• Mast cells are found in mucous membranes and connective tissues and are important for wound healing and defense against pathogens via the inflammatory response.
• The mast cell granules are composed of serotonin, heparin, and histamine. They secrete serotonin, heparin, and histamine when they are activated in the presence of the toxic substance or a pathogen.
• Hence Mast cells secrete 'serotonin, heparin, and histamine'.
• So, the correct answer is 'All the above'.

• The cells of the immune system can be categorized as lymphocytes (T-cells, B-cells and NK cells), neutrophils, and monocytes/macrophages. These are all types of white blood cells.
• Neutrophils are phagocytic cells that are also classified as granulocytes because they contain granules in their cytoplasm. These granules are very toxic to bacteria and fungi and cause them to stop proliferating or die on contact.
  
• B lymphocytes provide humoral immunity and T lymphocytes provide cellular immunity.
• Macrophages are efficient phagocytic cells that can leave the circulatory system by moving across the walls of capillary vessels. The ability to roam outside of the circulatory system is important because it allows macrophages to hunt pathogens with less limits. Macrophages can also release cytokines in order to signal and recruit other cells to an area with pathogens.
• Hence Neutrophils, Lymphocytes, and Macrophages are involved in body defense.
• So, the correct answer is 'All the above'.

Passive immunity is the transfer of active humoral immunity of ready-made antibodies (exogenous). Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially when high levels of antibodies specific to a pathogen or toxin are transferred to non-immune persons through blood products that contain antibodies.

Active immunity is the induction of immunity after exposure to an antigen. Antibodies are created by the recipient and may be stored permanently due to the activation of memory cells which recognizes the same antigen in future and destroy them. Active immunity can occur naturally when a microbe or other antigen is received by a person who has not yet come into contact with the microbe and has no pre-made antibodies for defence. The immune system will eventually create antibodies for the microbe, but this is a slow process.

Antivenum which is injected to body in case of snakebite, contain antibodies that disable a particular venom's toxins. Antivenom is less able to reverse the damage already happened in the body. So, quick injection is a must. So, the statement A is correct.

Innate immunity includes anatomical and physiological barriers, inflammation, complement activation, cytokine secretion, phagocytosis. Cytokines play an important role in the innate immune response. It fights directly against the virus by preventing its replication. At the same time, cytokines activates NK cells and macrophages, which produce more cytokines. Also, virus infected cells secrete proteins called interferons , which protect non-infected cells from further infection.

In the case of tetanus toxoid, when it administered in a newborn, the vaccine contains detoxified tetanus toxins (which does not lose their immunogenic properties). The administration of the tetanus toxoid stimulates the body to produce protective antibodies against the toxins.
In the case of administration of anti-venin against snake bite or anti-tetanus serum the, the doses contain antibodies which go and attack their target antigens in the body. The passage of IgG across placenta also helps the fetus to fight infections directly.
So, the correct answer is ' Administration of tetanus toxoid to newly born'.

Active immunity is the type of immunity in which antibodies are produced against a specific antigen. This type of immunity takes time in order to produce enough antibodies to fight the pathogen. This immune response is long lived and is more effective. It results in the immunological memory which remains for life time. It is used for prophylaxis to increases body resistance. Passive immunity is a type of immunity in which antibodies are obtained from outside the body. An example includes antibodies obtained from mother through the  breast milk. This immune response is short lived and less effective. It does not have any immunological memory. It is used for treatment of acute infection.

Artificially acquired active immunity can be induced by a vaccine, a substance that contains antigen. A vaccine stimulates a primary response against the antigen without causing symptoms of the disease. 

Toxoid is a type of vaccines obtained from the toxin and are inactivated and made harmless by chemical or heat treatment. Eg- tetanus toxoid is obtained by Clostridium tetani. These vaccines provide artificial active acquired immunity. So, the correct answer is 'Artificial active acquired immunity'.

Short-lived immunity acquired from mothers to foetus across placenta or through mother's milk to the infant is categorized as passive immunity. In passive immunity there is transfer of preformed antibodies in host to a recipient. In this case, the resistance is passively transferred from the mother to the foetus through placenta or through mother's milk. So, the correct answer is 'Passive immunity'.

Passive immunity is the transfer of active humoral immunity in the form of ready-made antibodies, from one individual to another. Passive immunity can occur naturally when maternal antibodies are transferred to the fetus through the placenta and can also be induced artificially when high levels of human (or horse) antibodies specific for a pathogen or toxin are transferred to non-immune individuals. Passive immunization is used when there is a high risk of infection and insufficient time for the body to develop its own immune response, or to reduce the symptoms of ongoing or immunosuppressive diseases.

Live, attenuated vaccines contain a living microbe that has been weakened in the lab so it cannot cause disease. Because a live, attenuated vaccine is the closest thing to a natural infection, these vaccines elicit strong cellular and antibody responses and often confer lifelong immunity with only one or two doses. Vaccines against measles, mumps, and chickenpox, for example, are made by this method. 

A vaccine is the killed or live suspension of microorganism which helps to activate the immune response. The agent stimulates the body's immune system to recognize the agent as a threat, destroy it and keep a record of it so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.

Innate immunity refers to non-specific defense mechanisms that are present in the human body by birth to protect against the antigen. These mechanisms include physical barriers such as skin, chemicals in the blood and immune system cells that attack foreign cells in the body.

Answer is option A i.e. "Direct cell membrane- to- membrane contact"
Antigen-presenting cells (APC) are immune system cells that present antigens on the cell surface which then communicate with appropriate T cell receptors by membrane-to-membrane contact. Antigen-presenting cells are involved in both the innate and adaptive immune responses.

The natural way of destroying the bacterial infection is the production of the antibodies by the plasma B-cells in the body. The production of HCl also kills the harmful bacteria. The WBCs are the cells which can phagocyte and destroy the bacterial cells.

. The mucous membrane is the epithelial tissue which lines several organs and secretes mucus. It stops pathogens and dirt from entering the body and to prevent bodily tissues by pushing disease causing agents towards body opening, it also prevents organs from  dehydration

Innate immune system is quick and non specific, it is the 1$^{st}$ immediate response of the body on infection. It is also referred to as primary response that would eliminate certain number of pathogens keeping the infection in check. 

Passive immunity is the transfer of active humoral immunity in the form of ready-made antibodies, from one individual to another. Passive immunity can occur naturally when maternal antibodies are transferred to the fetus through the placenta or can be induced artificially, when high levels of human antibodies specific for a pathogen or toxin are transferred to non-immune individuals.
Emil Adolf von Behring and Kitasato Shibasaburo discovered the antitoxin-based immunity to diphtheria and tetanus and antitoxin became the first major success of modern therapeutic immunology.

A. The lymph node is in the form of fibres called reticulum with white blood cells attached to it,  lymph sinus is lined by reticular cells and macrophages.

• Passive immunity is the protection against disease through antibodies produced by another human being or animal.
• Passive immunity doesn't last as long as active immunity.
• Emil von Behring was awarded the first Nobel Prize in Physiology or Medicine in 1901 for his discovery of serum therapy for diphtheria. He showed that serum from rabbits immunized with tetanus toxin could prevent tetanus in rabbits because it contains antibodies against tetanus.
• Hence the method of passive immunity was discovered by Von Behring.
•  So, the correct answer is 'Von Behring'.

Passive immunity is the transfer of active humoral immunity of ready-made antibodies (exogenous). Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially when high levels of antibodies specific to a pathogen or toxin are transferred to non-immune persons through blood products that contain antibodies. Passive immunization can be provided when people cannot synthesize antibodies, and when they have been exposed to a disease against which a person is not immunized.

Vaccination is the administration of antigenic material (a vaccine) to stimulate an individual's immune system to develop acquired immunity to a pathogen. Upon administration of attenuated or inactivated antigen (vaccine), the antibodies are produced by plasma cells and memory cells are activated. When the same antigen attacks the person in future, the memory cells recognize the antigen and promotes phagocytosis.

A lymphocyte is one of the subtypes of white blood cell in a vertebrate's immune system. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic acquired immunity), and B cells (for humoral, antibody-driven adaptive immunity). The total number of lymphocytes present in healthy humans is approximately 1 trillion.

Active immunity is the induction of immunity after exposure to an antigen. Antibodies are created by the recipient and may be stored permanently due to the activation of memory cells which recognizes the same antigen in future and destroy them. Active immunity can occur naturally when a microbe or other antigen is received by a person who has not yet come into contact with the microbe and has no pre-made antibodies for defence.

The adaptive immune system, also known as the acquired immune system is composed of highly specialized, systemic cells and processes that eliminate pathogens or prevent their growth. The acquired immune system is one of the two main immunity strategies found in vertebrates. Acquired immunity creates immunological memory after an initial response to a specific pathogen, and leads to an enhanced response to subsequent encounters with that pathogen.

The inflammatory response (inflammation) occurs when tissues are injured by bacteria, trauma, toxins, heat, or allergy. The damaged cells release chemicals including histamine, bradykinin, and prostaglandins. Histamine is released from mast cells. These chemicals cause blood vessels to leak fluid into the tissues, causing swelling. 

• In our body, neutrophils, macrophages and lymphocytes are involved in the second line of defence. They produce non-specific immune responses such as fever and inflammatory response.
•  Neutrophils are a type of WBC and they form an essential part of the innate immune system. Neutrophils are one of the first-responders of inflammatory cells to migrate towards the site of inflammation.
• Macrophages are the type of WBC which engulfs and digests cellular debris, foreign substances, microbes etc..
• Lymphocytes are a type of WBC. It includes natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic adaptive immunity), and B cells (for humoral, antibody-driven adaptive immunity). 

• Acquired immunity is the immunity obtained either from the development of antibodies in response to exposure to an antigen, as from vaccination or an attack of an infectious disease or from the transmission of antibodies, as from mother to fetus through the placenta or the injection of antiserum.
  
• Active acquired immunity results from the development of antibodies in response to an antigenic stimulus, as from exposure to an infectious disease or through vaccination.
• Hence, resistance developed in an individual as a result of antigenic stimulus is due to Active acquired immunity.

• Gammaglobulin is a class of globulins which are immune molecules that are produced by the immune system in response to the invasion of the body of agents that are perceived by the system as being foreign. Most significant globulins are immunoglobulins.
  
• Artificially acquired passive immunity is a short-term immunization by the injection of antibodies such as gamma globulin that are not produced by the recipient's cells.
  
• Hence, a person injected with gammaglobulin against hepatitis B is artificially acquired passive immunity.

Passive immunity is the transfer of active humoral immunity of ready-made antibodies (exogenous). Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus or child through the placenta or milk, and it can also be induced artificially when high levels of antibodies specific to a pathogen or toxin are transferred to non-immune persons through blood products that contain antibodies. Passive immunity is short-lived, as long as antibodies persist in the body. It is seen in cases where a patient needs immediate protection from a foreign body and cannot form antibodies quickly enough independently.

Artificially acquired passive immunity is a short-term immunization achieved by the transfer of antibodies, which can be administered in several forms such as human or animal blood plasma or serum. Antivenine is an antisera which is obtained from the snake poison and it is transferred to the person with snake bite. It is used to provide immediate immunity in case of emergency.

• When the immune system is activated due to the viral infection or other immune response the white blood cells in our body produces Interferons (are a group of proteins called cytokines). Interferons inhibit viral multiplication
• The name of the interferons comes from their ability to interfere with the production of new virus particles. Interferons do not directly kill viral or cancerous cells; they boost the immune system response and reduce the growth of cancer cells by regulating the activity of several genes that control the secretion of numerous cellular proteins that affect growth.
• Hence cytokine that provides nonspecific immunity against viruses is Interferon.
• So, the correct answer is 'Interferons'.

The acquired immune system, also known as the adaptive immune system is a subsystem of the overall immune system that is composed of highly specialized, systemic cells and processes that eliminate pathogens or prevent their growth. Acquired immunity creates immunological memory after an initial response to a specific pathogen, and leads to an enhanced response to subsequent encounters with that pathogen. Acquired immunity distinguish between self and non-self and produce antibodies against the non-self antigen.

Cytokines are a broad category of small proteins that are important in cell signalling. Their release has an effect on the behaviour of cells around them. Proinflammatory cytokines are produced predominantly by activated macrophages and are involved in the up-regulation of inflammatory reactions. There is abundant evidence that certain pro-inflammatory cytokines such as IL-1β, IL-6, and TNF-α are involved in the process of pathological pain.

Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. They are essential for fighting infections and for subsequent immunity. Phagocytes provide the second line of defence which is considered as a non-specific defence because it destroys invaders in a generalized way without targeting specific pathogens. Phagocytes are activated when the pathogen is able to cross the first line of defence.

Neutrophils are the most abundant type of granulocytes. They form an essential part of the innate immune system. Neutrophils undergo a process called chemotaxis via amoeboid movement, which allows them to migrate toward sites of infection or inflammation. Neutrophils express and release cytokines, which in turn amplify inflammatory reactions by allowing other neutrophils, monocytes, eosinophils attachment to endothelium and dilation of capillaries is seen to increase the blood flow to the affected area.

Chemical barriers protect against infection. The skin and respiratory tract secrete antimicrobial peptides such as the β-defensins. Enzymes such as lysozyme and phospholipase A2 in saliva, tears, and breast milk are antibacterial substances. Vaginal secretions serve as a chemical barrier following menarche, when they become slightly acidic, while semen contains defensins and zinc to kill pathogens. In the stomach, gastric acid serves as a powerful chemical defence against ingested pathogens.

Innate immunity refers to nonspecific defence mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells such as neutrophils that attack foreign cells in the body.  Polymorphonuclear leukocytes (PMNL - neutrophils) are the granulocytes characterized by the presence of granules in their cytoplasm. They provide innate immunity.

• Passive immunity is the transfer of active humoral immunity of ready-made antibodies (exogenous) or preformed antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially when high levels of antibodies specific to a pathogen or toxin are transferred to non-immune persons through blood products that contain antibodies. People administered with preformed antibodies get 'Passive immunity'.
• So, the correct answer is 'Passive immunity'.

Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase is an antibacterial enzyme produced by animals and humans that forms part of the innate immune system. Lysozyme is abundant in secretions including tears, saliva, human milk, sweat and mucus. It is also present in cytoplasmic granules of the macrophages and the polymorphonuclear neutrophils. Lysozyme is an enzyme and hence, acts as a chemical barrier of infection.

The second line of defence involves non-specific cellular and molecular responses of the innate immune system because these defences do not differentiate between different types of the pathogen and respond the same way upon every infection. Mucous membrane, phagocytes and lysozyme are all involved in a non-specific defence mechanism, whereas, killer T-cells or cytotoxic T-cells provide the third line of defence.

• Physiological barriers to the entry of microorganisms in the human body are tears in eyes, saliva in mouth and HCl in the stomach.
• The enzymes lysozymes are found in tears and saliva inhibits the synthesis of peptidoglycan present in the cell wall of microorganisms especially eubacteria.
• Hence Tears acts as the physiological barrier to entry of microorganisms in the human body.
• So, the correct answer is 'Tears'.

• Naturally acquired active immunity occurs when a person is exposed to a live pathogen and develops a primary immune response. An example is acquired after smallpox infection.
•  Naturally acquired passive immunity occurs during pregnancy, in which certain antibodies are passed from the maternal blood into the fetal bloodstream in the form of IgG.
• Artificially acquired active immunity can be induced by a vaccine, a substance that contains antigen. An example of artificial active immunity is building up a resistance to a disease due to immunization.
•  Artificially acquired passive immunity is a short-term immunization by the injection of antibodies, such as gamma globulin, that is not produced by the recipient's cells. eg: Antirabies serum,
•  So, the correct answer is 'a - iii, b - iv, ci, d - ii'.
  

• Immunity to viral infection is caused by a variety of specific and nonspecific mechanisms. 
• The activation of different immune functions and the duration and magnitude of the immune response depending on how the virus interacts with host cells. 
•  Cytotoxic T lymphocytes, natural killer (NK) cells and antiviral macrophages can recognize and kill virus-infected cells.
•  Hence, Body cells infected by viruses are destroyed mostly by Natural Killer cells.
• So, the correct answer is 'Natural Killer cells'.

Passive immunity is the transfer of active humoral immunity of ready-made antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta and through colostrum (mother's milk), and it can also be induced artificially, when high levels of antibodies specific to a pathogen or toxin (obtained from animals) are transferred to non-immune persons through blood products that contain antibodies. Passive immunity is short lived.

• Cytokine inhibits viral replication.
• They form an innate immune system by forming physical barriers. 
•  Virus-infected cells secrete proteins called interferons which protect non-infected cells from a further viral infection is a cytokine barrier. Hence, The cytokine barrier among these is interferon.

Artificially acquired passive immunity explains this form of protection. When performed antibodies are directly injected into the body, it is known as artificial passive immunity. It provides the quick immune response. In case of snake bites, the injection which is given to the patients, contain performed antibodies against snake venom. This type of immunisation is passive immunisation.