Types of Immunity – Immunology – Immune System (Biotech)

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Types of Immunity

Types of Immunity – Immunology – Immune System (Biotech)

Types of Immunity

The various types of immunity that are commonly identified, characterized and studied at length are as stated below, There are 5 main Types of Immunity which are;

(i) Humoral Immunity

(ii) Cell-mediated Immunity,

(iii) Innate (or Natural Immunity),

(iv) Acquired Immunity, and

(v) Non-specific Immunity.

These different types of immunity shall now be treated individually in the sections that follows:

1. Humoral Immunity

Antibodies are immunoglobulin (Ig) molecules (e.g., serum proteins) and they are usually comprised of several categories designated as IgA, IgD, IgE, IgG, and IgM respectively.

However, it is pertinent to state here that each category essentially possesses certain specific characteristic features, such as: size, carbohydrate content, electrophoretic-migration velocity, quantum of antigen-combining sites, immunological response, and immunological objective.

Examples:

(a) IgM: Almost always enjoys the reputation of being the first class of antibody generated invariably in most humoral responses but normally gets switched over to the corresponding IgA, IgE, or IgG at the very early stage in the immune response.

(b) IgG: Most versatile important and abundantly available class of antibodies taking part in largest humoral immune reactions. Besides, it happens to cross the placenta thereby providing a newly born baby absolute temporary immunity against whatever immunogens the mother has earlier against IgG.

(c) IgA: Antibodies are invariably found in a plethora of such secretions as : tears, saliva and mucous membranes. These are quite frequently termed as our first-line-of-defense mechanism by virtue of the fact that most bacteria, viruses and fungi that eventually gain entry into the body do cross a mucous membrane.

(d) IgE: Antibodies are equally important in our body’s defense against the parasitic worm infections specifically. Prominently and predominantly several allergic manifestations give rise to the release of histamines e.g., allergy due to pollens, house dust, dust mite, human hair, food allergens etc., which in turn afford the apparent discomforts resulting into extrinsic asthma, hay fever, or hives, or excessive sneezing (during changes of season due to pollens in the air).

(e) Antibodies normally serve as surface receptors strategically located on certain immunologically active cells so as to enable them to bind immunogen.

Importantly, the different types of cells or entities that are held responsible for contributing immensely to the humoral immunity are as follows:

(i) B Lymphocytes (or B Cells),

(ii) Immunodominant peptides (IDPs),

(iii) Antigen-presenting cell (APC),

(iv) T Cell subsets

(v) Class II MHC (major histocompatibility complex) proteins.

It would be worthwhile to have a closer and detailed description and functionalities of each of the cell or entity cited above in the selections that follows;

(i). B Lymphocytes [or B cells]

The B lymphocytes or B cells are so named because they were first and foremost found in the Bursa of Fabricius of birds.

It has been observed that in ‘birds’ the multipotent stem cells actually originate in the bone marrow usually migrate to the bursa, and here they virtually get differentiated into specific antibody synthesizing cells.

In fact, antibody molecules are normally generated by the plasma cells which are vividly differentiated from B cells.

It has been found that B cells are concentrated in various parts of the body, such as : spleen, mucous-associated lymphoid tissue, and regional lymph nodes, where they actually await contact by the foreign epitopes which promptly initiate the process of conversion into the plasma cells.

Types of Immunity

Interestingly, the characteristic features of B cells may be enumerated as stated below:

1. B cells possess essentially surface immunoglobulins (sIgs) plus a number of receptors.

2. The sIg form an integral part of B cells; and, therefore, act as receptor for antigens.

3. B cells may also have immunoglobulines (Ig) in their cytoplasm.

4. Intially, when B cells are ‘immature’, sIg molecules which are exhibited specially belong to IgM category that do not cross link with other IgMs.

5. IgD molecules appear prominently on the surface showing extremely high levels with B cell marching ahead to its developmental pathway.

6. Activation of B cell initiates loss of IgDs together with other receptors appearing in the membrane that eventually enhance the phenomenon of activation.

7. B cells may undergo activation by the aid of lipopolysaccharide preparations from Gramnegative organisms e.g., E. coli, Salmonlla. Besides, activation may be followed by the appearance of a plethora of surface receptors for Igs, ocystalline fragment (Fc) component of heavy Ig chain, and also for Epstein-Barr virus.

8. Antigen critically trigers selection of an appropriate antibody-producing cell and this selection is exclusively based upon the surface receptors.

9. Receptors which are found to be absolutely specific for an antigenic determinant not only provoke but also interact with B cells to initiate strategic proliferation and generate a clone of blast cells, most of which are capable of giving rise to the same antibody.

10. A portion of the prevailing blast cells get segregated and pass into the plasma cells (i.e., antibody secreting cells), while others do remain behind in lymphoid tissue in the form of memory cells.

11. B cell is characterized by a genetic composition which enables it to produce only one specificity of antibody ; and this is accomplished via random rearrangment of genes which essentially control and monitor both gross and minute antibody structure.

12. B cells are produced continuously in vivo throughout life since the life-span of a mature B cell is only a few days unless contacted by the immunogen for that it remains specific.

(ii). Immunodominant Peptides (IDPs)

It is, however, a universal truth that ‘antibody production’ invariably takes place through the earnest cooperation and interaction of various types of cells.

It has been duly observed that either macrophages or other cells having identical lineage encounter predominantly the extracellular foreign immunogen present in the blood or lympth, undergoes phagocytocis, and ultimately meets complete destruction.

Importantly, in the course of the ‘phagocytic phenomenon’ the ensuing macrophase critically identifies and recognizes epitope structures present on the immunogen, and notably protects them in the shape of short peptide chains having 10-18 amino acid lengths that are usually referred to as immunodominant peptides (IDPs).

Types of humoral Immunity

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(iii). Antigen-Presenting Cell (APC)

The antigen has to be presented correctly and precisely onto the surface of an antigen-presenting cell (APC) which is normally a macrophage, or similar cell. Most commonly the antigen is presented in association with self Ia molecules duly coded by MHC-genes.

(iv). T Cell Subsets

T Cells or thymus-derived T lymphocytes do play extremely vital and equally important roles in the domain of immune response mechanism, particularly in the cell-mediated immunity (CMI).

In true sense, the different types of immunological functionalities mediated by them usually fall under two distinct categories, namely:

(a) effector responses; and (b) regulatory responses.

Examples of Effector Functions (Responses):

The various types of effector functions are as stated under:

(i) tuberculin reaction (or delayed hypersensitivity response),

(ii) destruction of tissue grafts, and

(iii) lymphokines vis-a-vis their ability to perform ‘killer functions’ of other cells due to the secretion of soluble chemical mediators,

Kinds of T Cell Subjects: There are in all four kinds of T cell subsets that have been duly identified based on their surface markers, namely,

(a) T helper cells (TH cells or CD4 cell)

(b) Cytotoxic T cells [CTLs or Tc]

(c) Supperssor-inducer T cells [Ts]

(d) Delayed Hypersensitivity T cells [TDH].

(a) T helper cells (TH cells or CD4 cell)

The T helper cells (TH) are of two types, namely:

(i) Th1 cells

(ii) Th 2 cells.

(i). Th 1 cells:

It is, however, pertinent to state here that since the remarkable discovery of subsets of T helper cells i.e., Th 1 and Th 2, gained wide recognition and acceptance for the first time in 1986, a tremendous and copious volume of work has been duly accomplished and concepts with newer ideas conceived.

Salient features of Th 1 cells:

The various salient features of these cells are as follows:

(i) They activate macrophages,

(ii) Their clones specifically give rise to interleukin (IL-2), interferon (IFN-γ), and tumour necrosis factor (TNF-β),

(iii) They produce strong delayed-type hypersensitivity reaction (DTH-reaction),

(iv) They invariably generate cytokines for its own proliferation e.g., autocrine growth factor IL-2 for Th 1.

(v) They produce cytokines which cross-regulates each other’s development and activity viz., IL-4 and IL-10 suppressing Th 1.

(ii). Th 2 cells:

The salient features of Th 2 cells are as given below:

(i) Their clones give rise to IL-3, IL-4, IL-5, IL-10 and IL-13. However, it was earlier believed that IL-10 to be the product of Th 2, and later on it was demonstrated to be the product of both Th 1 and Th 2.

(ii) They generate weak DTH reaction involving relatively higher level of ‘antibody production’. (iii) They product cytokines for its own proliferation viz., autocrine growth factor IL-4 for Th 2. (iv) They also produce cytokines, which essentially cross-regulate each other’s development and activity e.g., IFN-γ suppressing Th 2.

(v). Class II MHC (Major Histocompatibility Complex) Proteins

The articulated and geometrical progression development specifically in the field of major histocompatiability complex (MHC) came into being from the extensive and intensive studies carried out solely upon transplantable tumours that are capable of growing in nearly 100% of either homologous or allogenic recipients.

In fact, this particular observation was anchored as a solid and strong evidence not only towards the host-tumour compatibility but also for the high levels of resistance attainable by different types of immunization characteristics more or less strengthened the belief that tumour cells invariably are associated with both extremely specific and apparently strong foreign antigens.

Surprisingly, the aforesaid observation was not found to be correct and genuine based on the indepth study of highly inbred strains of mouse.

It was, however, further revealed that such ‘tumours’ which either are produced or propagated via highly inbred strains do not necessarily afford active support to the various prevailing immunization procedures.

2. Cell-Mediated Immunity (CMI)

The cell-mediated immunity (CMI) essentially involves the particular system that is directly responsible and associated with not only the critical ‘rejection of certain organ transplants’ (e.g., heart, kidney, liver, eyes etc.,) but also the ‘defence mechanisms’ against intracellular microorganisms, endogenous neoplastic (tumour) growths, and host cells infected with various viruses.

Importantly, this specific classification of the relatively huge immune system, e.g., humoral immunity, depends heavily and exclusively upon the ensuing immunogen (antigen) stimulation for its much desired activation.

Another school of thought explains CMI as inadequacy and insufficiency of antibody mediated immune response that may eradicate effectively infections caused by a host of pathogenic microorgansisms, parasites, and viruses as well that would ultimately develop within the host cells, eventually offered a tremendous impetus to scientists across the globe in the specialized field of ‘immunology’ to evolve a corrective response mechanism that could go a long way in the management and control of such most dreadful and serious infections.

Types of cell mediatedImmunity
Fig: Human Defence System- Immunity

Therefore, justifiably the prevailing mechanism is invariably termed as the ‘cell-mediated response’ that not only effectively controls fatal infections emanated by viruses, microorganism and protozoa that multiply within the host cells significantly, but also affords paramount attraction of non-specific immune cells by lymphokines solely secreted by lymphocytes.

In a nut shell, the effectiveness of the cell-mediated immune response must be adequately normal because it predominantly is responsible for providing general and overall protection to these critical situations, namely : transplantation immunity, hypersensitively reactions, auto-immunity and neoplastic immunity as well.

Genesis of CMI T-effector cells which is solely responsible for CMI, in fact, originate from the precursor cells (stem cells) generated in bone marrow, very much akin to the instance of the antibody-forming cells in humoral immunity.

Interestingly, in CMI, the effector cells should precisely complete the differentiation process taking place in the thymus before undergoing circulation more or less freely in the vascular net work or regional lympth nodes or collecting in the spleen eventually.

Diferentiatied cells thus produced are essentially of different types, that are exclusively dependant upon the type and nature of the prevailing cell mediated response; besides, the extraordinary T-cell receptor (TCR) structures invariably observed on their surface, that is genuinely acquired via random genetic recognition is the ensuing course of differentiation at two distinct sites, namely:

(a) in bone marrow; and (b) maturation in thymus.

In actual practice, however, one may divide CMI responses into two distinct major categories based on the realistic requirement of different effector cell populations, namely: A.

Based on the Requirement of TC cells being converted to Cytotoxic T Lymphocytes (CTLS) having Direct Toxic Effect on Target The CMI essentially engaging CTLS is found to be profusely effective as a reasonably solid defence against a plethora of neoplastic (cancerous) growths, virus infections, and also tissue transplants.

On a serious note one may observe that the prevailing ‘immune system’ fails to realize that CTLS are virtually meant to protect the body, and must not destroy it in the long run.

Salient Feature:

The various salent features are namely:

(1) CTL-resposne may be sub-divided into two distinct phases eg.,(a) sensitization phase ; and (b) effector phase.

(2) Sensitization phase : Usually commences when a macrophage or other antigen-presenting cell (APC) specifically detects cells sloughed off from the growing tumour (neoplasm) transplanted tissue, or previously killed by the infective virus.

It is quite normal for any dead, damaged, partially damaged or even changed cells to become disconnected from the parent tissue, and travels ultimately to either liver or spleen for final destruction.

(3) APC will afford phagocytization of the ‘target cell’ as it would duly take cognizance of the viral, tumour or foreign epitopes available on the surface of the cell in its normal passage via the vascular or the lymphatic systems.

2.1. Immunosuppression

Immunosuppression may be defined as ‘a phenomenon wherein an organism’s ability to form antibodies in response to an antigenic stimulus is substantially reduced or suppressed’.

As on date a good number of ‘immunosuppression drugs’ are being used in order to enhance and prolong the ‘life-expectancy’ of a transplanted organ e.g., heart, kidney, eyes etc.

It has been duly observed that when small doses of immunosuppressive drugs are employed, the recipient’s immune system usually overcomes the drug, and rejection is indicated vividly by the gradual loss and efficiency of ‘organ function’ based on the following common symptoms and actions irrespective of the organ, namely:

(a) fibrous thickening of the innermost small arteries of the transplant,

(b) consequent administration of relatively larger doses of immunosuppressive drugs as an extreme alternative measure, and

(c) for each individual patient an altogether different and suitable ‘immunosuppressive therapy programme’ is usually slated for.

However, another perception of ‘immunosuppression’ legitimately promulgates the ensuing ‘immune response’ as a coordinated mechanism essentially involving the due recognition of antigen exclusively by the immunocompetent cells, termed as anti-T lymphocyte serum (ATS); besides, occasionally the macrophages that are found to function as an antigen-presenting cells (APC). Immunosuppression are of two types:

(a) non-specific

(b) specific.

2.2. Privileged Graft Sites

It is a well-known fact that the very problems of ‘graft rejection’ are not usually encountered in all transplants. Obviously, there are certain specific organs present in the human body which do accept foreign tissue quite readily, without any rejection whatsoever, and virtually 100% success rate achievable in all transplantations. In medical terminology, such specific areas in the body are termed as immunologically privileged graft sites. Interestingly, one such site is the cornea of the eye.

2.3. Graft-Vs-Host Disease (GVHD)

Keeping in view the prevalent common situation whereby the recipient happens to reject the ‘donor graft’, the reverse may also take place when the said donor graft essentially comprises of immunocompetent cells.

In immunological environment this specific phenomenon is invariably known as graft-Vs-host disease (or reaction). The net result is normally displayed and characterized by the ‘graft’ immunologically rejecting the host.

Examples: The above very commonly occurring phenomemon may be explicitely explained with the help of the following typical example:

(1) A patient being administered with fresh bone marrow from a non-identical donor, evidently in such a situation the patients existing immune cells shall attack the ‘grafted cells’, whereas the grafted bone marrow cells will attack normal body tissues very much present within the patients body.

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Hence, in this specific instance it is absolutely necessory to first completely destroy the entire immune cells of the patient by subjecting him to whole-body X-Ray irradiation treatment just prior to his receiving the bone-marrow graft so as to protect the ‘graft’ from any external attack.

Besides, it is equally important and vital that the patient’s tissues may also have to be adequately protected from attack by the prevailing immune cells in the bone marrow graft.

If by any chance this protection to the patient’s tissues are not accomplished duly, there could be a serious lethal condition developed ultimately usually termed as graft-Vs-host disease (GVHD).

In order to avoid, manage and control this condition from the very grass-root that pre-operative corrective measures should be swung into action to suppress the immune responsiveness of the grafted cells by the help of appropriate drugs.

However it is quite necessary and important that the dosage regimen of the drug must be reduced slowly and gradually over a certain length of time till the ‘graft’ is rendered fully tolerant and totally acceptable in its new environment.

Ultimate prevention of GVHD essentially needs an extremely sensitive and careful balancing of the immunosuppressive drugs therapy for relatively longer span wherein the patient is required to be confined in an isolated sterile environment.

It has been observed that the immunosupressive drugs do possess two serious drawbacks, namely: (a) rendering the patient quite vulnerable to infection.

(b) exerting enormous unpleasant side-effects.

However, the discovery of a new drug cyclosporin A has considerably eased the situation which is found to be both more effective and having relatively fewer side-effects.

3. Innate (or Natural Immunity)

Natural immunity of an animal is also invariably termed as innate immunity, or native immunity or inherited immunity. Natural immunity may be defined as ‘an inherited (in built) resistance to infection(s) and is concerned with species, races or individuals’.

In fact, as to date the exact mechanism of its defence has not yet been fully established and understood explicitely, inspite of its natural presence in an individual and not acquired through any prior exposure to the infectious agent.

In other words, the natural immunity solely related to a general or non-specific type of resistance, that ultimately affords prevention of infection caused by different kinds of pathogenic microorgansisms (or pathogens).

Interestingly, the degree and extent of the prevailing ‘natural immunity’ largely differs in various organisms, which may be expatiated further with the help of the following typical examples.

Examples: Mumps may be infected in humans, whereas the kennels like dogs and cats are practically immune to this disease.

NOTE: The extent of ‘natural immunity’ may alter between various species, races, strains, and sexes; and, may be controlled by nutrition, hormones and a large number of other factors.

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4. Acquired Immunity

Acquired immunity is a type of immunity which develops in an individual subject in response to an immunogen (antigen).

It may, however, be acquired naturally by intentional or accidental exposure to an infectious disease or artifically acquired by receiving active immunizing agents, such as: vaccines.

It has been observed that ‘acquired immunity’ usually gets developed during the life-span of an individual and hence critically refers to the immunity, that a particular individual predominantly exhibits against a specific pathogen.

Interestingly, it is quite often related to the very existence of either antibodies or interferons (ITFs) present in the circulating blood in vivo.

The acquired immunity may be justifiably classified into two distinct categories solely based on the antibodies, for instance:

(4.1) actively acquired immunity,

(4.2) passively acquired immunity.

(4.1). Actively Acquired Immunity

  • (a) Actively acquired natural immunity
  • (b) Actively acquired artificial immunity

The actively acquired immunity could be either due to, ‘natural’ or ‘artificial’ derived sources.

(a) Actively acquired natural immunity (AANI):

It is precisely caused due to any disease-producing infection (i.e., pathogenic organism based) from which a human being recovers ultimately.

It is observed that during the course of infection, the quantam of antibody production for the causative specific pathogenic entity gets stimulated in vivo to such an extent so that when there is a subsequent infection either by the same or antigenically compromised or antigenically related pathogen.

The resultant antibodies thus generated do assist overwhelmingly in boosting up the body’s defense mechanism profile appreciably.

(b) Actively acquired artificial immunity (AAAI):

It is regarded universally as the most widely accepted and common mode of accomplishing immunization of ‘vaccination’.

In actual practice, the antigens (immunogens) are adiminiistered into the body in a meticulously controlled amount as to stimulate the production of immunoglobulisn (Ig).

With the advent of geometrical progression of research in the field of immunology based on ultra-modern preparation and screening/evaluation methodologies, both attenuated and killed strains of microorganisms and viruses are being employed both intensively and extensively for affording immunization against a plethora of dreadful diseases in man, for instance: small pox, typhoid, measles, poliomyletis, yellow fever and the like.

Vaccines are being developed and prepared by the aid of a large variant of methods and technologies involving the well known ‘recombinant DNA technology’ (treated elsewhere in this text book comprehensively).

4.2. Passively Acquired Immunity

As a striking coincidence the passively acquired immunity can also be acquired either by natural or artificial manner.

(a) Passively acquired immunity for ‘natural’ means. It essentially involves the actual progressive transfer of antibodies from mother to her yet unborn baby in the womb via the placenta in the course of the ‘latter stage of pregnancy’.

(b) Passively acquired immunity by ‘artificial means’. It specifically concerns to the original production of ‘antibodies’ in some altogether different entity (humans or lower mammals); and subsequently followed by the calculated dosage regimens of these ‘antibodies’ in affected patients via sterile injection.

Some renowned organizations viz., Hoffkine Institute, Mumbai; Central Vaccine Institute, Kasauli ; and several other Drug Manufacturing Units, are actively engaged in the commercial production of ‘antibodies’ in horses and cows by active immunization for subsequent usage for passively acquired immunity in humans.

5. Non-Specific Immunity

The vertebrates, particularly the mammals, are not only privileged but also fortunate enough in possessing a primitive immune system that essentially confers in them non-specific immunity.

It is, however, pertinent to state here that the cells as well as the macromolecules of the prevailing system may render a clear cut basic distinction between either self or non-self determinants.

Furthermore, these self and non-self determinants fail to distinguish specifically one antigen (immunogen) from another; and, therefore, are perhaps in a better and more vulnerable position to attack ‘foreign material’ in a non-specific manner.

Interestingly, the cells that are particularly engaged in causing non-specific immunity are nothing but white blood corpuscles (cells) (WBC’s), but do specialize in the process of phagocytosis.

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