Active And Passive Immunity Compare And Contrast Essays

Active Immunity vs Passive Immunity
(Similarities and Differences between Active and Passive Immunity)

The acquired immunity is the immunity acquired by an organism during its life. The acquired immunity against a particular microbe may be induced by the host’s response to the microbe or by the transfer of antibodies or lymphocytes specific for the microbes. Based on the above criteria, the acquired immunity is categorized into two types – (1) Active Immunity and (2) Passive Immunity.

Active Immunity: The active immunity is the direct response of your body against the pathogens. It is induced by the exposure to a foreign antigen such as the antigen of microbes. It is an adaptive response of the individual after contact with specific pathogen or antigen.

Passive Immunity: The passive immunity is the immunity conferred to an individual by the transfer of serum or lymphocytes from a specifically immunized individual. Passive immunity is a useful method for conferring resistance without waiting for the development of the active immune response.

The present post discusses the Similarities and Differences between the Active and Passive Immune Systems with a Comparison Table.

Similarities between Active Immunity and Passive Immunity

Ø  Both active and passive immunity are acquired immunities.

Ø  Both can be natural and artificial.

Ø  Both types of immunity involve lymphocytes.

Ø  The antimicrobial components in both the systems are antibodies.

Ø  Both are induced by the antigens.

Ø  Both systems are specific.

Difference between Active and Passive Immunity

Sl. No.Active ImmunityPassive Immunity
1Produced actively by the immune system of the host.Produced passively by the immune system of the host.
2Antibody production is induced by the infection or by immunogensAntibodies are not produced, but directly transferred
3Active immunity involves both cell mediated and humoral immunity.Passive immunity is due to the presence of ready-made antibodies.
4Natural active immunity is by clinical infectionNatural passive immunity is by the transfer of antibodies through placenta
5Artificial active immunity is induced by vaccinationArtificial passive immunity is induced by injection of antibodies
6A lag-period is present Lag period is absent
7Active immunity is effective only after the lag-periodPassive immunity is immediately mediated (since lag-period is absent)
8Active immunity is durable.Passive immunity is only transient
9Active immunity offers effective protection against microbes.Passive immunity is less efficient in offering complete protection
10Immunological memory is presentImmunological memory
11In active immunity, the subsequent doses with the antigen causes booster effectIn passive immunity, the subsequent dozes is less effective due to the immune elimination
12Negative phase may occurs in active immunityNegative phase is absent
13Active immunity is not applicable to immune deficient individualsPassive immunity is applicable to immune deficient individuals

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Adaptive immunity

The immune system uses many different receptors to interrogate the environment. These are usually proteins and are found in the blood, in tissue fluids or bound to the cell surface. The antibody receptor, also called an immunoglobulin (Ig), was the first antigen-specific receptor to be characterized and is commonly drawn as a Y-shaped cartoon. It is formed by the combination of two identical heavy and two identical light chains. Such an Ig comprises three globular domains connected by more flexible linkers; the two binding domains, coded for by variable regions, have identical specificity for antigen (Figure 1). The globular Ig structure is a widely adapted template that is used by many molecules both inside and outside the immune system. The structure is known as the Ig domain (or fold) and proteins that contain such a domain are members of the ‘immunoglobulin superfamily’.

Figure 1Antibody structure

Antibody cartoons are often drawn in a Y-shape, as on the left. This picture represents an IgG molecule, made from two identical heavy (red and blue) and two identical light (yellow and green) chains. The top of the Y (called the Fab region) contains two variable regions that each bind the same antigen. The bottom of the Y is the constant (Fc) region, which interacts with cell receptors, complement, etc. On the right is a crystal structure of an IgG2 antibody. The heavy chains (red and yellow) and light chains (blue and green) are identical. This antibody is ∼10 nm from bottom to top. (From TimVickers.

Antibodies are soluble and do two things. At one end, they bind firmly to a target (antigen), whereas at the other, they signal to immune cells. Antibodies are divided into a number of different families, called isotypes (Table 2) and their production is a carefully regulated process involving cell–cell interactions that control which antibodies are made. Antibody research in the first half of the 20th Century focused on antibodies that could be purified from serum. Once the amino acids that made up the different chains were defined, attention turned to understanding how antibodies were produced by individual cells. It was discovered that, for most antibodies, their generation depended on co-operation between at least two types of cell: a cell that processed and presented targets (antigens) from the environment [antigen-presenting cell (APC)] and a lymphocyte that recognized the target antigen on the APC. This lymphocyte (now called a T-cell or T-lymphocyte) directed either the production of antibodies or killed the cell presenting the antigen (Figure 2).

Figure 2The adaptive immune response to infection

Infection, detected by APCs, triggers specific T-cells that co-ordinate killing and antibody production which stop the infection.

One early idea to explain how T-cells determined what to respond to was that the immune system only presented antigens from infections, but this was wrong. To the surprise of many immunologists, studies in the 1980s which defined the receptor molecules on the surface of cells that controlled this targeting process revealed that essentially all cells presented antigens. A healthy immune system surveys these antigens constantly, but this does not provoke a response. This challenges the impression that the immune system spends most of its time doing nothing. Quite the opposite: it is constantly reviewing the environment, checking whether anything is amiss. The immune system has a carefully developed sense of self which it generates through a process of education.

The lymphocyte's antigen receptors recognize a family of cell-surface molecules on APCs that are collectively known as major histocompatibility (MHC) determinants. In humans, these are also called human leucocyte antigens (HLAs). These molecules are a combination of material derived from the environment (the antigen), bound in the flexible jaws of the MHC molecule which hold it in place. The rest of the MHC molecule acts as a scaffold orientating the MHC–antigen complexes at a cell's surface where they can be scanned by a lymphocyte (Figure 3). In this way, cells display a constantly changing picture of the proteins that they are making as antigens loaded into their MHC molecules, and these antigens come from inside and around cells. This sampling strategy is effective because disguise is very difficult at the molecular level. A bacterial cell makes many proteins that do not resemble those made by an animal cell; a cell making a virus inside does not look like a normal cell.

Figure 3Antigen processing I

A protein molecule (on the left) is digested by the cell and a fragment from it (shown in green) is loaded into an MHC molecule that is then displayed on the surface of the cell (on the right), oriented so that it can be scanned by T-cells. The MHC molecule is ∼7 nm tall above the cell membrane. Not to scale. Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute

The commonest type of antigens presented by MHC molecules are peptides, short stretches of amino acids (eight to 30) which are processed from proteins. The source of this material may be the internal or external environment of the cell that presents it. Two types of MHC molecule, class I and class II, are involved in this process. All cells, except for red blood cells, present a selection of antigens from the proteins that they are making bound by MHC I. Each cell wears a collection of MHC–antigens on its surface that act as bar codes, identifying that cell to the immune system. Some immune cells called APCs are specialized to present antigen, and load antigens into MHC I and MHC II. They are continuously hoovering up proteins from the environment, using enzymes to digest them to produce short peptides, and loading them into the jaws of the MHC within the cell, before these molecules are displayed on the surface. In APCs, these two parallel processing pathways provide a real-time survey of whatever the environment contains (Figure 4).

Figure 4Antigen processing II

Internally produced and externally captured proteins are loaded on to MHC molecules inside APCs. Internally produced proteins are presented by MHC class I molecules, which are found on all the cells in the body. Externally acquired proteins are presented by MHC class II molecules on specialized APCs. Once loaded, molecules are exported to the cell surface.

All MHC molecules work in the same basic way, as a scaffold to present antigen, but as a species we have in our heritage a repository of many different MHC molecules (1122 common and well-documented different alleles as of 2013). This diversity at the level of the population of all humans, is narrowed down in the individual, who inherits about nine MHC molecules at random from their parents. Your own MHC molecules define your tissue type which is the major barrier to successful organ transplantation, and the reason we keep records of the tissue type of potential donors. The chance of meeting a stranger who shares the same MHC molecules as you is tiny, which is why, among a panel of 20 million potential donors, 2–5% of individuals will not find an exact match. Even an individual with the commonest set of HLA genes found in the U.K., who needed a transplant, would only find a few hundred potential donors among this panel. This is also why your best chance of finding a compatible donor is searching among your close relatives, with whom you share genes.

Of course, the complexity of this system did not arise to frustrate transplant surgeons. The immune system uses it, because each of these many different MHC molecules presents a unique selection of antigens processed from the same underlying proteins. For example, the antigens that are presented from the liver of one person will be different from the antigens presented from the liver of an unrelated person. In this way, the representation of self established by an individual's MHC, presenting its self-proteins to its own lymphocytes, is a very private system of identification that is difficult to copy, allowing the immune system to discriminate between foreign tissue transplants, invading infections and cancerous cells.

The sensor that interacts with the MHC–antigen complex is called the T-cell receptor (TCR or TcR). This receptor is part of a complicated structure called the TCR complex (it is formed by the combination of six different protein chains in four pairs). Two TCR chains (α and β) make up the sensor that examines MHC molecules (Figure 5), and the other chains are used for signalling the results of that examination into the T-cell. They are often omitted from cartoons that show the TCR at the cell surface. The signals that the TCR generates within the T-cell depend on the affinity of the interaction of the α and β sensor chains with the MHC–antigen. In essence, each TCR measures the affinity of this interaction and provides a read-out to the T-cell which determines the subsequent response of that cell. An activated TCR sends signals across the cytoplasm of the cell to the nucleus where it initiates programmes that change the pattern of genes that are being expressed and therefore some of the proteins made by the cell.

Complementing the wide variety of possible MHC molecules, TCRs need a comparable level of diversity to ensure the availability of suitable receptors. This is achieved through a modular approach to building up the receptor. When a new TCR is being made, parts of the α chain and β chain, which are used for recognition, are generated at random from a pool of hundreds of gene segments. These are joined together in a process that also adds random mutation. This means that at any time there are many more possible ways and individual can make TCRs that there are T-cells in the body. Each T-cell carries multiple copies of a single unique TCR.

Because making a TCR involves random recombination, this is quite a wasteful process, and only a small fraction of the T-cells produced will have TCRs with optimal biochemical properties. To test for these characteristics, T-cells first pass through a specialized organ called the thymus (which gives T-cells their name, T for thymus). For the T-cell to survive selection, the TCR must be useful. It must be able to ignore self-antigens in MHC complexes, but must still bind the MHC molecule strongly enough to interact with the peptide antigen. If this binding is too weak, the cell is discarded. TCRs that produce strong activation by self-antigens are dangerous and they are also removed by screening (Figure 6). To match these stringent requirements, the immune system is prepared to throw away more than 19 out of 20 of the T-cells that it makes. The result is a population of T-cells with a repertoire of TCRs that recognize self-antigen weakly, have the potential to recognize non-self-antigen strongly, and can safely be exported from the thymus.

Figure 6TCR and T-cell selection

(A) The DNA encodes a large number of different possible sequences for the TCR. The code for the part of the TCR that recognizes antigen are selected at random from these sequences. Different structural elements are combined to make the final TCR. The resulting receptor is expressed at the cell surface and tested in the thymus. (B) Cells bearing each TCR are put through a number of screening tests. If the TCR cannot work with the individual's MHC molecules, or if it is useless or dangerous, it is destroyed. If it passes these tests, it is exported from the thymus.

In a healthy person, these exported T-cells move continuously between lymph nodes and the blood, testing APCs for signs of infection. This baseline surveillance requires that the T-cells actively engage with APCs, using their TCR to interrogate MHC–peptide receptors. Usually they do not encounter activation signals and they move on. After several weeks, they will be replaced by younger cells, newly exported from the thymus, carrying their own unique TCRs.

But when an individual develops an infection, and antigens from this infection begin to be displayed on APCs that are scanned by T-cells, a few of the millions of different T-cells will have TCRs that trigger activation of the T-cell. First this stimulates the cell to divide, producing daughter cells with the same TCR. Instead of the blood only holding a handful of infection-specific T-cells, this expansion leads to it having first thousands and then millions. These cells also acquire an ability to act (Figure 2). CD8+ T-cells may kill infected cells directly, and CD4+ T-cells help to make antibodies; both send out signals to attract macrophages and neutrophils to the site of the infection. As the infection progresses, responding cells become more specialized, developing different effector functions that optimize how the immune system attacks it.

The other crucial recognition system of the adaptive immune response, antibodies that are produced by B-cells, go through a process of maturation and selection that serves to greatly increase the strength with which they bind their target antigen. Antibodies are very different from T-cells because, although they start as a cell-surface receptor [the B-cell receptor (BCR)], they are later secreted and can function well in many places that T-cells do not. Once they have been produced, they are an efficient early defence in cases of reinfection, able to bind to pathogens that breach external barriers. They can also neutralize soluble poisons (toxins) that some organisms produce, which is very important, for example, in the response to diphtheria. Antibodies circulate in the blood, are found within the mucus that lines our gastrointestinal organs and also in interstitial tissue fluids. Mothers pass antibodies to their children through their breast milk.

Like TCRs, antibodies are adapted to specific infections by selection from a pool of randomly generated candidates, through a series of selection steps called affinity maturation that promote optimal function (Figure 7). But, unlike TCRs, antibodies can recognize whole proteins before they have been broken down into peptides. The majority of antibodies recognize proteins in their native state, folded up, with different chains and loops contributing to a patch on their surface to which the antibody binds. Because of this antibodies often focus on the outside of pathogens. And, as a defence, some pathogens, from the influenza virus to the malaria parasite, have developed processes that continuously change how they look at the surface, as a way to escape the immune system.

Figure 7Affinity maturation

Antibodies that fit quite well are selected early in the immune response. The antibody receptor (BCR) is mutated within daughter cells. Many of these mutations bind the antigen worse than the parent antibody, and cells producing these antibodies die. Some antibodies bind the antigen better than the parent, and these cells live.

Antibody production starts in specialized immune system tissues. When a pathogen invades, antigens from it are carried to areas inside lymph nodes or the spleen by APCs. B-cells that have a BCR that can bind to these antigens are activated, take in and process antigen and load it into their MHC molecules. This prepares them to receive signals that stimulate growth and affinity maturation from T-cells that recognize these same antigens. B-cells and T-cells attract each other and this mutual attraction facilitates encounters between rare antigen-specific B- and T-cells. The help that T-cells provide to the B-cell promotes secretion of antibody, changes in the isotype of antibody that is secreted, and stimulates mutation of the genes that code for the BCR. Because the mutations occur at random, most of them actually impair the ability of the BCR to bind antigen, another expensive process in which the immune system is prepared to throw away many cells to select the best one. A fraction of the mutations improve the binding affinity of the BCR, making it stronger. Cells with BCRs of a higher affinity compete better for reducing amounts of antigen and are selected to live, whereas their less effective siblings die. By repeating this process several times, the affinity of the antibodies that are being made can increase by many orders of magnitude.

This dramatic increase in specificity is a key part of immune memory, but in recent years it has also become applied to many new ‘biologics’; therapies using antibodies to target diseases as diverse as rheumatoid arthritis and lung cancer. The high specificity that antibodies offer have made them an extremely flexible and effective medical technology.

Isotype switching permits antibodies to be directed into different roles. By rearranging the genes that specify the heavy chain of the antibody, Igs can be optimized for different environments and functions. The different isotypes are named according to their heavy chains (IgM, IgD, IgG, IgE and IgA). Because the antigen-recognizing domains of the antibodies do not change when a molecule with a new heavy chain is produced, each clone of B-cells maintains its specificity. A summary of the functions of different isotypes is given in Table 2.

Table 2.The five main types of antibody isotype which have different roles in the immune response

Innate immunity

Discrimination is also a key aspect of the innate immune system. This is an area of research that has grown explosively since studies at the close of the 20th Century identified families of receptors that sampled the environment for the presence of molecules associated with pathogens (called pathogen-associated molecular patterns; PAMPs). One of the first pathways of this kind to be worked out in detail exploited the discovery that a receptor cloned from insects, and implicated in their sensitivity to infection, was related to the gene for a similar protein that could be found in mammalian cells. This molecule, called Toll-like receptor 4 (TLR4) is a key mediator of the effects of sepsis in patients critically sick with infections. When it binds to bacteria, TLR4 triggers the release of cytokines that stimulate the whole immune system producing fever and, in the seriously ill, shock. TLR4 signalling can trigger the activation and recruitment of neutrophils and macrophages that can kill or limit the spread of infection at an early stage, allowing time for the adaptive immune response to develop.

Receptors that recognize PAMPs are found both outside and inside cells. As a group, these innate immune system receptors survey the environment for everything from viruses to fungal infections. The scope of this sensing network is an indication of the importance that an early reaction to infection plays in the survival of its victim. Innate immune responses slow infections down, giving the rest of the immune system time to catch up. Disorders of the innate immune response cause ‘autoinflammatory’ diseases, often manifesting as spontaneous bouts of illness and fever.

The discrimination of appropriate targets that require an immune response from those that do not is the key to immunity. Unleashing the immune system is a risky business. If the reaction is too strong, it can kill its host. If it is not strong enough, the infection may do the same. To carry out this difficult balancing act, the immune system comes with many checks that operate as the response to a pathogen progresses. One of the first mechanisms is a process of double-checking, before even starting to respond, called ‘co-stimulation’. The need for co-stimulation was deduced once it was understood that the immune system could generate antigen-specific receptors at random and throughout life. By continuously making new and unique T-cells and B-cells, there is a constant risk of producing an autoreactive

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