DIFFERENCES OF THE IMMUNE SYSTEM OF A VERTEBRATE AND INVERTEBRATE - BIOLOGY - 2025

The immune system of a vertebrate animal and an invertebrate has several differences, because each group has its own morphological anatomical characteristics. What immunologically distinguishes a vertebrate from an invertebrate is the type of defense system they use. Invertebrate animals have an innate immune system, made up of soluble and cellular components.

On the other hand, vertebrates are the only animals to present an acquired or adaptive immune system, composed of antibodies and B and T lymphocytes. In the innate immune system there is no "memory" that allows it to recognize the pathogens that have infected prior to the animal. On the contrary, the acquired immune system has special structures that perform this function.

Lymphocyte T. Source: NIAID / NIH, via Wikimedia Commons

Both systems, regardless of the animal's cellular structure, its diversity or its degree of development, have mechanisms that defend them from pathogens. In this way they protect them from bacteria and viruses that could cause irreversible damage to the host.

These defense actions vary significantly throughout the phylogenetic scale. The trend is that as you move up the ladder, immune responses are more complex, specific, and effective.

Differences in the immune system of vertebrates and invertebrates

Innate and acquired immunity

Invertebrates have a natural or innate immune system, the mechanisms of which defend them from infections caused by infectious agents. It is made up of cells with phagocytic capacity and humoral components.

Bee, an invertebrate

In this innate system, the host animal does not have an “immune memory” of response to the infectious attacks that it has already received. This implies that the cells of this system identify and act against bacteria in a generic way, not giving the host long-term immunity against them.

The natural immune system acts immediately, with responses such as nodule formation, phagocytosis, agglutination and encapsulation of the pathogen.

Vertebrate animals also have an innate immune system. This has the same characteristics that invertebrates have, except that phagocytic cells are more developed and exist in greater diversity.

However, vertebrates also have an acquired immune system. All, except agnates, produce antibodies, have T lymphocytes and molecules of the major Histocompatibility complex (HCM).

Golden frog, a vertebrate

This allows them to recognize a great variety of antigenic structures, having the ability to "remember" previous exposures. In addition, they can respond more effectively to subsequent exposures to the same infection.

Major histocompatibility complex

The vast majority of vertebrate animals, unlike invertebrates, have MHC (major histocompatibility complex) molecules, which participate in specific immune responses, both cellular and humoral. These molecules play an important role, as they help T lymphocytes to recognize antigens.

In addition to this, the genes of the major histocompatibility complex, absent in invertebrates, give vertebrates a greater or lesser susceptibility to the attack of an infectious disease.

Receivers

The innate immunity of invertebrates identifies patterns of molecules of the pathogen that are not present in the host's cells. These molecules are called the pathogen-associated pattern of molecules (PMAO).

This pattern is recognized by pattern recognition receptors (PRRs) and by Toll-like receptors (TLRs); they are proteins that identify a wide spectrum of pathogens, stimulating responses that are generally inflammatory.

PRRs are found in cells of the native immune system, acting in the identification of molecules associated with microbes. When detected, they initiate an immune-type response.

The acquired immune system, typical of vertebrates, has more sophisticated defense mechanisms. These are dynamically interconnected with those of the innate immune system.

The functional and anatomical unit of the acquired system is the lymphocyte. This is a type of leukocyte, whose function is to regulate the adaptive immune response, reacting to the presence of foreign materials, such as tumor cells and microorganisms.

There are T lymphocytes, B and NK cells, which are responsible for destroying the cells that are infected. Types T and B have specific receptors that are responsible for producing antibodies.

Lymphatic system

In vertebrate animals, the lymphatic system is responsible, among other things, for immune responses to pathogens that could attack the body.

This anatomical structure carries lymph. It is made up of primary lymphoid organs, within which are the thymus, lymph nodes, and bone marrow. In these lymphocytes are generated, differing into T and B lymphocytes.

The secondary lymphoid organs are the spleen, lymph nodes, and mucosa-associated lymphoid tissues. In these tissues, T and B lymphocytes come into contact with pathogens and their antigens, triggering their activation and multiplication to destroy them.

Invertebrate animals lack a lymphatic system. In mollusks and arthropods the immune system resides in the hemolymph. In it are hemocytes, which are the phagocytic cells of the innate immune system.

Humoral response

Among the soluble factors of the immune system, invertebrate animals do not have specific structures, like the antibodies of vertebrates. However, they have substances produced to a greater degree by hemocytes. An example of these compounds are opsonins, proteins that function as an opsonizer.

In arthropods there are peptides, such as linear and cyclic peptides, that react to the presence of microbes and fungi. Insects, echinoderms, and mollusks have lysozyme.

Invertebrate IL-1 stimulates phagocytosis of hemocytes, in addition to participating in encapsulation and nodule formation.

Vertebrate animals are the only ones that have the ability to generate specific antibodies to the diversity of pathogens that could attack them.

Regarding the amount and type of immunoglobulins, there is greater complexity and variety as one moves up the phylogenetic scale

Vertebrate animals have immunoglobulin M, except for agnates which have heavy chain antibodies with thioester linkages.

Physical chemical barriers

In invertebrates, gelatinous barriers can be found, such as the mucous-like secretions of mollusks and annelids. They also exist of high hardness, such as the arthropod exoskeleton.

Among the barriers that try to prevent the entry of pathogens into the host, there are cyclic peptides (drosomycin, linear peptides (anti-Gram bacterial peptides and cecropins), agglutinins, among others.

The variety of barriers in vertebrates differs between fish, amphibians, birds, or mammals. A common barrier in all these animals is the skin, which covers and protects the body. This can be found covered in scales, hair and feathers.

Surrounding the natural body orifices, such as the nostrils, are defense structures, such as mucus, cough, and lysozyme, which is found in tears and saliva.

Other antimicrobial substances in vertebrate animals are, among others, the acidic pH that exists in the stomach and the microbial flora of the intestine.

References

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