Assignment Code: BBCCT-127/TMA/2023

Course Code: BBCCT-127

Assignment Name: Immunology

Year: 2023

Verification Status: Verified by Professor

Part-(A)

Marks: 50

Q1) Write short note on the following:

Q1) (a) Natural killer cells.

Ans) NK cells are large, granular lymphocytes that kill tumour cells on their own, even if they haven't been exposed to the tumour before. Later, it became clearer that NK cells are a separate lineage of lymphocytes with roles like killing other cells and making cytokines. They make up between 5 and 10% of the body's lymphocytes. They are very important for killing virus-infected cells in the body, controlling early signs of cancer, and preventing graft rejection. They are also found in the placenta during pregnancy.

Q1) (b) Lymph nodes.

Ans) These organs are bean-shaped structures made up of reticular networks that are 80 percent full of lymphocytes, macrophages, and dendritic cells. At the points where lymphatic vessels meet, there are lymph nodes. They are the first organs to fight antigens that get into tissue spaces because they are found everywhere. Antigens are brought to these nodes by the lymphatic fluid that flows through them. The phagocytic and dendritic cell network inside these nodes will be exposed to these antigens.

Q1) (c) Interleukins.

Ans) Interleukins are a subset of cytokines, which are molecules that send messages between cells and change how cells act. Like other cytokines, interleukins are not stored in cells. Instead, they are quickly and briefly released when a stimulus, like an infectious agent, is detected. Once an interleukin is made, it goes to its target cell where it binds to a molecule on the surface of the cell called a receptor. This interaction sets off a chain of signals inside the target cell, which change how the cell acts.

Q1) (d) Adjuvant.

Ans) Adjuvants are molecules that don't cause an immune response on their own, but when injected with other substances, they make the immune response stronger. One example of an adjuvant is aluminium hydroxide. Adjuvants are often added to a vaccine to make it work better. They are injected with an antigen to help the immune system make antibodies that fight the antigen.

Q2) (a) How a pathogen is killed by oxygen dependent and oxygen independent pathways?

Ans) Key parts of the host's defence against bacterial disease are recognising the pathogen and then sending neutrophils to the site of the infection. Neutrophil recruitment is a multistep process that includes sending neutrophils from the bloodstream to faraway sites of infection and/or injury, getting neutrophils from bone marrow reserves, and, if needed, increasing the production of blood cells.

Neutrophils kill microorganisms that they have eaten in two ways: with and without oxygen. When bacteria that cause disease are eaten by cells, powerful antimicrobial ROS are made. These include superoxide radicals, hydrogen peroxide, hypochlorous acid, hydroxyl radicals, and chloramines. Also, cytoplasmic granules join with phagosomes that contain bacteria and add antimicrobial peptides and proteases to the vacuole lumen. So, the neutrophil's strong antimicrobial activity comes from a combination of highly proteolytic and degradative enzymes, cationic molecules, and reactive oxygen species.

Q2) (b) Illustrate the Properties of B and T-cell epitopes?

Ans) How well an antigen-binding site on B cell antibody molecules works as a B cell epitope depends on what kind of antigen it is. When B cells and antigens interact, they form a weak, non-covalent bond between an antibody, a membrane receptor, and an antigen. This is called a binary complex. So, for the binding site of the antibody and the epitope to form a binary complex, they need to have shapes that match. This criterion puts some limits on the way the epitope works.

T cells don't directly recognise soluble native antigens. But antigens can be recognised when they are changed into antigenic peptides and put on MHC molecules. The antigenic peptide that the T cell recognises forms a complex with the MHC molecule and the antigenic peptide and the T cell receptor. Most T cell epitopes are inside the cell and become antigenic peptides when they are processed inside APCs or changed self-cells.

Q3) Discuss classical pathway of complement activation.

Ans) Paul Ehrlich came up with the word "complement" to describe the part of blood serum that helps antibodies do their job. The complement system is a group of soluble and cell-bound proteins, mostly glycoproteins, that are made by hepatocytes in the liver, epithelial cells in the digestive tract and genitourinary tract, monocytes, and macrophages. They move around in the blood as inactive proenzymes, which are also called zymogens. After proteolytic cleavage of the inhibitory fragment exposes the active site, they change into active molecules. So, the complement system, also called the complement cascade, is a part of the immune system that helps antibodies and other immune cells fight off pathogens by causing inflammation and attacking the cell membrane of the pathogen.

Complement System Performs the following Functions:

1. Cells and pathogens like bacteria and viruses are killed by it.

2. Through opsonization, it helps the phagocytosis of different antigens on particles.

3. Complement receptors connect to many immune system cells. This causes certain cell functions, inflammation, and the release of substances that control the immune system.

4. It also helps get rid of immune complexes that build up in the body.

There are three established pathways of complement activation namely:

1. The Classical pathway

2. The Alternative pathway

3. The Lectin pathway

The Classical pathway starts when an antigen forms a soluble complex with an antibody or when an antibody binds to an antigen on a target, like a bacterium. In this case, it's important to know that the initiation step involves a C1 macromolecular complex.

Three parts make up the C1 macromolecular complex: 6 units of C1q, 2 units of Cir, and 2 units of C1s. This C1 complex binds to at least two Fc sites on the antibody with its globular heads. This keeps the C1 complex and antibody together in a stable way.

Q4) (a) Differentiate between Antigen and Immunogen.

Ans) The main difference between antigen and immunogen is that antigen is any structure that binds to parts of the immune system, like antibodies, B cells, and T cells, while immunogen is a type of antigen that can cause an immune response.

Antigen and immunogen are two types of molecules that stick to parts of the immune system. Antigens can be proteins, polysaccharides, lipids, or nucleic acids, while immunogens are usually proteins and large polysaccharides. But lipids and nucleic acids can also cause an immune response when they attach to proteins or large polysaccharides.

An antigen is a substance that binds to antibodies or receptors on the surface of B cells and T cells. An immunogen is an antigen that can cause an immune response. So, this is what makes antigen and immunogen different.

Q4) (b) Differentiate between Immunoglobulin G and Immunoglobulin M.

Ans) Antibodies protect the body from viruses, bacteria, and allergens. They are made by the immune system. Different immunoglobulins or antibodies are made by the body so that it can react to different particles. For example, there is a different antibody for mononucleosis than for chickenpox. In the case of autoimmune diseases, the body sometimes makes antibodies against itself by mistake, thinking that healthy tissues and organs are foreign particles.

Immunoglobulin M

1. These are primarily seen in lymph and blood fluid.

2. These are the first antibodies the body synthesises when it combats a new infection or to new “non-self” antigens which render a short-term protection.

3. It increases for a few weeks and then dips due to the initiation of production of IgG.

Immunoglobulin G

1. About 75%–80% of the immunoglobulins in the blood are IgG. This is the most common type of antibody.

2. They are visible in the blood and other body fluids.

3. They help keep viruses and bacteria from getting into the body.

4. After an immunisation or infection, it can take time for these antibodies to form.

5. These antibodies lay the groundwork for long-term protection against microorganisms.

6. These antibodies are the only ones that can get through a pregnant woman's placenta to protect her unborn child.

Q5) Describe ways of antibody diversification.

Ans) Human antibodies come from different stages of Ig (Immunoglobulin) development, such as the pre-immune repertoire, in which antibodies are made to check the body for foreign bodies, and the post-immune repertoire, in which antibodies against foreign antigens are chosen and matured. There are four main places where antibodies can be different in the pre-immune repertoire. The first source of pre-immune diversity is the recombination of VH, DH, and JH chains to make a functional VH chain and of VL and VJ chains to make a functional light chain. To make a functional VH gene, one of 39 functional VH genes is paired with one of 27 functional D genes and 6 functional JH chains. This is done by chance. Second, the junctional diversity created by the recombination process adds even more diversity to the VDJ joining process in 4 ways.

1. There are three open reading frames in the D genes that can be used to translate the genes in either direction, giving a total of six possible peptide fragments.

2. During the process of rearrangement, a hairpin is made, which, when broken, can lead to the addition or removal of N nucleotides, giving the DNA more variety.

3. As part of the VDJ joining process, N nucleotides can be added or taken away. During the recombination process, it is possible that the coding sequences for a few amino acid residues will be lost.

4. TdT (Terminal deoxynucleotidyl Transferase) can add or take away N nucleotides, especially on either side of the D segment in the VDJ junctions that make up CDR-H3 in the functional V-region. It is thought that these N changes can lead to more than 107 different types of CDR-H3, with CDR lengths ranging from a few amino acid residues to more than 25. Somatic rearrangements, combinatorial diversity within chains, inherent mutagenesis that happens during assembly, and the combination of heavy and light chains can lead to more than 1016 different antibodies before they are made.

   
   

Part-(B)

Marks: 50

Q6) Give an overview of T- cell development.

Ans) T cells are important for building and keeping up immune responses, homeostasis, and memories. T cells have a receptor that can recognise different antigens from pathogens, tumours, and the environment. This receptor also helps the immune system remember things and stay tolerant of itself. Many inflammatory and autoimmune diseases are also thought to be caused in large part by T cells. The in vivo functional role of T cells in immunity and immunopathology, as well as the underlying mechanisms, have been mostly figured out with the help of mouse models.

This has led to the development and improvement of immune-based cures and immunotherapies for humans. But the power and usefulness of mouse models for testing hypotheses depends on limiting the research to one type of infection or disease change over a set amount of time in sterile, pathogen-free conditions. Humans, on the other hand, are constantly exposed to both good and bad microorganisms and carry chronic pathogens, but they can live for many decades without getting major infections, even when they are very old. To figure out how the unique longevity and stability of human immunity works, T cells need to be studied in the complex environment of the human body, in different places, at different ages, and in many different people.

T lymphocytes come from progenitors in the bone marrow that move to the thymus to mature, be chosen, and then be sent out to the rest of the body. Peripheral T cells are made up of different subsets, such as naive T cells, which can respond to new antigens, memory T cells, which are activated by antigens in the past and keep long-term immunity up, and regulatory T (Treg) cells, which keep immune responses in check.

Q7) Write short note on the following:

(a) Role of MHC I and II.

Ans) Class I MHC molecules bind to peptides that are mostly made when the proteasome breaks down cytosolic proteins. The MHC I: peptide complex is then put into the outside of the cell's plasma membrane through the endoplasmic reticulum. The epitope peptide is stuck to parts of the class I MHC molecule that are outside of cells. So, the job of the class I MHC is to show cytotoxic T cells the proteins inside of cells. But class I MHC can also present peptides made from proteins from outside the body. This is called cross-presentation.

Like MHC class I molecules, MHC class II molecules are heterodimers. However, unlike MHC class I molecules, MHC class II molecules are made up of two homogeneous peptides, an A chain and a B chain, which are both written in the MHC. The sub-designations a1, a2, etc. refer to different domains within the HLA gene. Each domain is usually coded by a different exon within the gene, and some genes have even more domains that code for things like leader sequences, transmembrane sequences, etc. These molecules have parts outside of the cell, as well as a transmembrane section and a tail in the cytoplasm.

The A1 and B1 parts of the chains come together to make a membrane-distal peptide-binding domain, while the A2 and B2 parts, which are the only extracellular parts of the chains, come together to make a membrane-proximal immunoglobulin-like domain. The antigen-binding groove is made up of two -helix walls and a B-sheet. This is where the antigen or peptide binds.

Q7) (b) CD4+Helper T-cells.

Ans) CD4+T cells are very important for an effective immune response to pathogens that is well-controlled. Antigen-MHC complexes activate naive CD4+T cells, which then change into different subtypes based mostly on the cytokine environment of the microenvironment. Other subsets of T-helper cells have been found besides T-helper 1 and T-helper 2. These include T-helper 17, regulatory T cell, follicular helper T cell, and T-helper 9, each of which has a unique cytokine profile.

To differentiate into a certain phenotype, a set of cytokine signalling pathways must be activated, along with lineage-specific transcription factors and epigenetic changes at the right genes. The cytokines that the differentiated cells release are what make these cells do what they do. This paper will focus on how cytokines tell CD4+T cells to change and how a network of transcription factors makes this happen.

Q8) Discuss Type III and Type IV Type Hypersensitivity.

Ans) Type-lll hypersensitivity is an immune reaction that happens when the innate immune cells take too long to get rid of antigen-antibody complexes, causing them to build up in tissues and blood vessels. Immune complex disease is another name for this kind of hypersensitivity. When antigen-antibody complexes get stuck in tissues or blood vessels, the Fe region of IgG receptors calls neutrophils to the complex. This turns on the complement system and brings neutrophils to the complex. In the clinic, serum sickness and the Arthus reaction are linked to type-lll hypersensitivity reactions.

Type-IV hypersensitivity is also called delayed hypersensitivity or cell-mediated hypersensitivity. In this type, the immune reaction takes a few days to happen and is caused by lymphocytes, while in other types, antibodies cause the immune reaction. When allergens are in the body, they turn on T cells, which damage tissue through monocytes and macrophages. Grave's disease, which you will learn more about in the next section, contact dermatitis, some morbilliform reactions, and severe exfoliative dermatoses are all linked to type-V hypersensitivity. Depending on the type of T cell activated and the type of effector cells recruited, type-IV hypersensitivity can further be divided into four subtypes:

1. Type IVa: TH1 and monocytes are activated. Cytokines involved include IL-1, IL-2 and IFNY.

2. Type IVb: TH2 and eosinophils are activated. Cytokines involved include IL-3, IL-4 and IL-5.

3. Type IVc: CD8+ T cells are activated. Cytokines like granzyme B, perforin, Fas Ligands are involved.

4. Type IVd: Cells activated include CD4+, CD8+ T cells and neutrophils. Cytokine IL-8 and GM-CSF (Granulocyte macrophage-colony stimulating factor) are involved.

Q9) Differentiate between the following:

i) Endogenous and Exogenous antigen.

Ans) The main difference between exogenous and endogenous antigens is that exogenous antigens come from outside the body, while endogenous antigens are made inside the body. The main types of antigens in the body are exogenous and endogenous. They are sorted by where they came from. Also, exogenous antigens enter the body when they are eaten, breathed in, or injected, while endogenous antigens are made by the normal breakdown of cells.

Antigens that come from outside the body are called exogenous antigens, while antigens that come from inside the body are called endogenous antigens. Endogenous antigens are made by the cell as part of normal cell metabolism or when the cell is infected by bacteria or viruses. So, this is the main difference between antigens that come from outside the body and antigens that come from inside the body.

ii) DNA vaccines and Purified macromolecules as vaccines

Ans) DNA vaccination is a way to protect against disease by injecting plasmid DNA that codes for antigenic proteins into a person's muscle. This way, some cells will take up the DNA and directly make the antigenic protein that it codes for. Antigens are made by the cells and shown on their surfaces. In other words, the body's own cells become factories that make vaccines. They make the antigens that are needed to trigger both an antibody response and a cell-mediated response.

The surprising thing about a DNA vaccine that is injected is that it is expressed by muscle cells much better than in tissue culture. The DNA seems to either stay in an episomal form for a long time or become part of the chromosomal DNA. Some of the risks that come with attenuated or killed whole organism vaccines can be avoided with vaccines made of specific, purified macromolecules from pathogens.

Q10) Describe the Immunological basis of graft rejection.

Ans) Antibody-mediated, hyperacute vasculitis rejection can happen after a liver or other organ transplant in people who already have antibodies against the donor's major histocompatibility complex class I–encoded antigens. Most of the time, acute allograft rejection starts when a lot of the recipient's T cells recognise alloantigen from the donor. When MHC histoincompatible tissues are transplanted, they cause a strong immune response from T cells that is cytopathic. Donor alloantigen are processed by antigen-presenting cells in this way that rejection is caused by T cells. Donor and recipient APCs take in donor MHC molecules.

After processing inside the cell, MHC peptide fragments are sent to the T cells of the recipient. Antigen presentation happens when these antigenic fragments of peptides fit into a groove on the MHC molecules on the surface of the APC. Acute cellular rejection is the type of immune rejection that has been studied the most. Acute cellular rejection is characterised by a sudden loss of function in the allograft. A biopsy of the transplanted tissue shows that host T cells and other mononuclear leukocytes have invaded the transplanted tissue and that these cells have damaged the graft.

Even though immunosuppressive therapy is often used, acute rejection does happen sometimes. Acute cellular rejection involves both CD4+ and CD8+ T cells, but CD4+ T cells are mostly in charge of the rejection response. Even though CD4+ T cells are important in rejection, many activated CD8+ T cells and other mononuclear leukocytes enter the transplant at the time of rejection. It is not clear what their role is. B-cell infiltration is a sign that transplanted kidneys are being rejected quickly and severely.

During rejection, allografts also contain cells of the innate immune system, such as natural killer cells. The missing self-theory says that NK cells always have inhibitory receptors that are specific for self-MHC class I antigens. This is how they can recognise alloantigen, and their role in the rejection of bone marrow transplants has been known for a long time.