Immunity Questions in English

(A) Antibodies are specialized proteins produced by the immune system,specifically by $B$-lymphocytes (plasma cells),in response to the presence of antigens.
These antigens are typically foreign substances such as bacteria,viruses,or other pathogens that cause infections.
Therefore,antibodies function to identify and neutralize these pathogens,thereby protecting the body against infection.

(B) Cell-mediated immunity $(CMI)$ is primarily mediated by $T$-lymphocytes.
$B$-lymphocytes are responsible for humoral immunity by producing antibodies.
Erythrocytes (red blood cells) are involved in oxygen transport,and thrombocytes (platelets) are involved in blood clotting.
Therefore,$T$-lymphocytes are the cells responsible for cell-mediated immunity.

(B) An $Antigen$ is a molecule or molecular structure,such as may be present at the outside of a pathogen,that can be bound by an antigen-specific antibody or $B$-cell antigen receptor. The presence of antigens in the body triggers the immune system to produce antibodies,thereby eliciting an immune response.

(A) Antibodies are specialized proteins produced by plasma cells (differentiated $B$-lymphocytes) in response to an antigen.
These proteins belong to a class of serum proteins known as immunoglobulins $(Ig)$.
Chemically,antibodies are $Gamma$-globulins,which are glycoproteins that play a crucial role in the immune system by identifying and neutralizing foreign objects like bacteria and viruses.

(A) Active immunity is a type of immunity that develops when the immune system of an individual produces antibodies in response to the presence of an antigen.
This can occur naturally after an infection (disease) or artificially through vaccination.
Therefore,it is the resistance developed by the host's own immune system after exposure to a pathogen or antigen.

(C) Antibodies are specialized proteins produced by $B$-lymphocytes in response to pathogens.
These are also known as immunoglobulins $(Ig)$.
They have a structure composed of four polypeptide chains: two light chains and two heavy chains,held together by disulfide bonds.
Therefore,antibodies are essentially proteinaceous in nature.

(B) Killer $T-$cells,also known as Cytotoxic $T-$cells ($CD8^+ T-$cells),are responsible for destroying infected or cancerous cells.
They release proteins called perforins and granzymes.
Perforins create pores in the plasma membrane of the target cell,which leads to the lysis of the cell or allows granzymes to enter and induce apoptosis (programmed cell death).
Therefore,the correct option is $B$.

(B) The immune system consists of various types of lymphocytes that perform specific functions.
$T$-lymphocytes (or $T$-cells) are responsible for cell-mediated immunity.
Among these,Killer $T$-cells (also known as Cytotoxic $T$-cells) are specialized cells that directly recognize,attack,and destroy infected host cells or foreign antigens.
Helper $T$-cells,on the other hand,primarily function by secreting cytokines to activate other immune cells like $B$-cells and macrophages,rather than directly destroying antigens themselves.
$B$-cells are primarily involved in humoral immunity by producing antibodies.

(A) Opsonization is a process in which pathogens or antigens are coated with molecules such as antibodies (opsonins).
This coating process makes the antigens more visible and easier for phagocytic cells,such as macrophages and neutrophils,to recognize and engulf them.
Once the phagocyte binds to the opsonized antigen,it proceeds to digest the pathogen,thereby enhancing the immune response.

(C) The condition where an individual lacks both functional $T-$ cells and $B-$ cells is known as $SCID$ (Severe Combined Immunodeficiency).
$SCID$ is a genetic disorder characterized by the disturbed development of functional $T-$ cells and $B-$ cells,which results in a severely compromised immune system.
Because these individuals have no adaptive immune response,they are extremely susceptible to infections and must be kept in a sterile,germ-free environment (often referred to as a 'bubble') to survive.

(B) Active immunity is developed when the host's immune system produces antibodies in response to an encounter with antigens,such as living or dead microbes or other proteins.
In the case of vaccination or natural infection,the body is exposed to weakened or killed pathogens (germs).
This exposure triggers the immune system to recognize the pathogen and produce specific antibodies and memory cells,providing long-term protection.
Therefore,active immunity is obtained by the infection of weakened germs or through vaccination.

(D) $IgM$ is the largest immunoglobulin in size. It typically exists as a pentamer,meaning it consists of five monomeric units joined together by a $J$-chain. Due to this pentameric structure,it has a high molecular weight and is the first antibody produced during an initial immune response.

(A) When pre-formed antibodies (such as globulins) are injected directly into a person's body to provide immediate protection against a specific pathogen like the hepatitis virus,it is known as artificially acquired passive immunity.
This is 'passive' because the person's own immune system does not produce the antibodies.
It is 'artificially acquired' because the antibodies are introduced through medical intervention (injection) rather than through natural processes like placental transfer or breastfeeding.

(C) An autoimmune disease occurs when the immune system mistakenly attacks the body's own healthy cells and tissues.
Rheumatoid arthritis is a classic example of an autoimmune disorder where the immune system attacks the synovial membranes of the joints.
Cancer is a disease of uncontrolled cell division.
Asthma is an allergic response to environmental allergens.
Erythroblastosis foetalis is a condition caused by $Rh$ incompatibility between the mother and the fetus.

(D) Interferons are a type of cytokine barrier in innate immunity.
They are proteins secreted by virus-infected cells.
These proteins protect non-infected cells from further viral infection.
Since the options provided do not include viruses,the correct answer is $D$ (None of the above).

(A) Interferons $(IFNs)$ are a group of signaling proteins made and released by host cells in response to the presence of several viruses.
They belong to the class of glycoproteins known as cytokines.
When a cell is infected by a virus,it secretes interferons,which induce an antiviral state in neighboring uninfected cells,thereby preventing further viral replication.
Thus,they are classified as antiviral proteins.

(B) 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.
Specifically,interferons are glycoproteins that protect non-infected cells from further viral infection.
When a cell is infected by a virus,it secretes interferons,which bind to neighboring uninfected cells and induce the production of antiviral proteins,thereby preventing the spread of the virus.
Therefore,interferons primarily act against viral infections.

(C) When a person steps on a rusted nail,there is a high risk of infection by the bacterium $Clostridium$ $tetani$,which causes tetanus. To prevent this,the doctor administers $ATS$ ($Antitetanic$ $Serum$). $ATS$ contains preformed antibodies that provide passive immunity against the tetanus toxin. Therefore,the correct option is $C$.

(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 part of the innate immune system and provide a non-specific defense mechanism by inhibiting viral replication in neighboring cells.
Therefore,the correct option is $B$.

(A) Interferons are a group of signaling proteins made and released by host cells in response to the presence of several viruses.
They are classified as $Antiviral \text{ proteins}$ because they inhibit viral replication within the host cells.
When a cell is infected by a virus, it secretes interferons, which signal neighboring cells to heighten their antiviral defenses.

(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 antiviral agents by inhibiting viral replication within the host cells.
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 infection.

(C) When a host cell is infected by a virus,it responds by producing specific antiviral proteins known as interferons $(IFNs)$. These proteins are released by the infected cells to protect neighboring healthy cells from further viral infection by inhibiting viral replication.

(D) The correct answer is $D$. Interferons are host-specific proteins. This means that an interferon produced by one species of animal is generally ineffective when introduced into a different species. Therefore,interferons from other animals cannot be used in humans and are typically only effective in the specific animal species from which they were derived.

(D) The molecules of interferon $(IFN)$ are classified into three main types based on their cellular origin:
$1$. Leucocytic interferon $(IFN-\alpha)$: Produced by leucocytes.
$2$. Fibroblastic interferon $(IFN-\beta)$: Produced by fibroblasts.
$3$. Immune interferon $(IFN-\gamma)$: Produced by lymphocytes.
Therefore,all the mentioned types are present in humans.

(C) Interferons are a group of signaling proteins made and released by host cells in response to the presence of several viruses.
Based on the cell type that produces them,interferons are classified into three main types:
$1$. $Alpha$ $(IFN-\alpha)$: Produced by leucocytes ($WBC$s),hence called Leucocytic interferon.
$2$. $Beta$ $(IFN-\beta)$: Produced by fibroblasts,hence called Fibroblastic interferon.
$3$. $Gamma$ $(IFN-\gamma)$: Produced by $T$-lymphocytes and natural killer cells,hence called Immune interferon.
Since the question asks for the interferon synthesized by $WBC$s,the correct answer is Leucocytic interferon.

(B) The principle of grafting,particularly in the context of tissue or organ transplantation,relies on the compatibility of the donor and recipient tissues.
This compatibility is primarily determined by the Major Histocompatibility Complex $(MHC)$ genes,which encode cell surface proteins that the immune system uses to distinguish 'self' from 'non-self'.
Therefore,the success of a graft depends on the similarity of the immune system markers between the donor and the recipient to prevent immune rejection.
Thus,the correct answer is $B$.

(A) An allograft is a tissue or organ transplant between two genetically non-identical individuals of the same species.
An autograft involves transplanting tissue from one part of an individual's body to another part of the same individual.
$A$ xenograft involves transplanting tissue between individuals of different species.
Therefore,grafting between two members of the same species is classified as an allograft.

(B) Innate immunity consists of four types of barriers:
$1$. Physical barriers: Skin and mucous membranes prevent the entry of microorganisms into the body.
$2$. Physiological barriers: Acid in the stomach,saliva in the mouth,and tears from eyes prevent microbial growth.
$3$. Cellular barriers: Certain types of leukocytes (WBCs) like $PMNL$-neutrophils and monocytes,and macrophages in tissues,phagocytose and destroy microbes.
$4$. Cytokine barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.
Therefore,skin and mucous membranes are classified as physical barriers.

(D) $T$-lymphocytes (or $T$-cells) are a type of white blood cell that play a central role in cell-mediated immunity. The main types of $T$-cells include:
$1$. Helper $T$-cells $(CD4^+)$: These cells help activate other immune cells.
$2$. Cytotoxic $T$-cells $(CD8^+)$: These cells destroy infected or cancerous cells.
$3$. Suppressor (or Regulatory) $T$-cells: These cells regulate the immune response and prevent autoimmunity.
'Repressor' $T$-cells are not a recognized classification of $T$-lymphocytes in immunology. Therefore,the correct option is $D$.

(D) The correct statements are $i$ and $ii$.
$1.$ Kidney transplantation involves the introduction of foreign tissue into the recipient's body. To prevent the recipient's immune system from attacking the transplanted organ,immunosuppressants are administered to suppress the immune response for a long period.
$2.$ The rejection of a transplant is primarily mediated by $T$-lymphocytes,which are the key components of cell-mediated immunity $(CMI)$.
$3.$ $B$-lymphocytes are primarily involved in humoral immunity (antibody production) and are not the primary cause of graft rejection.
$4.$ Interferons are antiviral proteins and are not responsible for the rejection of organ transplants.

(C) The five major classes of immunoglobulins (antibodies) found in humans are $IgG$,$IgA$,$IgM$,$IgD$,and $IgE$.
$IgS$ is not a recognized class of immunoglobulin.
Therefore,$IgS$ is not found in human serum.

(B) The secretory antibody is $IgA$.
$IgA$ is the primary immunoglobulin found in mucous secretions,including saliva,tears,breast milk,and colostrum.
It plays a crucial role in the immune function of mucous membranes by preventing the attachment of pathogens to epithelial surfaces.

(B) When a person is bitten by a snake, the venom contains toxins that require immediate neutralization.
In this medical emergency, pre-formed antibodies (antivenom) are injected directly into the patient's body.
Since the body does not produce these antibodies itself and they are introduced from an external source, this is classified as $Artificially$ $acquired$ $passive$ $immunity$.
Active immunity involves the body producing its own antibodies in response to an antigen, whereas passive immunity involves receiving ready-made antibodies.

(C) An antigen is a foreign substance,typically a protein or polysaccharide,that enters the body and is recognized by the immune system as 'non-self'.
Upon recognition,the immune system is triggered to produce specific proteins called antibodies to neutralize or eliminate the antigen.
Therefore,an antigen is defined as a substance that stimulates the production of antibodies.

(A) $IgG$ is the only antibody that can cross the placenta from the mother to the fetus. Therefore,its presence at birth is expected as it is derived from the mother.
$IgM$ is a large pentameric molecule that cannot cross the placental barrier.
If $IgM$ is detected in the blood of a newborn,it indicates that the fetus has produced it in response to an intrauterine infection,as the fetus's own immune system has been activated.

(B) Emil von Behring was awarded the first Nobel Prize in Physiology or Medicine in $1901$ for his work on serum therapy,specifically for his development of antitoxin treatment for diphtheria,which is a form of passive immunity.

(B) The $D.P.T$ vaccine stands for Diphtheria,Pertussis,and Tetanus vaccine.
It contains inactivated toxins or killed bacteria that stimulate the host's immune system to produce specific antibodies.
Since the body actively produces its own antibodies in response to the antigen present in the vaccine,it is an example of active immunity.

(A) The human body possesses an innate immunity system that acts as the first line of defense to prevent the entry of pathogens.
This system includes physical and physiological barriers.
Skin and mucous membranes lining the respiratory,gastrointestinal,and urogenital tracts are the primary physical barriers that trap microorganisms and prevent their entry into the body.
Therefore,skin and mucous membranes constitute the first line of defense.

(C) An antibody molecule is represented as $H_2L_2$,where $H$ stands for heavy chain and $L$ stands for light chain.
Each antibody molecule has two antigen-binding sites.
These sites are located at the variable regions of the antibody,which are formed by the interaction between the $N$-terminal ends of one heavy chain and one light chain.
Therefore,the antigen-binding site is located between one heavy chain and one light chain.

(B) Immunodeficiency is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent.
$1$. $A$ defective thymus gland leads to a deficiency in $T$-lymphocytes,which are crucial for cell-mediated immunity.
$2$. Without a functional thymus,the maturation of $T$-cells is impaired,resulting in a severe immunodeficiency disorder.
$3$. While the $AIDS$ virus $(HIV)$ also causes immunodeficiency,the most fundamental biological cause related to the development of the immune system itself is the failure of the thymus gland.

(B) Passive immunity is the type of immunity where pre-formed antibodies are received by the body rather than being produced by the individual's own immune system.
$1$. The fetus receives antibodies $(IgG)$ from the mother through the placenta.
$2$. The infant receives antibodies $(IgA)$ from the mother's colostrum (initial milk).
Since the body does not actively produce these antibodies,it is classified as passive immunity.

(C) vaccine is a biological preparation that provides active acquired immunity to a particular infectious disease. It typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe,its toxins,or one of its surface proteins. Therefore,vaccines consist of inactivated or attenuated (weakened) pathogens or their components,which stimulate the immune system to recognize the agent as a threat,destroy it,and keep a record of it so the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.

(D) Primary lymphoid organs are the sites where lymphocytes originate and/or mature and differentiate into antigen-sensitive lymphocytes.
In humans,the bone marrow and the thymus are the primary lymphoid organs.
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,Peyer's patches of the small intestine,and the appendix.

(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: Certain types of leukocytes (WBCs) like $PMNL$-neutrophils,monocytes,and natural killer cells in the blood,as well as macrophages in tissues,phagocytose and destroy microbes.
$4$. Cytokine barriers: Virus-infected cells secrete proteins called interferons,which protect non-infected cells from further viral infection.
Therefore,saliva and tears are examples of physiological barriers.

(C) The initial milk secreted by the mother during the first few days of lactation is called colostrum.
Colostrum is rich in antibodies,specifically $IgA$ (Immunoglobulin $A$).
$IgA$ provides essential passive immunity to the newborn infant,protecting them from various infections during their early development.

(C) An antibody,also known as an immunoglobulin $(Ig)$,is a large,$Y$-shaped protein produced by plasma cells (differentiated $B$-lymphocytes) of the immune system.
Its primary function is to identify and neutralize foreign objects such as bacteria and viruses.
Antibodies specifically recognize and bind to unique molecules called antigens present on the surface of pathogens,thereby triggering an immune response.

(D) Immunity is the ability of the host to fight against disease-causing organisms,conferred by the immune system.
It is primarily based on the 'memory' of the immune system.
When the immune system encounters a pathogen for the first time,it produces a primary response of low intensity.
Subsequent encounters with the same pathogen elicit a highly intensified secondary or anamnestic response due to the memory of the first encounter.

(A) The immune system consists of lymphoid organs,tissues,cells,and soluble molecules like antibodies.
Primary lymphoid organs like the bone marrow and thymus are sites where lymphocytes originate and/or mature.
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.
The brain is part of the central nervous system and is not a lymphoid organ; therefore,it does not play a direct role in providing immunity.

(C) $NK$ cells stand for Natural Killer cells.
These are a type of cytotoxic lymphocyte critical to the innate immune system.
They play a major role in the host-rejection of both tumors and virally infected cells.

(A) Neutrophils and monocytes are types of white blood cells that act as phagocytes.
They are part of the innate immune system,specifically the cellular barrier.
These cells engulf and destroy pathogens (bacteria,viruses,etc.) through a process known as phagocytosis.
Therefore,the correct process is phagocytosis.