First of all we need to learn basics of avian immunology
Avian Immunology has some similarities and some dissimilarities from mammals. Most of the research on immune system is done in mammals mainly humans. In scientific literature little information is available about birds. But fundamental lines of defense are same like chickens possess primary and secondary immune systems.
- Primary immune system is innate i.e. present from birth and it is nonspecific means it gives same response to every infection.
- Secondary immune system is acquired with time in life of bird either through vaccination or through exposure to specific infections. It means for every infection special immunity is developed. For example immunity developed against New Castle Disease virus has no value against Gumboro (or IBD).
Primary (nonspecific) immune response
Inflammation is primary immune response. Inflammation is the first response of the body to the infection and it is a process which begins following a sublethal injury to tissue and ends with complete healing or death. It is characterized by five cardinal signs. Cardinal signs of inflammation are:
- Redness
- Heat
- Swelling
- Pain
- Loss of function
Inflammation is beneficial in most of cases expect in prolonged or unending process and inflammation of immunological origin i.e. hypersensitivity and autoimmunity. It provides basic foundation of pathogenesis and pathology. It is now said that immunity is the resistance of body, while inflammation is the process by which the immune mechanism are implemented. The basis of inflammation remains same be it mammal or birds. For details, one can refer other immunology books. However, there are some differences in reaction in birds which are briefly listed below:
- Adult bird has greater capacity to react in comparison to embryos and chicks.
- In birds, there is increased permeability in venules. While in mammals it occurs in capillary/ arterioles.
- In poultry, Heterophils are the first line of defense and comes first in inflammation followed by mononuclear cells. However, in mammals the sequence is not clear.
- In poultry, Basophils are seen in significant numbers at the site in early stage of inflammation. However, in mammals basophils are rarely seen.
- Formation of perivascular foci due to infiltration of lymphoid cells causing cuffing around blood vessels. This is very common in avian inflammatory reaction. However, in mammals it has been seen only in some specific inflammations of brain.
- Giant cells are the feature of avian inflammation and are observed in acute and chronic inflammation while in mammals these are seen only in chronic inflammation.
- In birds, thrombocytes act as phagocytic cells.
- Biphasic vascular permeability response occurs in chicken.
- In chicken, inflammation caused by phytohaemagglutinin or concanavalin-A (Con-A) is characterized by skin response with infiltration of heterophils, monocytes and basophils. While in mammals, there are no neutrophils and basophils in such reactions.
Secondary (Specific) Immune Response
Specific immunity encompasses two aspects of immunity. These are humoral immunity, which is carried out by antibodies, and cell-mediated immunity, which refers primarily to T cell activity. Lymphocytes are the cellular components of specific immunity.
There are various subpopulations of lymphocytes which are morphologically indistinguishable but which differ in site of development, tissue location, phenotypic expression of cell surface molecules, and functional abilities. Chickens lymphocytes consist of B cells, which develop in the bursa of Fabricius, and T cells, which develop in the thymus.
Inflammation acts as a double-edged sword during viral infections. While inflammation can promote the recruitment of immune cells, uncontrolled and exacerbated inflammation, as observed in many A.I. cases, is associated with systemic edema and extensive tissue damage.
Both B cells and T cells express antigen receptors on their surface. Each B cell and T cell has a homogeneous set of antigen-receptors specific for a given antigen. Collectively, the B cell and the T cell compartment each have a repertoire of nearly 109 different antigen-specificities.
Leaving behind complex jargon and terminologies we would talk about practical importance of avian immunology. Note: although antigen can be any foreign protein but here we are referring virus particles as an antigen.
Just remember following points:
- Heterophils fights with bacteria and lymphocytes fights with viruses
- Wandering lymphocytes in the body have numerous different inherent receptors (upto 10^9 as cited above) which can recognize almost every antigen present on earth.
- When any antigen/virus enter inside the body it is identified by immune system as foreign invader and presented to lymphocytes.
- B and T cells do not work on their own just by interacting with antigens means lymphocytes do not directly attached to these viruses.
- To interact with viruses lymphocytes mainly rely on antigen presenting cells (APC) to recognize any invader.
- These APCs present antigen to lymphocytes then out of numerous receptors present on lymphocytes only a particular receptor would interact with antigen and antigen-receptor bonding occurs.
- Now clonal expansion of lymphocyte occurs that means lymphocyte with this particular receptor divide extensively and population of these lymphocytes increases significantly to fight with virus.
- In following picture only cell with red receptors would recognize antigens then this cell gets multiplied and make its own clones to provide more and more red receptors to bind with antigen and neutralize these antigens. Red receptors are nothing but antibodies and neutralization means nothing but destruction of virus particle.
These clonally expanded cells fights with infection. Some of these cells left in the blood without interacting with virus. These cells are called memory cells and get multiplied quickly when same virus attacked again. Vaccination of poultry birds works on the principle of memory cell generation.
Difference between B and T cells
Difference between B and T cells: B cells made receptors and release outside the cells which are called soluble receptors or antibodies. T cell made receptors and express them on their surfaces.
B cells and antibodies
B cells produces 2 types of receptors one is membrane bound called BCR and other are soluble receptors called antibodies (which are release into the blood). Structurally BCR and antibodies are same. When a B cell encounters the antigen for which it has a specific BCR, it can bind to the antigen via the BCR. The binding of the antigen to the BCR provides a first signal for B cell activation.
Important point: In most B cell-antigen interactions, the B cell requires a second signal from T cells (helper T cells), in the form of soluble factors or through direct T cell-B cell contact, before it can become fully activated.
Once activated, the B cell multiplies and differentiates either into antibody producing plasma cells or memory cells. B cell response that requires a second signal from T cells is called a T-dependent response. Certain antigens, or so-called T independent antigens, are able to activate B cells without additional factors from T cells (T independent response)
What are antibodies ?
Antibodies (soluble receptors), rather than the B cell itself, are the actual effectors of the humoral immune response. Due to their ability to specifically interact with antigen, antibodies can greatly enhance the effectiveness of non-specific immune components. For example, antibodies increase the ability of phagocytic cells to capture and eliminate antigens, activate the complement system, and prevent antigens from binding to and infecting cells.
In chickens, three classes of antibody molecules (immunoglobulins; Ig) have been identified. These are IgM, IgG (also known as lgY),and IgA.
T cells and its functions
Similar to B cells, each T cell has a homogeneous set of antigen receptors (T cell receptor; TCR) specific for a given antigen. Depending upon type of protein receptors involved (alpha / beta / gamma / delta) TCR are of 3 types
1. TCR1 (gamma / delta)
2. TCR2 (alpha / beta1)
3. TCR3 (alpha / beta2)
T cells first express their TCR during thymic T cell development. At this stage, most T cells already start to express CD4 and CD8 cell surface molecules. In healthy egg-type and broiler chickens, double-positive (CD4+ CD8+) lymphocytes in the thymus (thymocytes) constitute the majority of thymocytes (> 60%). The majority of surviving od3TCR thymocytes will mature into alpha beta TCR+CD4+CD8- or alpha beta TCR+CD4-CD8+ T cells and enter the periphery.
In chickens, as in mammals, mature CD4+CD8- T cells constitute the subset of T cells which recognize exogenous antigens (phagocytosed antigens) in association with MHC class II molecules on antigen presenting cells (e.g. macrophages). Moreover, CD4+CD8- T cells include the cytokine producing regulatory T cell subset (helper T cell) required for the antigen-specific activation of B cells, other T cells, and non-specific immune cells (e.g. macrophages)
T cells that express CD4 recognize antigens in context with MHC class II, whereas T cells that express CD8 recognize antigens in the context with MHC class I. During antigen recognition, CD4 binds to the extracellular domains of MHC class II and CD8 binds to the extracellular domains of MHC class I.
CD4+ T cell (Th or helper T cells) functions include activating other immune cells, releasing cytokines, and helping B cells to produce antibodies. They help to shape, activate and regulate the adaptive immune response. Cytotoxic T lymphocytes (Tc or cytotoxic T cells) kill their target cells primarily by releasing cytotoxic granules into the target cell. These cells recognise their specific antigen (such as fragments of viruses) only when presented on MHC Class I molecules present on the surface of all nucleated cells.
Important point: Marek’s disease virus mainly infect T cells due to which they loss their protective immune function and bird undergoes immunosuppression. Infection with virulent strains of serotype 1 MDV causes an early cytolytic infection (3 to 7 days post infection [dpi]), primarily in B lymphocytes with temporary immunosuppression and T-cell activation. Once activated, T lymphocytes (basically CD4+ cells) themselves become susceptible to infection, which can be lytic, but after about 7 dpi the virus enters latency. In susceptible genotypes, a further cytolytic phase may occur about 2 weeks after infection, resulting in a permanent immunosuppression.
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Ethnoveterinary Medicine (EVM) and Viral Challenges in Poultry – Lessons from Field
Phytobiotics as strategic antiviral compounds – an advanced herbal science
Warning
Do not go for every herbal-labelled product that claims successful treatment
This is just an example that how we can explore potential antiviral herbs. During the above exploration, I came across many surprisingly unique points of avian immunology along with herbs and molecules that could effectively regulate it. With the help of the above information, I designed a product named VIROCON a few years back which controls mortalities in severe viral outbreaks. I mean it is really possible to save birds sometimes through knowledge of immunology and special intervention in the light of scientific evidence.
Note: above-quoted information is part of a booklet I created named viral diseases of poultry a basic field level investigation. It contains my own opinion backed by proven research and available scientific knowledge. Viral diseases in poultry are a non-ending subject in which research is going on and according to some experts, we do not even know 50% of all the diseases affecting poultry. New emerging viral diseases and modifications in existing strains make it more difficult to treat and control viral outbreaks by poultry health service providers. This is not the end of the document, I am exploring new things and adding to this document in future, INSHALLAH.
Dr Ibne Ali running ‘Ali Veterinary Wisdom‘ with the sole purpose of educating poultry and livestock farmers. We try to help farmers by advising them during disease outbreaks, regarding the prevention of viral diseases, biosecurity planning, feed formulation, toxicity management and farm auditing.
