Immunity Questions in English

(D) Lymphoid organs are the sites where origin and/or maturation and proliferation of lymphocytes occur.
Primary lymphoid organs include the bone marrow and thymus,where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
Secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen,which then proliferate to become effector cells.
Examples of secondary lymphoid organs include the spleen,lymph nodes,tonsils,Peyer's patches of the small intestine,and the appendix.
Since the thymus is a primary lymphoid organ,it is not included in the secondary lymphoid organs.

(A) Antibodies are specialized proteins produced by the immune system to identify and neutralize foreign objects like bacteria and viruses.
In the human body,$B$-lymphocytes (or $B$-cells) are the specific type of white blood cells responsible for the production of antibodies.
When $B$-cells encounter an antigen,they differentiate into plasma cells,which then secrete large quantities of antibodies into the blood and lymph.
$T$-lymphocytes are involved in cell-mediated immunity but do not produce antibodies directly.
Leucocytes is a general term for all white blood cells,and Erythrocytes are red blood cells,which do not produce antibodies.

(B) An auto-immune disease occurs when the body's immune system fails to distinguish between 'self' and 'non-self' cells,leading to the destruction of healthy body tissues.
Rheumatoid arthritis is a classic example of an auto-immune disease where the immune system attacks the joints.
Typhoid and Malaria are infectious diseases caused by pathogens (Salmonella typhi and Plasmodium,respectively).
The Common cold is a viral infection caused by rhinoviruses.
Therefore,the correct option is $B$.

(C) Assertion $(A)$ is correct: Vaccines work on the principle of memory of the immune system. They induce the production of antibodies and generate memory $B$ and $T$ cells that recognize the pathogen upon subsequent exposure.
Reason $(R)$ is incorrect: In vaccination,either weakened (attenuated) or inactivated (dead) pathogens,or antigenic proteins of the pathogen,are introduced into the body,not 'activated' pathogens. Introducing activated (virulent) pathogens would cause the disease.

(A) Interferons $(IFNs)$ are a group of signaling proteins made and released by host cells in response to the presence of several viruses.
They are a type of cytokine that acts as an innate immune response.
When a cell is infected by a virus,it secretes interferons,which signal neighboring cells to heighten their antiviral defenses,thereby preventing the spread of the viral infection.
Therefore,they are classified as antiviral proteins.

(B) Interferons are a group of signaling proteins made and released by host cells in response to the presence of several viruses.
They act as a type of cytokine.
When a cell is infected by a virus,it secretes interferons,which signal neighboring cells to heighten their antiviral defenses,thereby inhibiting further viral multiplication.
Thus,they provide innate immunity against viral infections.

(C) Antibodies,also known as immunoglobulins,are specialized $Y$-shaped proteins produced by the immune system in response to the presence of antigens (foreign substances like bacteria or viruses).
These proteins recognize and bind to specific antigens to neutralize them or mark them for destruction by other immune cells.
Therefore,antibodies are classified as proteins.

(D) Antibodies,also known as immunoglobulins $(Ig)$,are specialized proteins produced by the immune system to identify and neutralize foreign objects like bacteria and viruses.
Structurally,antibodies are $Y$-shaped molecules composed of four polypeptide chains: two heavy chains and two light chains.
These polypeptide chains are covalently linked to carbohydrate moieties,making them glycoproteins.
Therefore,the correct answer is $D$.

(B) The immune system is primarily composed of specialized cells that recognize and eliminate pathogens.
Lymphocytes are the key cells of the immune system.
They are a type of white blood cell $(WBC)$ that includes $B$-lymphocytes and $T$-lymphocytes.
$B$-lymphocytes produce antibodies to fight pathogens,while $T$-lymphocytes help activate other immune cells and directly destroy infected cells.
Therefore,the correct option is $B$.

(B) Passive immunity was discovered by $Emil \text{ } von \text{ } Behring$ and $Shibasaburo \text{ } Kitasato$ in $1890$. They demonstrated that immunity could be transferred from an immunized animal to a non-immunized one through the serum, which contained antibodies. $Emil \text{ } von \text{ } Behring$ was awarded the first Nobel Prize in Physiology or Medicine in $1901$ for his work on serum therapy, particularly its application against diphtheria.

(C) Interferons $(IFNs)$ are a group of signaling proteins made and released by host cells in response to the presence of several pathogens,such as viruses,bacteria,parasites,or tumor cells.
However,they are most characteristically known for being produced by virus-infected cells to protect non-infected neighboring cells from further viral infection.
Therefore,among the given options,the synthesis of interferons is primarily associated with the response to viral infections.

(C) $T$-lymphocytes are a type of white blood cell that plays a central role in cell-mediated immunity.
They originate in the bone marrow and mature in the thymus gland.
There are three main functional types of $T$-lymphocytes:
$1$. Helper $T$-cells $(T_h)$: These activate other immune cells.
$2$. Cytotoxic $T$-cells $(T_c)$: These directly kill infected or cancerous cells.
$3$. Suppressor $T$-cells $(T_s)$: These regulate the immune response and prevent autoimmunity by suppressing the activity of other immune cells.
Therefore,option $C$ is the correct statement.

(C) Innate immunity consists of four types of barriers:
$1$. Physical barriers: Skin and mucus coating of the respiratory,gastrointestinal,and urogenital tracts.
$2$. Physiological barriers: Acid in the stomach,saliva in the mouth,and tears from the eyes prevent microbial growth.
$3$. Cellular barriers: Polymorphonuclear leukocytes ($PMNL$-neutrophils),monocytes,and natural killer cells in the blood,as well as macrophages in tissues.
$4$. Cytokine barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.
Therefore,saliva and tears are classified as physiological barriers.

(C) In the immune system,$T$-lymphocytes are a type of white blood cell that plays a central role in cell-mediated immunity. The letter '$T$' in '$T$-lymphocytes' stands for 'Thymus'. This is because these cells originate from stem cells in the bone marrow and then migrate to the thymus gland,where they mature and differentiate into functional $T$-cells.

(A) Tetanus is caused by the bacterium $Clostridium$ $tetani$,which produces a potent toxin. When a person is at risk of developing tetanus (e.g.,after a deep wound),they require immediate protection. This is achieved through passive immunization by administering preformed antibodies (antitoxins). This provides immediate immunity,which is crucial because the body does not have enough time to produce its own antibodies through active immunization.

(A) Statement $(i)$ is correct: The body's immune system recognizes the transplanted organ as 'non-self' or foreign,so the recipient must take immunosuppressants for life to prevent rejection.
Statement $(ii)$ is correct: The cell-mediated immune response,primarily driven by $T$-lymphocytes,is the main mechanism responsible for the rejection of transplanted organs (graft rejection).
Statement $(iii)$ is incorrect: $T$-lymphocytes,not $B$-lymphocytes,are responsible for graft rejection.
Statement $(iv)$ is incorrect: Graft rejection is based on the recognition of self vs. non-self antigens ($MHC$ molecules),not on interferons.
Therefore,statements $(i)$ and $(ii)$ are correct.

(A) The correct answer is $A$.
$1$. In the treatment of viper snake bites,'passive immunity' is utilized,where preformed antibodies (antivenom) are injected directly into the patient's body.
$2$. $T$-lymphocytes do not produce antibodies themselves; instead,they assist $B$-lymphocytes in the immune response.
$3$. An antibody molecule consists of two light chains and two heavy chains,not four light chains.
$4$. The rejection of a transplanted organ (graft rejection) is primarily mediated by cell-mediated immunity,which is governed by $T$-lymphocytes,not $B$-lymphocytes.

(A) The human body has innate immunity consisting of four types of barriers:
$1$. Physical barriers: Skin and mucus coating of the respiratory,gastrointestinal,and urogenital tracts.
$2$. Physiological barriers: Acid in the stomach,saliva in the mouth,and tears from eyes.
$3$. Cellular barriers: Polymorphonuclear leukocytes ($PMNL$-neutrophils),monocytes,and natural killer cells.
$4$. Cytokine barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells.
Option $A$ is correct because $PMNL$ and monocytes are examples of cellular barriers.
Option $B$ is incorrect because these are examples of passive immunity (pre-formed antibodies).
Option $C$ is incorrect because saliva and tears are physiological barriers,not physical.
Option $D$ is incorrect because mucus coating is a physical barrier,while $HCl$ is a physiological barrier.

(B) Cell-mediated immunity $(CMI)$ 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.
$T$-lymphocytes are responsible for $CMI$,whereas $B$-lymphocytes are responsible for humoral immunity (antibody-mediated immunity).
Therefore,the correct option is $B$.

(C) The body's immune system is capable of differentiating between 'self' and 'non-self' cells.
When an organ like a kidney is transplanted from a donor,the recipient's immune system recognizes the foreign tissue as 'non-self' due to the presence of different antigens on the surface of the donor cells.
This rejection is primarily mediated by $T$-lymphocytes,which are responsible for cell-mediated immunity $(CMI)$.
$CMI$ is the branch of the immune system that does not involve antibodies but rather the direct action of specialized cells to destroy foreign grafts or infected cells.
Therefore,the correct answer is cell-mediated immunity.

(B) The immune system of higher vertebrates has the ability to differentiate between self and non-self cells.
When the immune system fails to recognize self-cells and starts attacking them, it leads to a condition known as an $Autoimmune \text{ } disease$.
Examples include $Rheumatoid \text{ } arthritis$ and $Type-1 \text{ } diabetes$.

(B) The rejection of transplanted tissues or organs is primarily caused by the $T$-lymphocytes of the recipient's immune system.
This specific type of immune response is known as the $Cell-mediated$ $immune$ $response$ $(CMIR)$.
$T$-cells recognize the transplanted tissue as 'non-self' or foreign and initiate an attack to destroy the graft,leading to rejection.

(A) $MALT$ stands for Mucosa-Associated Lymphoid Tissue.
It is a major component of the immune system located within the lining of the mucosal surfaces of the body,such as the respiratory,digestive,and urogenital tracts.
It is estimated that $MALT$ constitutes about $50\%$ of the total lymphoid tissue in the human body.

(D) The rejection of a transplanted organ,such as a kidney graft,is primarily mediated by the recipient's immune system recognizing the donor tissue as foreign.
This process is driven by $T$-lymphocytes,which are the key components of the cell-mediated immune response.
Specifically,cytotoxic $T$-cells identify the $MHC$ (Major Histocompatibility Complex) antigens on the surface of the transplanted cells as non-self and initiate an attack to destroy the graft.
Therefore,the cell-mediated immune response is responsible for graft rejection.

(C) An antibody molecule is $Y-$shaped and consists of four polypeptide chains: two identical heavy $(H)$ chains and two identical light $(L)$ chains. The antigen-binding site,also known as the paratope,is located at the variable region of the antibody. This site is formed by the interaction between the variable domain of one heavy chain $(V_H)$ and the variable domain of one light chain $(V_L)$. Therefore,the antigen-binding site is found between one heavy and one light chain.

(A) The $Y$-shaped antibody molecule is composed of $4$ polypeptide chains: two identical light chains and two identical heavy chains,held together by disulphide bonds.
The antigen-binding site (paratope) is formed at the $N$-terminal end of the antibody molecule,where the variable region of a heavy chain and the variable region of a light chain come together.
Therefore,the antigen-binding site involves both the light chain and the heavy chain.

(C) An antigen is a foreign substance that enters the body and triggers the immune system to produce specific antibodies.
These antibodies then neutralize the antigen through an antigen-antibody reaction,which is a fundamental mechanism of the body's adaptive immunity.

(B) $IgM$ is a pentameric immunoglobulin,meaning it consists of five monomeric units joined together by a $J$-chain.
It is the largest antibody and is the first immunoglobulin to be produced during the primary immune response to an antigen.
$IgG$ is a monomer,$IgA$ is typically a dimer in secretions,and $IgE$ and $IgD$ are monomers.

(A) An autoimmune disorder occurs when the immune system mistakenly attacks the body's own healthy cells.
$A$. Myasthenia gravis is a chronic autoimmune neuromuscular disease that causes weakness in the skeletal muscles.
$B$. Osteoporosis is a condition where bones become weak and brittle due to loss of tissue.
$C$. Muscular dystrophy is a group of genetic diseases that cause progressive weakness and loss of muscle mass.
$D$. Gout is a form of arthritis caused by the accumulation of uric acid crystals in the joints.
Therefore,the correct option is $A$.

(A) The success of an organ transplant depends on the proper matching of histocompatibility antigens present on the surface of all body cells.
Because the transplanted tissue contains foreign antigens,it is recognized as 'non-self' by the recipient's immune system.
These antigens stimulate the recipient's $T-$cells and antibodies to attack the graft,leading to rejection.
Therefore,patients undergoing organ transplantation are administered immunosuppressive drugs to suppress the immune response and prevent the rejection of the transplanted tissue.

(C) The Assertion is correct because wounds can be a portal of entry for $Clostridium$ $tetani$ spores,which cause tetanus.
However,the Reason is incorrect. Anti-tetanus serum $(ATS)$ provides passive immunity by introducing pre-formed antibodies directly into the body,rather than stimulating the body to produce its own antibodies (which would be active immunity). Therefore,the correct choice is $C$.

(B) Antigens are substances that,when introduced into the body,stimulate the production of antibodies.
They are mostly proteins but can also be carbohydrates,lipids,or nucleic acids.
Every antigen possesses specific sites called antigenic determinants or epitopes,which allow the immune system to recognize them.
The recognition ability of the immune system (specifically innate immunity) is present from birth and does not require prior exposure to the antigen.
Therefore,both the Assertion and the Reason are correct,but the Reason does not explain why antigens have antigenic determinants; it describes the nature of the immune system's recognition capability.
Thus,the correct option is $B$.

(D) Interferons are not antibodies; they are a class of glycoproteins known as cytokines produced by cells in response to viral infections,not bacterial infections. Therefore,the Assertion is incorrect.
Interferons do not primarily stimulate inflammation at the site of injury; their main function is to inhibit viral replication and induce an antiviral state in neighboring cells. Therefore,the Reason is also incorrect.
Since both the Assertion and the Reason are false,the correct option is $D$.

(D) Antibodies isolated from the serum of hyperimmunized animals are typically polyclonal,meaning they are heterogeneous because they are produced by multiple $B$-cell clones in response to an antigen.
Conversely,monoclonal antibodies $(Mabs)$ are produced by a single clone of hybridoma cells,making them homogeneous immunological reagents with defined specificity.
Since the antibodies separated from serum are generally heterogeneous,the Assertion is incorrect,while the Reason is correct.

(C) Interferons are antiviral proteins produced by virus-infected cells. They protect non-infected cells from further viral infection by inhibiting viral protein synthesis.
While interferons can now be produced using genetic engineering,they are naturally synthesized by the body's cells in response to viral infections. Therefore,the claim that they can be synthesized 'only' by genetic engineering is false.
Thus,the Assertion is correct,but the Reason is incorrect.

(A) Artificially acquired passive immunity is a type of immunity where pre-formed antibodies or sensitized lymphocytes are introduced into the host's body.
This provides immediate protection but is short-lived because the host's immune system does not produce these antibodies itself.
Bone marrow transplantation involves the transfer of stem cells that can differentiate into immune cells,which is a form of adoptive immunotherapy.
Since the introduced cells (lymphocytes/stem cells) are produced outside the host and provide immunity,it is classified as artificially acquired passive immunity.
Therefore,both the Assertion and the Reason are correct,and the Reason correctly explains the Assertion.

(C) The innate immune system is present from birth and provides non-specific defense against pathogens.
It acts as the first line of defense through various barriers such as physical,physiological,cellular,and cytokine barriers.
Natural killer $(NK)$ cells are a type of lymphocyte that belongs to the innate immune system.
These cells are capable of recognizing and destroying infected or abnormal cells through phagocytosis and the release of cytotoxic granules,thereby providing immediate protection before the acquired immune system is activated.

(C) Passive immunity is defined as the immunity conferred by the introduction of pre-formed antibodies into the body.
Since these antibodies are produced outside the host (exogenous) and then administered,the host's own immune system is not actively involved in their production.
Therefore,the exogenous supply of antibodies provides passive immunity.
Examples include antibodies transferred from mother to fetus through the placenta $(IgG)$ or through colostrum $(IgA)$.

(N/A) Primary lymphoid organs are the sites where lymphocytes originate and/or mature and become antigen-sensitive. These include the $Bone \, marrow$ and the $Thymus$.
Secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells. These include the $Spleen$, $Lymph \, nodes$, $Tonsils$, $Peyer's \, patches$ of the small intestine, and $Appendix$.

(N/A) Differences between Innate and Acquired Immunity:

Innate Immunity	Acquired Immunity
$(1)$ It is present from birth.	$(1)$ It develops after birth upon exposure to pathogens.
$(2)$ It provides non-specific defense barriers.	$(2)$ It involves specific $T$-cells,$B$-cells,and antibodies.
$(3)$ Example: Skin,mucous membranes,fever.	$(3)$ Example: Vaccination,immunity after recovery from disease.

$(b)$ Differences between Active and Passive Immunity:

Active Immunity	Passive Immunity
$(1)$ Host produces antibodies in response to antigens.	$(1)$ Ready-made antibodies are directly introduced.
$(2)$ It is slow but long-lasting.	$(2)$ It is fast but temporary.
$(3)$ Example: Natural infection or vaccination.	$(3)$ Example: Colostrum $(IgA)$,anti-tetanus serum.

(N/A) An antibody molecule is $Y$-shaped and consists of four polypeptide chains: two small chains called light chains and two longer chains called heavy chains. The structure is represented as $H_2L_2$. The chains are held together by disulfide bonds $(-S-S-)$. Each antibody molecule has two antigen-binding sites located at the tips of the $Y$-shaped structure,which are specific to particular antigens.

(N/A) Our body is protected against most of these foreign agents (pathogens or antigens). The ability of the host to fight against these disease-causing organisms,conferred by the immune system,is called $immunity$.
Immunity is of two types:
$(i)$ $Innate$ $immunity$
$(ii)$ $Acquired$ $immunity$

(N/A) Innate immunity is a non-specific type of defense that is present at the time of birth. This is accomplished by providing different types of barriers to the entry of foreign agents into our body. Innate immunity consists of four types of barriers:
$(i)$ Physical barriers: Skin on our body is the main barrier which prevents the entry of micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal, and urogenital tracts also helps in trapping microbes entering our body.
$(ii)$ Physiological barriers: Acid in the stomach, saliva in the mouth, and tears from the eyes all prevent microbial growth.
$(iii)$ Cellular barriers: Certain types of leukocytes $(WBC)$ of our body like polymorpho-nuclear leukocytes $(PMNL-neutrophils)$, monocytes, and natural killer cells (a type of lymphocyte) in the blood, as well as macrophages in tissues, can phagocytose and destroy microbes.
$(iv)$ Cytokine barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.

(N/A) Acquired immunity is pathogen-specific. It is characterized by memory.
When our body encounters a pathogen for the first time,it produces a response called the primary response,which is of low intensity.
Subsequent encounters with the same pathogen elicit a highly intensified secondary or anamnestic response. This is due to the fact that our body retains a memory of the first encounter.
The primary and secondary immune responses are carried out by two special types of lymphocytes in our blood: $B$-lymphocytes and $T$-lymphocytes.
$B$-lymphocytes produce an army of proteins in response to pathogens to fight them; these proteins are called antibodies.
$T$-cells do not secrete antibodies themselves but help $B$-cells produce them.
Each antibody molecule has four peptide chains: two small 'light chains' and two longer 'heavy chains',represented as $H_2L_2$.
Different types of antibodies are produced in our body,such as $IgA, IgM, IgE,$ and $IgG$.
Because these antibodies are found in the blood,this response is called the humoral immune response.
The second type of acquired immune response is the cell-mediated immune response or cell-mediated immunity $(CMI)$,which is mediated by $T$-lymphocytes.
$CMI$ is responsible for graft rejection in organ transplants,as the body can differentiate between 'self' and 'non-self' cells.

(N/A) $(1)$ 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. Injecting microbes deliberately during immunization or infectious organisms gaining access to the body during natural infection induces active immunity.
$(2)$ Passive immunity: When ready-made antibodies are directly given to protect the body against foreign antigens or agents,it is called passive immunity. The yellowish fluid colostrum secreted by the mother during the initial days of lactation has abundant antibodies $(IgA)$ to protect the infants. The fetus also receives some antibodies $(IgG)$ from the mother through the placenta during pregnancy.

(N/A) The principle of vaccination or immunization is based on the property of 'memory' of the immune system. In vaccination,a preparation of antigenic proteins of pathogen or inactivated/weakened pathogen (vaccine) is introduced into the body.
The antibodies produced in the body against these antigens neutralize the pathogenic agents during actual infection. The vaccines also generate memory $B$ and $T$-cells that recognize the pathogen quickly on subsequent exposure and overwhelm the invaders with a massive production of antibodies.
If a person is infected with some deadly microbes to which quick immune response is required,as in tetanus,we need to directly inject the preformed antibodies or antitoxin (a preparation containing antibodies to the toxin).
Even in cases of snakebites,the injection given to the patient contains preformed antibodies against the snake venom. This type of immunization is called $passive$ $immunization$.
Recombinant $DNA$ technology has allowed the production of antigenic polypeptides of pathogen in bacteria or yeast. Vaccines produced using this approach have been made on a large scale and have been increasingly available for immunization,e.g.,Hepatitis-$B$ vaccine produced from yeast.

(N/A) Autoimmunity is the system of immune responses of an organism against its own healthy cells and tissues.
Any disease that results from such an aberrant immune response is termed an "autoimmune disease".