs

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Fish immune system is a complex network of cellular and humoral components that work together to defend his body against foreign invaders.

Fish have an immune system that is similar to that of other vertebrates, including mammals.

The immune system of fish is composed of several components, including the skin, scales, mucous membranes, and lymphoid organs.

The article provide you a detailed overview of fish immune system, specially how does fish immune system works with the help of its immune components.

Fish Immune System

What is Immune System?

Immune system is animal body’s innate or acquired defense mechanism that detects something unfamiliar to body, such as a virus bacteria, fungus, parasite and any other foreign components.

The immune system is constantly monitoring our bodies searching for any invader that it may not recognize as part of its own self.

Within this system there are different types of particular cellular or humoral components that recognize and destroy harmful substances before they can cause infection or illness.

Organs Involved in Fish Immune System

Lymphoid Organs of Fish
Source: ResearchGate

Skin

Fish skin is covered with a protective mucus layer that helps to prevent the entry of pathogens. Fish scales also provide physical protection from invasion.

Gills

Fish gills are lined with mucous membranes that help to filter pathogens and other foreign particles from the water the fish is breathing.

Spleen

The spleen is an organ that is responsible for filtering the blood and removing dead or damaged cells. It also produces white blood cells and antibodies that help to fight off infections.

Kidney

The kidney is an organ that helps to filter the blood and remove waste products. It also plays a role in maintaining the proper balance of electrolytes in the blood.

Thymus

The thymus is an organ that plays a role in the development of T-cells, which are important cells in the immune system.

Lymphoid Tissue

Fish also have lymphoid tissue, such as the head kidney and the gut-associated lymphoid tissue (GALT) which are responsible for producing immune cells and antibodies.

Before discussing the mechanism of fish immune system, get an idea about two types of immunity

Innate Immunity: Innate immunity includes physical and chemical barriers such as scales and mucus, as well as non-specific immune responses such as the activity of phagocytes and the complement system.

Adaptive immunity includes the production of antibodies by B cells and the development of memory cells.

The innate immune system and the acquired immune system are two different systems that work together to protect the body from pathogens and other harmful invaders.

The innate immune system is the first line of defense against pathogens and other harmful invaders. It is non-specific and always present, meaning it does not change or adapt in response to specific pathogens.

The innate immune system includes physical and chemical barriers, such as the skin and mucous membranes, as well as cellular components such as phagocytes, natural killer cells, and granulocytes, that work together to prevent infection and remove pathogens from the body.

The acquired immune system, also known as the adaptive immune system, is the second line of defense against pathogens. It is specific and develops over time, meaning it can adapt and change in response to specific pathogens.

The acquired immune system includes cells and molecules such as T cells, B cells, and antibodies that can recognize and respond to specific pathogens.
The innate immune system acts quickly, but it is not specific to a certain pathogen, meaning it doesn’t differentiate between pathogens and normal cells.

The acquired immune system takes longer to respond but it is specific to the pathogen it is recognizing, meaning it can distinguish between pathogens and normal cells.

Both systems work together to protect the body from pathogens, with the innate immune system providing a rapid response to invaders and the acquired immune system providing a targeted response to specific pathogens. The acquired immune system also has a memory, meaning it can remember pathogens it has encountered before and respond faster and more effectively next time.

Fish have a complex immune system that includes both innate and adaptive mechanisms.

Mechanisms of Fish Immune System

Fish Immune Defense and Immune Organs
Source: ResearchGate

Broadly fish body defense mechanism divided into non-specific and specific immune system.

Lets learn about these defenses!

Non-Specific Immune System of Fish

Nonspecific defense mechanisms provide a general protection against a wide range of pathogens.

This type of immunity does not require prior exposure to a pathogen and it is activated immediately when a pathogen is detected.

Physical barriers, such as skin and scales, mucous membranes, complement system are examples of nonspecific immunity.

In fish body, non-specific immune defenses act with two components; cellular and humoral component.

No-Specific Cellular Components

They destroy the cell through phagocytic activity

Have a look on phagocytosis process!

Phagocytosis Process

Phagocytosis is a process in which cells, such as white blood cells called phagocytes, engulf and destroy pathogens, foreign particles, and other unwanted materials.

It is an important mechanism of the immune system that helps to remove pathogens and other harmful substances from the body.

The process of phagocytosis can be divided into several steps:

Recognition

The phagocyte recognizes the pathogen or other foreign material through receptors on its surface. These receptors can bind to specific molecular patterns on the surface of pathogens, such as the complement protein C3b that binds to the surface of pathogens through the complement cascade.

Chemotaxis

Once the pathogen is recognized, the phagocyte moves towards it through a process called chemotaxis. Chemotaxis is the movement of a cell in response to a chemical signal.

Adhesion

The phagocyte adheres to the pathogen or other foreign material through the use of adhesion molecules on its surface.

Engulfment

The phagocyte surrounds the pathogen or other foreign material by extending its plasma membrane around it, forming a phagosome. The pathogen or other foreign material is now enclosed within the phagosome.

Digestion

The phagosome then fuses with lysosomes, which contain enzymes that can break down the pathogen or other foreign material. The pathogen is then digested and killed.

Elimination

The remnants of the pathogen are then eliminated from the cell and the organism.

Phagocytosis is a key mechanism of the innate immune system, it helps to remove pathogens and other harmful substances from the body. It is also important in the removal of dead cells and other debris from the body.

No-Specific Humoral Components

Fish possess a nonspecific humoral immunity system that includes various components such as complement, lysozyme, and bactericidal/permeability-increasing protein (BPI). These components act in a nonspecific manner by providing a general defense against a wide range of pathogens, rather than targeting specific pathogens.

Complement is a group of proteins that work together to enhance the activity of antibodies and phagocytes, helping to clear pathogens from the body. Lysozyme is an enzyme that breaks down the cell walls of bacteria, thereby killing them. BPI is a protein that binds to the cell walls of gram-negative bacteria and neutralizes lipopolysaccharides, which are toxic molecules that can cause inflammation and septic shock.

Overall, the nonspecific humoral immunity of fish provides a first line of defense against pathogens and helps to prevent infection.

Complement system: Fish also have a complement system, which is a group of proteins that work together to help fight off pathogens. The complement system helps to destroy pathogens by lysing their cells, and also helps to recruit other immune cells to the site of infection.

All of these mechanisms work together to provide a first line of defense against pathogens that may enter the fish’s body. They are rapidly activated and provide an immediate response to the presence of pathogens. These mechanisms are considered as the non-specific defense mechanisms, because they respond to a wide range of pathogens, not specific to one pathogen.

Complement System

The complement system is a group of proteins found in the blood and other body fluids of fish, as well as in other organisms, that work together to enhance the activity of antibodies and phagocytes, helping to clear pathogens from the body.

The complement system has several functions, including:

Opsonization

The complement system can coat pathogens with complement proteins, making them more easily recognized and phagocytosed (engulfed and destroyed) by white blood cells called phagocytes.

Chemotaxis

The complement system can attract phagocytes to the site of an infection, helping to increase the number of cells present to fight the pathogen.

Membrane Attack Complex (MAC)

The complement system can form a complex of proteins on the surface of certain pathogens that creates a hole in the membrane, causing the pathogen to lose its integrity and die.

Inflammation

The complement system can also promote inflammation, which is the body’s response to injury or infection. Inflammation helps to bring more white blood cells to the site of an infection and can also help to contain the infection by forming a physical barrier.

The complement system in fish operates in a similar way as in mammals, but it is less complex and less diverse. The major pathways of the complement system in fish are the classical, alternative, and lectin pathway.

Each pathway is activated by different triggers and leads to the activation of different complement proteins. The final common pathway in all three pathways lead to the formation of the membrane attack complex (MAC) which is the main mechanism of complement-mediated killing in fish.

Overall, the complement system is an important component of the nonspecific humoral immunity in fish and plays a crucial role in the defense against pathogens by enhancing the activity of antibodies and phagocytes, promoting inflammation, and killing pathogens.

The complement cascade is the sequence of reactions that occur when the complement system is activated. There are three main pathways that activate the complement cascade: the classical pathway, the alternative pathway, and the lectin pathway.

The classical pathway is activated by the binding of antibodies to pathogens. This pathway results in the activation of C1, C2, and C4, leading to the formation of the C3 convertase enzyme complex. This complex cleaves C3 into C3a and C3b. C3b binds to the surface of the pathogen, creating a C3 convertase enzyme complex that further cleaves C3 into C3a and C3b. C3b also binds to the surface of the pathogen and acts as an opsonin, making the pathogen more susceptible to phagocytosis.

The alternative pathway is activated in the absence of antibodies. In this pathway, the first component activated is C3. The activation of C3 leads to the formation of a C3 convertase enzyme complex. This complex cleaves C3 into C3a and C3b. The C3b molecule binds to the surface of the pathogen, creating a C3 convertase enzyme complex that further cleaves C3 into C3a and C3b. C3b also binds to the surface of the pathogen and acts as an opsonin, making the pathogen more susceptible to phagocytosis.

The lectin pathway is activated by the binding of mannose-binding lectin (MBL) or ficolins to pathogen. This pathway results in the activation of the MBL-associated serine protease (MASP) which cleaves C4 and C2 leading to the formation of the C3 convertase enzyme complex. This complex cleaves C3 into C3a and C3b. C3b binds to the surface of the pathogen, creating a C3 convertase enzyme complex that further cleaves C3 into C3a and C3b. C3b also binds to the surface of the pathogen and acts as an opsonin, making the pathogen more susceptible to phagocytosis.

All three pathways ultimately lead to the formation of the Membrane Attack Complex (MAC) which is the main mechanism of complement-mediated killing in fish. This complex creates a hole in the membrane of pathogens, causing it to lose its integrity and die.

It’s worth to mention that fish complement system is less complex and diverse than mammals, however, the three pathways mentioned above are well conserved in fish.

Lysozyme

Non-Specific vs Specific Immune System of Fish

In fish, as in other animals, the immune system can be divided into two main types: nonspecific immunity and specific immunity.

Both types of immunity are important for the protection of fish against pathogens.

Nonspecific immunity provides a rapid response to the presence of pathogens, while specific immunity provides a more targeted response that is specific to the pathogen.

Specific Immune System of Fish

Specific immunity refers to the defense mechanisms that are specific to a particular pathogen. This type of immunity is acquired after exposure to a pathogen, and it provides a more targeted response to the pathogen.

Specific immunity includes the production of antibodies, which are proteins that bind to specific pathogens and help to neutralize or remove them.

Fish have the ability to produce antibodies through the B-cells, which are cells that are found in the head kidney and other lymphoid organs.

Cellular

Cellular Components of Fish Immune System

Fish possess a variety of cellular components that make up their defense mechanism against pathogens and other harmful invaders.

Some of the main cellular components of fish defense mechanism include:

Phagocytes

These are white blood cells that engulf and destroy pathogens, foreign particles, and other unwanted materials. They include neutrophils, macrophages, and monocytes.

Natural Killer cells

These are a type of white blood cell that can recognize and kill cells that are infected with viruses or cancerous cells.

T cells

These cells play a crucial role in the adaptive immune response of fish, they include T helper cells, T cytotoxic cells, and T regulatory cells.

B cells

These cells produce antibodies that can bind to specific pathogens and mark them for destruction by other cells of the immune system.

Mast cells

These cells play an important role in the inflammatory response to infection and injury. They release histamine and other mediators that promote inflammation and attract other cells of the immune system to the site of infection.

Granulocytes

These are a type of white blood cell that includes neutrophils, eosinophils and basophils. They are characterized by the presence of granules in their cytoplasm that contain enzymes and other molecules that can destroy pathogens.

All these cells work together to protect the fish from infection and disease, by recognizing and responding to pathogens through various mechanisms such as phagocytosis, killing infected cells, and releasing mediators that promote inflammation and attract other cells of the immune system to the site of infection.

Humoral Immunity

Fish, like other vertebrates, have both humoral and cellular components of the immune system that work together to protect them from pathogens.

Humoral immunity: Humoral immunity refers to the production of antibodies that are produced by B-cells in the head kidney, spleen and other lymphoid organs. These antibodies bind to specific pathogens and help to neutralize or remove them. Antibodies can also activate the complement system which helps to enhance the destruction of pathogens.

Cellular immunity: Cellular immunity refers to the activity of immune cells, such as T-cells and macrophages, that directly attack and destroy pathogens. T-cells are important cells in the immune system that are responsible for recognizing and responding to specific pathogens. They are produced in the thymus, and they can be divided into two main types: CD4+ and CD8+ T-cells. CD4+ T-cells help to coordinate the immune response, while CD8+ T-cells directly attack and destroy infected cells. Macrophages are cells that are able to engulf and digest pathogens, and also present antigens to other immune cells.

Both humoral and cellular immunity play important roles in the protection of fish against pathogens. Humoral immunity provides a quick response to pathogens by producing antibodies, while cellular immunity provides a more targeted response through the activity of immune cells. The two types of immunity work together to provide a comprehensive protection against pathogens and maintain the overall health of the fish.

Fish have several first line defense mechanisms that work to protect them from harmful pathogens:

Physical barriers: Fish have physical barriers such as skin and scales that provide a barrier to protect the fish from harmful pathogens. The skin is also covered with a protective mucus layer that helps to prevent the entry of pathogens.

Mucous membranes: Fish also have mucous membranes in their gills, mouth, and other organs that help to filter pathogens and other foreign particles from the water they are breathing or the food they are eating.

Innate immune system: Fish also have a unique immune system component called the “innate immune system” which help them to identify and respond to pathogens quickly. The cells of the innate immune system, such as neutrophils, monocytes, macrophages, and natural killer cells, are able to recognize and respond to pathogens without prior exposure.

Stress Factors Which Influence Fish Immune Functions

Fish, like all living organisms, are subject to a variety of stress factors that can influence their immune functions.

Some of the main stress factors that can impact fish immune function include:

Temperature

Fish are ectothermic animals, meaning their body temperature is dependent on the environment.

Changes in water temperature can affect the activity of the immune system, making fish more susceptible to infection at higher or lower temperatures.

Salinity

Fish living in environments with changes in salinity, such as estuaries, can be exposed to different levels of salinity which can affect the function of the immune system.

Photoperiod

Photoperiod, which refers to the length of daylight hours, can be a stress factor that influences the fish immune system. In fish, changes in photoperiod can affect the function of the hypothalamic-pituitary-interrenal (HPI) axis, which is responsible for the regulation of the stress response.

Fish are sensitive to changes in photoperiod, and variations in the length of daylight hours can affect the timing of reproduction, feeding, and other physiological processes. For example, when fish are exposed to short photoperiods, it may disrupt the normal circadian rhythm and lead to stress, which can suppress the immune system.

Additionally, changes in photoperiod can also affect the levels of melatonin and cortisol, which are hormones that play a role in the regulation of the immune system. Short photoperiods can lead to an increase in melatonin levels and a decrease in cortisol levels, which can alter the immune response and make fish more susceptible to infection.

Moreover, changes in photoperiod can also affect the activity of phagocytes, the white blood cells that are responsible for engulfing and destroying pathogens. Studies have shown that the activity of phagocytes can be decreased in fish exposed to short photoperiods, which can make fish more susceptible to infection.

In summary, photoperiod can be a stress factor that influences the fish immune system, by disrupting the normal circadian rhythm, the HPI axis, the levels of melatonin and cortisol, and the activity of phagocytes.

Therefore, it is important to consider the effects of photoperiod on fish immune system and take appropriate measures to minimize their impact, such as maintaining consistent lighting conditions in aquaculture systems.

Pollution

Exposure to pollutants such as heavy metals, acid rain, pesticides, and organic compounds, polychlorinated biphenyls (PCBs) can disrupt the function of the immune system and make fish more susceptible to infection.

Nutrition

Proper nutrition is essential for the proper functioning of the immune system.

Deficiencies in essential nutrients such as vitamins and minerals can affect the function of the immune system and make fish more susceptible to infection.

Crowding

Crowding in fish farms and aquaculture can increase the risk of infection and disease transmission and lead to stress, which can suppress the immune system.

Transport and handling

The process of transporting fish from one location to another can be stressful and can lead to changes in the immune system, making fish more susceptible to infection.

Pathogens

Fish may be exposed to a variety of pathogens such as bacteria, viruses, and parasites, which can disrupt the function of the immune system and make fish more susceptible to infection.

Hierarchy

All these stress factors can disrupt the balance of the fish immune system and make fish more susceptible to infection.

Therefore, it is important to understand the effects of these stress factors on fish immune functions and take appropriate measures to minimize their impact.

Active Immunity Vs Passive Immunity

Active immunity is the process by which an individual’s immune system produces its own antibodies in response to exposure to a specific antigen, such as a virus or bacteria.

This type of immunity occurs when the body is exposed to a live pathogen, or through vaccination, where a dead or weakened form of the pathogen is introduced to the body.

Active immunity can also happen when the body is exposed to a substance that is similar to the pathogen, this is called “memory immune response”, which is responsible for the immunity that persists after the primary infection or vaccination.

Passive immunity, on the other hand, is the process by which an individual receives pre-formed antibodies from another source, such as through the transfer of maternal antibodies to a fetus or newborn, or through the administration of antibodies in the form of immunoglobulin (Ig) therapy.

Passive immunity provides immediate protection, but the effects are temporary because the antibodies will eventually be cleared from the body.

Active immunity is more preferable than passive immunity because it is more sustained and provides long-term protection. Passive immunity is more effective in emergency cases, when there is a need for fast and immediate protection.

FAQs

What is recognition of Non-Self in fish immunity?

Fish, like all living organisms, have mechanisms for recognizing and responding to non-self entities, such as pathogens.

One key mechanism for recognition of non-self in fish is through the use of pattern recognition receptors (PRRs).

These receptors can recognize and bind to specific molecular patterns found on the surface of pathogens, such as lipopolysaccharides (LPS) on the cell walls of gram-negative bacteria, or double-stranded RNA (dsRNA) found in some viruses.

Once a pathogen is recognized by a PRR, it triggers a cascade of immune responses including the activation of the complement system, the production of antibodies, and the activation of phagocytes.

References