Immunity Questions in English

(N/A) The human immune system consists of lymphoid organs, tissues, cells, and soluble molecules like antibodies. Our immune system is unique in that it recognizes foreign antigens, responds to them, and remembers them. The immune system also plays an important role in allergic reactions, autoimmune diseases, and organ transplantation.
Lymphoid Organs: These are the organs where origin and/or maturation and proliferation of lymphocytes occur. The primary lymphoid organs are the bone marrow and the thymus, where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
After maturation, the lymphocytes migrate to secondary lymphoid organs like the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, and the appendix. The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells.
Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced. The thymus is a lobed organ located near the heart and beneath the breastbone. The thymus is quite large at the time of birth but keeps reducing in size with age and by the time puberty is attained, it reduces to a very small size. Both bone marrow and thymus provide micro-environments for the development and maturation of $T$-lymphocytes.
The spleen is a large, bean-shaped organ. It mainly contains lymphocytes and phagocytes. It acts as a filter of the blood by trapping blood-borne microorganisms. The spleen also has a large reservoir of erythrocytes.
Lymph nodes are small solid structures located at different points along the lymphatic system. Lymph nodes trap the microorganisms or other antigens, which happen to get into the lymph and tissue fluid. Antigens trapped in the lymph nodes are responsible for the activation of lymphocytes present there and cause the immune response.
There is lymphoid tissue also located within the lining of the major tracts (respiratory, digestive, and urogenital tracts) called Mucosa-associated lymphoid tissue $(MALT)$. It constitutes about $50$% of the lymphoid tissue in the human body.

Lymphoid organs are the organs where the origin,maturation,and proliferation of lymphocytes occur.
The primary lymphoid organs are the bone marrow and thymus,where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
After maturation,the lymphocytes migrate to secondary lymphoid organs such as the spleen,lymph nodes,tonsils,Peyer's patches of the small intestine,and the appendix.
The secondary lymphoid organs provide the sites for the interaction of lymphocytes with the antigen,which then proliferate to become effector cells.

(N/A) $B-$lymphocytes and $T-$lymphocytes are the two main types of lymphocytes involved in the adaptive immune system.
$1$. Origin and Maturation: Both originate from stem cells in the bone marrow. $B-$lymphocytes mature in the bone marrow,whereas $T-$lymphocytes migrate to the thymus gland to mature.
$2$. Function: $B-$lymphocytes are responsible for humoral immunity by producing antibodies. $T-$lymphocytes are responsible for cell-mediated immunity by directly attacking infected cells or activating other immune cells.
$3$. Response: $B-$lymphocytes recognize antigens directly,while $T-$lymphocytes recognize antigens only when presented by antigen-presenting cells (APCs) via the major histocompatibility complex $(MHC)$.

(B) The thymus gland is a primary lymphoid organ that plays a crucial role in the development of the immune system.
In old individuals,the thymus gland undergoes gradual degeneration (atrophy),which leads to a significant decrease in the production of thymosins.
Thymosins are peptide hormones essential for the differentiation of $T$-lymphocytes,which provide cell-mediated immunity.
Consequently,the reduction in thymosin levels results in a weakened immune response in elderly people.

(N/A) In higher vertebrates,the memory-based acquired immunity is based on the ability to differentiate between self and non-self (e.g.,pathogens). Although the exact mechanism is not fully understood,it is based on two corollaries:
$(i)$ Higher vertebrates can distinguish between foreign molecules and foreign organisms. Most of experimental immunology is based on this aspect.
$(ii)$ Sometimes,due to genetic or other unknown reasons,the body attacks its own cells,resulting in damage to the body. This is known as an $Auto-immune$ disease.

(N/A) Immunity: The overall ability of the host to fight the disease-causing organisms,conferred by the immune system,is called immunity.
$(b)$ Retrovirus: $A$ group of viruses that possess $RNA$ as their genetic material,which is enclosed within a protein coat and an envelope. They use the enzyme reverse transcriptase to convert their $RNA$ into $DNA$ inside the host cell.
$(c)$ Allergy: The exaggerated response of the immune system to certain antigens present in the environment is called allergy.

(N/A) $1.$ $\text{CMI}$: $\text{Cell-Mediated Immunity}$
$2.$ $\text{MALT}$: $\text{Mucosa-Associated Lymphoid Tissue}$
$3.$ $\text{AIDS}$: $\text{Acquired Immuno Deficiency Syndrome}$

(N/A) Both $B$-cells and $T$-cells are formed in the bone marrow through the process of hematopoiesis.
$B$-cells mature in the bone marrow,whereas $T$-cells migrate to the thymus to mature.
$B$-cells are responsible for humoral immunity (antibody-mediated immunity) by producing antibodies,while $T$-cells are responsible for cell-mediated immunity $(CMI)$ by activating phagocytes,antigen-specific cytotoxic $T$-lymphocytes,and releasing various cytokines.

(N/A) The mucus coating of the epithelium lining of the gut helps in trapping microbes entering the body.
Saliva in the mouth contains lysozyme,which kills the pathogens. Hydrochloric acid $(HCl)$ in the gastric juice secreted by the stomach prevents microbial growth.
All these barriers are part of the innate immunity,specifically physiological barriers.

(N/A) Interferons are natural proteins produced by the cells of the immune system in response to foreign agents such as viruses,tumor cells,and parasites.
They protect non-infected cells from further infection.
Interferons inhibit viral replication within host cells,activate natural killer cells and macrophages,and increase antigen presentation to lymphocytes.
They induce the resistance of host cells to viral infection.
When the antigen is presented to matching $T$-cells and $B$-cells,these cells multiply and remove the foreign substance.

(N/A) : Antigen binding site,
$B$ : Light chain,
$C$ : Heavy chain

(N/A) Lymph nodes are small,solid structures located at different points along the lymphatic system.
Lymph nodes trap microorganisms or other antigens that enter the lymph and tissue fluid.
Antigens trapped in the lymph nodes are responsible for the activation of lymphocytes present there,which leads to the production of antibodies.
This process initiates the immune response.
Additionally,they help in destroying cancer cells.

(A) Each antibody molecule is composed of four polypeptide chains: two identical heavy chains $(H)$ and two identical light chains $(L)$.
These chains are held together by disulfide bonds.
Due to the presence of two heavy chains and two light chains,the structure is represented as $H_2L_2$.

(B) When the body encounters a pathogen for the first time,it produces a response called the $Primary$ $response$.
$Primary$ $response$ is of low intensity.
- During this process,memory cells are formed.
- These memory cells are stored in the spleen and lymph nodes.
When the body encounters the same pathogen subsequently,the immune system recognizes it and elicits a $Secondary$ $response$ (also known as an $Anamnestic$ $response$).
$Secondary$ $response$ is of much higher intensity due to the rapid proliferation of these memory cells.

(N/A) The principle of vaccination is based on the property of 'memory' of the immune system.
In vaccination,a preparation of antigenic proteins of pathogens or inactivated/live but weakened pathogens is introduced into the body.
The antigens generate the primary immune response by producing antibodies. The vaccines also generate memory $B$-cells and $T$-cells.
When the vaccinated person is attacked by the same pathogens,the existing memory $B$-cells or $T$-cells recognize the antigen quickly and overwhelm the invaders with a massive production of lymphocytes and antibodies.
Hepatitis $B$ vaccine is produced from yeast.

(N/A) Organ transplantation is often the only remedy when vital organs like the heart,liver,or kidney fail to function.
Finding a suitable donor is critical because grafts from animals or unrelated humans are usually rejected by the recipient's immune system.
The body's immune system can distinguish between 'self' and 'non-self' cells. The cell-mediated immune response is primarily responsible for graft rejection.
Tissue matching and histocompatibility are essential for successful transplants,and patients typically require life-long immunosuppressants.
Identical twins share the same genetic makeup and histocompatibility antigens. Therefore,there is no risk of graft rejection,making them the ideal donors.

(N/A) An antigen is a foreign substance that elicits the formation of an antibody. An antibody is a protein that is synthesized by the immune system in response to the presence of an antigen. Antigen and antibody show a high degree of specificity in binding to each other. Two diagnostic kits based on antigen-antibody interaction are:
$(a)$ $ELISA$ (Enzyme-Linked Immunosorbent Assay) for the detection of $HIV$.
$(b)$ Pregnancy test kits (which detect $hCG$ hormone).

(D) The correct answer is $D$.
Active immunity is slow and takes time to give its full effective response because it involves the activation of the immune system and the production of antibodies by the host's body.
In contrast, passive immunity provides immediate relief as ready-made antibodies are introduced directly into the body.

(C) Active immunity is slow and takes time to give a full effective response. It is not fast. Passive immunity is fast because ready-made antibodies are directly introduced into the body. Therefore,statement $C$ is incorrect.

(B) The correct matching is as follows:
$(a)$ Main lymphoid organ: Bone marrow $(3)$, where all blood cells including lymphocytes are produced.
$(b)$ $MALT$ (Mucosa-Associated Lymphoid Tissue): It constitutes about $50\%$ of the lymphoid tissue in the human body $(4)$.
$(c)$ Lobe-like organ arranged near the heart: Thymus $(1)$, which is the site for $T$-lymphocyte differentiation.
$(d)$ Large bean-shaped organ: Spleen $(2)$, which acts as a reservoir of erythrocytes and filters the blood.
Therefore, the correct sequence is $(a-3), (b-4), (c-1), (d-2)$.

(A) The correct matching is as follows:
$(a)$ Physical barrier: The skin on our body is the main barrier which prevents the entry of microorganisms. Thus,$(a-1)$.
$(b)$ Physiological barrier: Acid in the stomach,saliva in the mouth,and tears from eyes prevent microbial growth. Thus,$(b-4)$.
$(c)$ Cellular barrier: Certain types of leukocytes (WBCs) like polymorphonuclear leukocytes ($PMNL$-neutrophils) and monocytes and macrophages in blood and tissues can phagocytose and destroy microbes. Thus,$(c-3)$.
$(d)$ Cytokine barrier: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection. Thus,$(d-2)$.
Therefore,the correct sequence is $(a-1, b-4, c-3, d-2)$.

(N/A) Allergy: It is an exaggerated or hypersensitive immune response to certain antigens present in the environment,such as dust,pollen,or animal dander. This response involves the release of chemicals like histamine and serotonin from mast cells.
Cell-mediated immunity $(CMI)$: This is an immune response that does not involve antibodies. Instead,it involves the activation of phagocytes,antigen-specific cytotoxic $T$-lymphocytes,and the release of various cytokines in response to an antigen. It is primarily responsible for the rejection of transplanted organs and defense against intracellular pathogens like viruses and certain bacteria.

(N/A) Active immunity: When a host is exposed to antigens,which may be in the form of living or dead microbes or other proteins,antibodies are produced in the host body. This type of immunity is called active immunity. It is slow and takes time to give its full effective response.
Passive immunity: When ready-made antibodies are directly given to protect the body against foreign agents,it is called passive immunity. This provides immediate relief,for example,the administration of tetanus antitoxin or antibodies received by a fetus from the mother through the placenta.

(N/A) Pathogens: Infectious disease-causing agents,such as bacteria,viruses,fungi,or parasites,are known as pathogens.
Immunity: The overall ability of the host body to fight against disease-causing organisms,conferred by the immune system,is called immunity.

(N/A) Innate immunity: It is a non-specific type of defense that is present at the time of birth. It provides protection against pathogens by various barriers such as physical,physiological,cellular,and cytokine barriers. It is always available to protect the body.
Acquired immunity: It is a pathogen-specific type of immunity. It is not present from birth but develops during an individual's lifetime after exposure to a specific pathogen or through vaccination. It is characterized by memory,meaning the body remembers the specific pathogen for a faster response upon subsequent encounters.

(B) -lymphocytes: These cells produce specific proteins called antibodies in response to pathogens and release them into the blood to neutralize them.
$T$-lymphocytes: These cells do not secrete antibodies themselves but help $B$-lymphocytes to produce antibodies and also mediate cell-mediated immunity.

(N/A) Antigen: These are substances that,when they enter the body,stimulate the production of antibodies. They are also known as immunogens.
Lymphoid organs: These are the organs where lymphocytes originate,proliferate,and mature. Examples include the bone marrow and thymus.

(N/A) $1$. Retrovirus: These are viruses that contain $RNA$ as their genetic material. They use the enzyme reverse transcriptase to convert their $RNA$ into $DNA$ inside the host cell.
$2$. Antibody: These are a class of proteins known as immunoglobulins $(Ig)$,which are produced by $B$-lymphocytes in response to specific antigenic stimulation to neutralize pathogens.

(A) The initial milk secreted by the mother during the first few days of lactation is called colostrum.
Colostrum is rich in essential nutrients and contains abundant antibodies,specifically $IgA$ (Immunoglobulin $A$).
These antibodies are crucial for providing passive immunity to the newborn infant,protecting them from various infections during their early life.

(A) The human body has several barriers to prevent the entry of pathogens.
Physical barriers are the first line of defense.
The skin and the mucous coating of the epithelium lining the respiratory,gastrointestinal,and urogenital tracts are the primary physical barriers.
Therefore,epithelial tissue acts as a physical barrier for immunity.

(A) Innate immunity consists of four types of barriers: physical,physiological,cellular,and cytokine barriers.
$1$. Cellular barriers include cells like Polymorphonuclear leukocytes ($PMNL$-neutrophils),monocytes,and natural killer $(NK)$ cells that phagocytose or destroy microbes.
$2$. Antibodies are part of the acquired (adaptive) immune system,which is produced by $B$-lymphocytes in response to specific antigens.
$3$. Therefore,antibodies secreted by lymphocytes are not part of the cellular barrier of innate immunity.

(C) Interferons are a type of cytokine barrier in innate immunity.
When a cell is infected by a virus,it secretes proteins called interferons.
These interferons protect the neighboring non-infected cells from further viral infection.
Therefore,virus-infected cells are the source of interferon secretion.

(B) When the body encounters a pathogen for the first time,it produces a $Primary \ response$ of low intensity.
When the same pathogen encounters the body for the second time,the body exhibits a highly intensified $Secondary \ response$ or $Anamnestic \ response$.
This response is based on the memory of the first encounter,which is maintained by memory $B$ and $T$ cells.

(B) The immune system is broadly classified into two types: $1$. Cell-mediated immunity $(CMI)$ and $2$. Humoral immunity (Antibody-mediated immunity).
$B$-lymphocytes are responsible for producing antibodies in response to pathogens,which circulate in the blood and lymph (the body fluids or 'humors').
Therefore,the immune response mediated by $B$-lymphocytes is known as Humoral immunity or Antibody-mediated immunity.
Cell-mediated immunity is primarily mediated by $T$-lymphocytes.

(B) The $H_{2}L_{2}$ molecule represents the basic structure of an antibody (immunoglobulin).
Antibodies are proteins produced by $B$-lymphocytes in response to pathogens.
These antibodies are secreted into the blood and lymph,which are the primary components of blood plasma.
Therefore,$H_{2}L_{2}$ molecules are primarily found in the blood plasma to circulate throughout the body and provide humoral immunity.

(C) The immune system is divided into two main types: cell-mediated immunity and humoral immunity.
Humoral immunity is mediated by antibodies present in the blood and lymph.
These antibodies are produced by $B$-lymphocytes in response to pathogens.
Therefore,antibodies are the key components responsible for the humoral immune response.

(C) $CMI$ stands for Cell-Mediated Immunity.
Cell-Mediated Immunity is primarily mediated by $T$-lymphocytes or $T$-cells.
$B$-cells are responsible for Humoral Immunity (antibody-mediated immunity).
Antibodies are the products of $B$-cells.
Interferons are cytokines that provide innate immunity against viral infections.
Therefore,$T$-cells are the key components of $CMI$.

(B) The rejection of a transplanted organ is primarily a result of the recipient's immune system recognizing the donor tissue as 'non-self'.
$1$. The primary mechanism involved in organ rejection is $Cell-Mediated Immunity$ $(CMI)$.
$2$. $T$-lymphocytes (specifically $T$-cytotoxic cells) recognize the foreign antigens on the surface of the transplanted organ cells.
$3$. These $T$-cells then attack and destroy the donor cells, leading to tissue damage and organ rejection.
$4$. While humoral immunity (antibodies) can also play a role, $Cell-Mediated Immunity$ is the dominant force in both acute and chronic rejection processes.

(D) Antigens are substances that can induce an immune response. Chemically,they are most commonly proteins or large polypeptides. However,they can also be composed of polysaccharides,lipids,or nucleic acids,especially when these molecules are conjugated with proteins. Therefore,antigens can consist of all the mentioned types of molecules. Thus,the correct option is $D$.

(D) Active immunity is the immunity developed by an individual's own immune system when it is exposed to antigens (such as living or dead microbes or other proteins).
When a pathogen enters the body,the immune system recognizes the foreign antigens and produces specific antibodies to neutralize them.
Option $A$ describes the primary immune response to a pathogen,and Option $C$ describes the production of antibodies against antigens,both of which are fundamental characteristics of active immunity.
Therefore,both $A$ and $C$ are correct descriptions of active immunity.

(C) Breastfeeding provides the infant with pre-formed antibodies,specifically $IgA$,which are transferred from the mother to the child. Since the infant's own immune system does not produce these antibodies in response to an antigen,this is a classic example of passive immunity. Passive immunity is defined as the transfer of active humoral immunity in the form of ready-made antibodies from one individual to another.

(B) Colostrum is the yellowish fluid secreted by the mother during the initial days of lactation. It contains abundant amounts of $IgA$ antibodies. These antibodies are crucial for providing passive immunity to the newborn infant,protecting them from various infections.

(A) The placenta acts as a barrier between the mother and the fetus,but it allows the passage of certain substances.
$IgG$ is the only class of antibodies that can cross the human placenta from the mother to the fetus.
This provides the fetus with passive immunity against various pathogens during the early stages of life.
$IgA$ is primarily found in colostrum (mother's milk) and provides immunity to the newborn after birth,not through the placenta.

(B) Passive immunity is a type of immunity where pre-formed antibodies are received by the body rather than being produced by the body's own immune system.
$A$. Transfer of antibodies through the placenta is a natural passive immunity.
$B$. Production of antibodies by the body against $HIV$ virus is an example of active immunity,as the body's immune system actively responds to the antigen.
$C$. Transfer of $IgA$ antibodies through colostrum/breastfeeding is a natural passive immunity.
$D$. Injection of antibodies (e.g.,anti-tetanus serum) is an example of artificial passive immunity.
Therefore,option $B$ is not included in passive acquired immunity.

(C) Vaccination is a process where a preparation of antigenic proteins of pathogen or inactivated/weakened pathogen (vaccine) is introduced into the body.
These antigens generate a primary immune response by producing antibodies in the body.
These antibodies neutralize the pathogenic agents during subsequent actual infection.
The vaccines also generate memory $B$-cells and $T$-cells that recognize the pathogen quickly on subsequent exposure.
Therefore,inactivated pathogens are the primary component introduced during vaccination.

(C) The treatment given against snake bites involves the injection of pre-formed antibodies (antivenom) into the patient's body.
Since the body does not produce these antibodies itself in response to an antigen,this is a form of passive immunity.
Passive immunity provides immediate relief by neutralizing the toxins present in the snake venom.

(C) Primary lymphoid organs are the sites where lymphocytes originate and/or mature and become antigen-specific. The primary lymphoid organs are the bone marrow and the thymus.
In these organs,immature lymphocytes differentiate into antigen-sensitive lymphocytes.
After maturation,the lymphocytes migrate to secondary lymphoid organs like the spleen,lymph nodes,tonsils,and Peyer's patches,where they interact with the antigen and then proliferate to become effector cells.
Therefore,the spleen,tonsils,and Peyer's patches are secondary lymphoid organs,not primary.

(C) Lymphocytes are the primary cells of the immune system.
They originate in the bone marrow.
Secondary lymphoid organs like the spleen,lymph nodes,tonsils,Peyer's patches of the small intestine,and appendix provide the sites for interaction of lymphocytes with the antigen,which then proliferate to become effector cells.
Therefore,the spleen is a secondary lymphoid organ where this interaction occurs.
While bone marrow and thymus are primary lymphoid organs where maturation occurs,the interaction with antigens typically happens in secondary lymphoid organs.

(C) The $Thymus$ is a primary lymphoid organ located near the heart and beneath the breastbone.
It plays a crucial role in the development and maturation of $T$-lymphocytes.
At the time of birth,the $Thymus$ is quite large in size.
However,as an individual grows older,the $Thymus$ gradually undergoes atrophy (involution),meaning it reduces in size and is replaced by adipose tissue.
Therefore,the correct answer is $Thymus$.

(B) $MALT$ stands for Mucosa-Associated Lymphoid Tissue.
It constitutes about $50\%$ of the lymphoid tissue in the human body.
It is located within the lining of the major tracts such as the respiratory,digestive,and urogenital tracts.