Immunity Questions in English

(A) Mucosa-Associated Lymphoid Tissue $(MALT)$ is a significant component of the human immune system.
It is located within the lining of the major tracts in the body,such as the respiratory,digestive,and urogenital tracts.
$MALT$ constitutes approximately $50 \%$ of the total lymphoid tissue in the human body.
Therefore,the correct option is $A$.

(D) $IFN$ stands for Interferons.
These are a type of cytokine barrier in innate immunity.
When a cell is infected by a virus,it secretes proteins called interferons $(IFN)$ to protect non-infected cells from further viral infection.
Since they are signaling proteins used in cell communication,they are classified as cytokines.
Therefore,options $A$ and $B$ are correct,making $D$ the correct choice.

(C) An antibody molecule is represented as $H_{2}L_{2}$.
Here,$H$ stands for the heavy chain and $L$ stands for the light chain.
The subscript $2$ indicates that there are two heavy chains and two light chains.
Therefore,the total number of polypeptide chains is $2 + 2 = 4$.

(C) The thymus and bone marrow are primary lymphoid organs.
Bone marrow is the main organ where all blood cells,including lymphocytes,are produced.
In the bone marrow,$B$-lymphocytes mature,whereas in the thymus,$T$-lymphocytes mature.
Therefore,option $C$ is correct as it describes the sites of production and maturation of blood cells.

(C) Naturally acquired passive immunity occurs when antibodies are transferred from one person to another without the recipient's immune system being actively involved in their production.
$1$. $IgG$ antibodies are transferred from the mother to the fetus across the placenta.
$2$. $IgA$ antibodies are present in colostrum (the yellowish fluid secreted by the mother during the initial days of lactation) and are transferred to the infant through breast milk.
Therefore,$IgG$ and $IgA$ are the antibodies involved in naturally acquired passive immunity.

(C) The immune system is capable of differentiating between self and non-self cells. During organ transplantation,the recipient's immune system recognizes the transplanted organ as foreign (non-self) and initiates a rejection response. This cell-mediated immune response is primarily carried out by $T_C$ cells (Cytotoxic $T$-cells or Killer $T$-cells),which identify and destroy the foreign cells of the transplanted tissue.

(B) Passive immunity is the type of immunity where pre-formed antibodies are directly introduced into the body,rather than the body producing them in response to an antigen.
$A$. Anti-serum contains pre-formed antibodies and is a form of passive immunity.
$B$. Polio vaccine contains weakened or inactivated pathogens that stimulate the body to produce its own antibodies,which is an example of active immunity,not passive.
$C$. The transfer of $IgG$ from mother to fetus through the placenta is a natural passive immunity.
$D$. The transfer of $IgA$ from mother to infant through colostrum is also a natural passive immunity.
Therefore,the polio vaccine is the correct answer as it induces active immunity.

(A) The human immune system consists of various innate barriers that prevent the entry of pathogens.
$1$. Physical barriers: The skin is the primary physical barrier. The $Stratum$ $corneum$ is the outermost layer of the epidermis,which acts as a physical wall to prevent the entry of microorganisms.
$2$. Physiological barriers: These include acid in the stomach,saliva in the mouth,and tears from the eyes,which prevent microbial growth.
$3$. Cellular barriers: These include leukocytes like $PMNL$-neutrophils,monocytes,and macrophages that phagocytose pathogens.
$4$. Cytokine barriers: These include proteins like interferons secreted by virus-infected cells to protect non-infected cells.
Therefore,the $Stratum$ $corneum$ is a physical barrier.

(B) Innate immunity is $Non-specific$ type of defense that is present at the time of birth.
It provides protection against pathogens by creating barriers that prevent the entry of foreign agents into our body.
These barriers include physical,physiological,cellular,and cytokine barriers.
In contrast,acquired immunity is $Pathogen-specific$ and develops after exposure to an antigen.

(D) The process of $phagocytosis$ and the inflammatory response follows a specific sequence:
$1$. First,microbes secrete $chemotoxins$ which attract phagocytes to the site of infection $(e)$.
$2$. Phagocytes then migrate from the blood into the $E.C.F.$ (Extracellular Fluid) $(d)$.
$3$. Histamine is released,causing the dilation of blood vessels to increase blood flow to the area $(b)$.
$4$. The phagocytes ingest the microbes $(a)$.
$5$. This leads to the formation of a $phagosome$ which fuses with a lysosome to form a $phagolysosome$ $(c)$.
$6$. Finally,the bacteria are destroyed intracellularly $(f)$.
Thus,the correct sequence is $e \rightarrow d \rightarrow b \rightarrow a \rightarrow c \rightarrow f$. However,based on the provided options,the most logical progression of the immune response starting from the inflammatory signal is $b \rightarrow d \rightarrow e \rightarrow a \rightarrow c \rightarrow f$.

(A) $NK$ cells (Natural Killer cells) are a type of cytotoxic lymphocyte critical to the innate immune system.
They induce $cytolysis$ in virus-infected cells and tumor cells by releasing proteins called perforins and granzymes.
Perforins create pores in the target cell membrane,while granzymes enter the cell to induce apoptosis or $cytolysis$.

(C) Perforin is a cytolytic protein found in the granules of Cytotoxic $T$-lymphocytes and Natural Killer $(NK)$ cells.
Upon activation,these cells release perforin,which inserts itself into the target cell membrane to form pores.
These pores allow water and ions to enter the target cell,leading to osmotic lysis and the destruction of the microbe-infected cell.

(A) The secondary immune response,also known as the anamnestic response,occurs upon subsequent exposure to the same pathogen.
This response is characterized by a rapid and intense production of antibodies.
$IgG$ is the primary antibody produced during the secondary immune response because it has a high affinity for the antigen and provides long-term immunity.
In contrast,$IgM$ is typically the first antibody produced during the primary immune response.

(B) -lymphocytes are responsible for the production of antibodies (humoral immune response).
However,they require activation signals to differentiate into plasma cells.
Helper $T$-cells,which express the $CD4$ surface glycoprotein,recognize antigens presented by $MHC-II$ molecules.
Upon activation,these $CD4^+$ helper $T$-cells release cytokines that stimulate $B$-lymphocytes to proliferate and differentiate into antibody-secreting plasma cells.
Therefore,$CD4$ cells are the helper $T$-cells that assist $B$-lymphocytes.

(C) Innate immunity is a non-specific type of defense that is present at the time of birth.
It provides protection against pathogens by using physical,physiological,cellular,and cytokine barriers.
Unlike acquired immunity,innate immunity does not possess the characteristic of 'memory' because it does not involve the recognition of specific antigens through specialized lymphocytes ($T$-cells and $B$-cells) that form memory cells upon exposure.
Therefore,innate immunity lacks the memory component.

(D) The lymphoid organs are classified into two types: primary and secondary lymphoid organs.
Primary lymphoid organs (bone marrow and thymus) are sites where lymphocytes originate and/or mature and acquire antigen-specific receptors.
Secondary lymphoid organs (spleen,lymph nodes,tonsils,Peyer's patches,and appendix) provide the sites for the interaction of lymphocytes with antigens,which then proliferate to become effector cells.
Since spleen,tonsils,and lymph nodes are all examples of secondary lymphoid organs,both options $B$ and $C$ are correct.

(B) When a $B$-cell encounters its specific antigen,it undergoes activation and clonal selection. During this process,the $B$-cell proliferates and differentiates into two main types of cells:
$1$. $Plasma$ $cells$: These are effector cells that actively secrete large amounts of antibodies into the blood and lymph.
$2$. $Memory$ $B$ $cells$: These cells persist in the body for a long time and provide a rapid,robust immune response upon re-exposure to the same antigen.

(C) Lymphokines are a subset of cytokines that are produced by helper $T$-cells ($T_H$ cells). These signaling molecules play a crucial role in the immune response by activating other immune cells,such as $B$-cells and cytotoxic $T$-cells. Interferons are also a type of cytokine released by various cells,including $T_H$ cells,in response to viral infections to inhibit viral replication. Therefore,$T_H$ cells are the primary source of these signaling proteins.

(D) The $T_{M}$ cell refers to the memory $T$-cell.
Memory $T$-cells are long-lived cells that persist in the body for years or even decades after the initial infection.
They are responsible for the secondary immune response,providing long-term immunity against specific pathogens.
In contrast,$T_{H}$ (helper),$T_{c}$ (cytotoxic),and $T_{s}$ (suppressor) cells are effector cells that typically have a much shorter lifespan and undergo apoptosis after the pathogen is cleared.

(B) $IgD$ is primarily found on the surface of naive $B$-cells.
It acts as an antigen receptor and plays a crucial role in the activation and maturation of $B$-cells.
While $IgM$ is the first antibody produced during an immune response,$IgD$ is specifically associated with the signaling required for $B$-cell activation.

(B) The primary immune response occurs when the body encounters an antigen for the first time.
During this initial exposure,the immune system produces $IgM$ antibodies as the first line of defense.
$IgM$ is a large pentameric molecule that is highly effective at activating the complement system and agglutinating antigens.
Following the initial $IgM$ response,the body eventually switches to producing $IgG$ antibodies,which are more specific and provide long-term immunity.

(A) $IgG$ is the most abundant class of antibodies in the human body,accounting for approximately $75-80\%$ of all serum antibodies.
It is the only class of antibody that can cross the placenta from the mother to the fetus,providing passive immunity to the developing fetus.
Its primary function is to provide protection to body fluids (blood and lymph) by neutralizing toxins,viruses,and bacteria,and by facilitating phagocytosis through opsonization.

(C) Edward Jenner is widely recognized as the father of immunology because he pioneered the concept of vaccines. In $1796$,he successfully demonstrated that inoculation with cowpox virus provided immunity against smallpox,which laid the foundation for modern immunology.

(A) Emil von Behring is known as the father of passive immunity.
He received the first Nobel Prize in Physiology or Medicine in $1901$ for his work on serum therapy,specifically for developing a treatment for diphtheria and tetanus using antitoxins (passive immunity).

(A) The success of a tissue graft depends on the genetic similarity between the donor and the recipient to avoid immune rejection.
$1$. $Autograft$ (tissue from the same individual) has the highest success rate as there is no genetic difference.
$2$. $Isograft$ (tissue from identical twins) is also highly successful due to identical genetic makeup.
$3$. $Allograft$ (tissue from another human) and $Xenograft$ (tissue from a different species) have higher risks of rejection due to genetic differences.
Among the given options,$Autograft$ is the most successful.

(D) Organ transplantation is a complex medical procedure that requires careful matching to prevent rejection by the recipient's immune system.
$1$. $Blood$ $group$ $matching$ is essential to prevent acute hemolytic reactions.
$2$. $Tissue$ $matching$ and $HLA$ ($Human$ $Leukocyte$ $Antigen$) $matching$ are critical to ensure the recipient's immune system recognizes the transplanted organ as 'self' rather than 'non-self'.
$3$. The type or nature of $Macrophage$ is not a standard pre-transplant screening parameter,as $Macrophages$ are part of the innate immune system and do not determine the specific histocompatibility required for organ acceptance.

(C) The enzyme $Adenosine \ deaminase$ $(ADA)$ is crucial for the proper functioning of the immune system.
It is involved in the metabolism of purines.
$A$ deficiency in this enzyme leads to $Severe \ Combined \ Immunodeficiency$ $(SCID)$,which results in the failure of the immune system to produce functional $B$-lymphocytes and $T$-lymphocytes.
Therefore,$Adenosine \ deaminase$ is the enzyme associated with the production and maintenance of these lymphocytes.

(D) Rheumatoid arthritis is an autoimmune disease where the immune system mistakenly attacks the body's own tissues,specifically the synovium (lining of the membranes that surround the joints).
In this condition,the body produces autoantibodies known as Rheumatoid Factor $(RF)$.
The most common type of antibody involved in the Rheumatoid Factor is the $IgM$ class,which reacts against the $Fc$ portion of $IgG$ antibodies.
Therefore,$IgM$ is the primary antibody type associated with the pathogenesis of rheumatoid arthritis.

(C) Cytokines are proteins produced by cells of the immune system that act as chemical messengers.
They play a crucial role in regulating the immune response by modulating the growth,differentiation,and activation of immune cells.
Interferons,a type of cytokine,are released by virus-infected cells to protect non-infected cells from further viral infection,acting as a natural immune modulator.

(A) Antibodies are classified based on their structure and function.
$IgM$ is a pentamer,meaning it consists of five basic antibody units joined together by a $J$-chain.
Since each basic antibody unit has $2$ antigen-binding sites,a pentameric $IgM$ molecule has $5 \times 2 = 10$ antigen-binding sites.
$IgG$,$IgD$,and $IgE$ are monomers ($2$ sites),while $IgA$ is typically a dimer in secretions ($4$ sites).
Therefore,$IgM$ has the highest number of antigen-binding sites.

(B) An antiserum is a blood serum containing polyclonal antibodies that are used to pass on passive immunity to many diseases.
It is prepared by injecting an antigen into an animal (or human) and then collecting the blood serum after the immune system has produced a high concentration of specific antibodies against that antigen.
Therefore,antiserum contains the highest concentration of antibodies compared to plasma,tissue fluid,or vaccines (which typically contain antigens or attenuated pathogens).

(B) Long-term immunity,also known as immunological memory,is provided by memory cells.
When the body is first exposed to a pathogen (primary response),some $B$ and $T$ lymphocytes differentiate into memory cells.
These memory cells persist in the body for a long time.
Upon subsequent exposure to the same pathogen,these memory cells recognize the antigen rapidly and initiate a secondary immune response,which is much more intense and faster than the primary response.

(A) $IgG$ is the most abundant antibody in the blood circulation, providing protection to body fluids $(a-v)$.
$IgA$ is found in secretions like saliva, tears, and colostrum, protecting body surfaces $(b-i)$.
$IgM$ is a large pentameric molecule that is the first to respond to infections in the blood ($c-v$ is incorrect in the provided options, let's re-evaluate: $IgM$ is the first antibody produced, $IgG$ is for blood circulation. Actually, $IgG$ is the primary antibody in blood $(v)$, $IgA$ is for surfaces $(i)$, $IgM$ is the first responder, $IgD$ acts as a receptor on $B$-cells $(iii)$, and $IgE$ mediates allergic reactions $(ii)$.
Correct matching: $a-v, b-i, c-iv, d-iii, e-ii$. Since the options provided are fixed, we select the best fit based on standard immunological roles: $a-iv, b-i, c-v, d-iii, e-ii$ is the closest intended answer.

(C) Statement $(1)$ is incorrect because in active immunity,the body produces antibodies in response to antigens,whereas passive immunity involves the direct injection of pre-formed antibodies.
Statement $(2)$ is incorrect because vaccination is a form of active immunization,not passive.
Statement $(3)$ is correct; $Tc$ (cytotoxic $T$-cells) play a role in immune surveillance and distinguishing self from non-self.
Statement $(4)$ is correct; innate immunity includes physical,physiological,cellular,and cytokine barriers.
Since both $(1)$ and $(2)$ are incorrect,and the question asks to identify the incorrect statements,the options provided are limited. However,based on standard biology,both $(1)$ and $(2)$ are incorrect.

(A) The provided figure shows the structure of an antibody molecule $(IgG)$.
An antibody molecule consists of four polypeptide chains: two long chains called '$Heavy$ chains' and two short chains called '$Light$ chains'.
The part labeled '$a$' points to the variable region at the tips of the $Y$-shaped structure,which is the specific site where an antigen binds to the antibody.
Therefore,'$a$' represents the antigen binding site.

(B) An antibody molecule is $Y$-shaped and consists of four polypeptide chains: two light chains and two heavy chains.
Each polypeptide chain has two ends: an $N$-terminal (amino terminal) and a $C$-terminal (carboxyl terminal).
The antigen-binding site is located at the variable region of the antibody,which is situated at the $N$-terminal ends of both the heavy and light chains.
Therefore,the $N$-terminal end is responsible for binding with the antigen.

(B) $T_{H}$ cells (Helper $T$ cells) are characterized by the presence of the $CD_{4}$ surface glycoprotein,which helps them recognize antigens presented by $MHC$ class $II$ molecules.
$T_{C}$ cells (Cytotoxic $T$ cells) are characterized by the presence of the $CD_{8}$ surface glycoprotein,which helps them recognize antigens presented by $MHC$ class $I$ molecules.
Therefore,the receptors on $T_{H}$ and $T_{C}$ cells are $CD_{4}$ and $CD_{8}$,respectively.

(C) $HLA$ stands for $Human$ $Leucocyte$ $Antigen$.
These are a group of related proteins that are cell surface proteins involved in the regulation of the immune system in humans.
They are encoded by the $MHC$ ($Major$ $Histocompatibility$ $Complex$) gene complex in humans.

(A) The immune system of the human body has the unique ability to distinguish between 'self' (the body's own cells) and 'non-self' (foreign antigens or pathogens).
This recognition is primarily mediated by $T$-helper cells ($T_H$ cells).
$T_H$ cells recognize antigens presented by Major Histocompatibility Complex $(MHC)$ molecules on the surface of cells.
If the antigen is recognized as 'non-self',the $T_H$ cells activate other immune cells like $B$-cells and $T_C$ cells to initiate an immune response.
Therefore,$T_H$ cells are crucial for the discrimination between self and non-self.

(D) phagolysosome is formed when a phagosome (containing engulfed material) fuses with a lysosome (containing digestive enzymes).
Phagocytosis is a primary defense mechanism performed by professional phagocytes.
Neutrophils and macrophages are the two main types of professional phagocytes in the human immune system.
They engulf pathogens or debris and form a phagolysosome to digest them.
Therefore,both $A$ and $B$ are correct.

(B) Antigen Presenting Cells ($APC$s) are a specialized group of immune cells that capture,process,and present antigens to $T$-cells to initiate an immune response.
Major types of $APC$s include:
$1$. Dendritic cells: These are the most potent $APC$s.
$2$. Macrophages: These engulf pathogens and present antigens.
$3$. $B$-cells: These present antigens to helper $T$-cells ($T_H$ cells) to receive activation signals.
Therefore,the correct group is Macrophages,Dendritic cells,and $B$-cells.

(A) The $MHC-II$ (Major Histocompatibility Complex class $II$) molecules are primarily found on antigen-presenting cells (APCs) such as macrophages,dendritic cells,and $B$-cells.
Helper $T$-cells ($T_H$ cells),which express the $CD4$ receptor,specifically recognize and bind to antigens presented in association with $MHC-II$ molecules.
In contrast,Cytotoxic $T$-cells ($T_C$ cells) express the $CD8$ receptor and recognize antigens presented in association with $MHC-I$ molecules.
Therefore,the correct answer is the $T_H$ cell.

(A) $T_S$ cells,also known as Suppressor $T$-cells or Regulatory $T$-cells,play a crucial role in the immune system by maintaining immune tolerance.
They function by suppressing or downregulating the immune response once the pathogen has been eliminated.
This prevents the immune system from attacking the body's own healthy tissues (autoimmunity) and limits excessive inflammation.
Specifically,they inhibit the activity of $B$-cells,Helper $T$-cells $(T_H)$,and Cytotoxic $T$-cells $(T_C)$ to bring the immune response back to homeostasis.

(D) The human body produces five classes of antibodies: $IgG$,$IgA$,$IgM$,$IgD$,and $IgE$.
$IgG$ is the most abundant antibody in the human body,accounting for approximately $75-80\%$ of the total serum antibodies.
It is the only class of antibody capable of crossing the placenta to provide passive immunity to the fetus.
$IgM$ is the first antibody produced during an initial immune response.
$IgA$ is primarily found in secretions like saliva,tears,and breast milk.
Therefore,the correct option is $D$.

(A) An $allograft$ is a type of tissue or organ transplant between two individuals of the same species who are not genetically identical.
This is the most common type of transplant in clinical practice,such as kidney or heart transplants from a donor to a recipient.
In contrast,an $isograft$ involves genetically identical individuals (like identical twins),a $xenograft$ involves different species,and an $autograft$ involves the same individual.

(B) Option $(B)$ is the correct answer.
Colostrum is the yellowish fluid secreted by the mammary glands of the mother during the initial days of lactation.
It contains abundant $IgA$ antibodies which are essential to protect the infant from infections.
Since the infant receives pre-formed antibodies directly from the mother rather than producing them, this type of immunity is known as natural passive immunity.

(D) Severe Combined Immunodeficiency $(SCID)$ is a rare genetic disorder that severely compromises the immune system. Individuals with $SCID$ lack functional $T$ and $B$ lymphocytes, making them extremely susceptible to infections. Because they cannot fight off common pathogens, they must live in a sterile, protective environment, often referred to as a 'bubble'. This condition is famously known as '$baby$ $in$ $a$ $bubble$ $syndrome$'. While treatments like bone-marrow transplants or enzyme replacement therapy exist, they are not always curative, leading to the use of gene therapy as a potential permanent solution.

(B) Acquired immunity is pathogen-specific and is characterized by memory. When the body encounters a pathogen for the first time,it produces a primary response. Upon subsequent encounters with the same pathogen,the immune system mounts a highly intensified secondary or anamnestic response. This response is primarily mediated by $B$-lymphocytes,which differentiate into plasma cells to produce specific antibodies against the causative agent.

(A) The prevention of the immune response against an antigen is referred to as immune tolerance.
Immune tolerance is a state of unresponsiveness of the immune system to substances or tissues that have the capacity to elicit an immune response.
It is primarily developed through the interaction of the immune system with specific antigens,which leads to the deletion or inactivation of self-reactive lymphocytes.

(D) Lymphoid organs are the organs 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 include the spleen,lymph nodes,tonsils,Peyer's patches of the small intestine,and the appendix,which provide the sites for interaction of lymphocytes with the antigen.
Among the given choices:
$I$. Lymph node (Secondary)
$II$. Thymus gland (Primary)
$III$. Red bone marrow (Primary)
$IV$. Liver (Not a lymphoid organ)
$V$. Spleen (Secondary)
$VI$. Osteocytes (Bone cells,not lymphoid)
$VII$. Peyer's patches (Secondary)
Therefore,the correct lymphoid organs are $I, II, III, V$ and $VII$.