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Immunity
❖ Definition
• Immunity is the ability of the body to defend itself against disease-causing organisms. Everyday our body
comes in contact with several pathogens, but only a few results into diseases. The reason is, our body
has the ability to release antibodies against these pathogens and protects the body against diseases. This
defence mechanism is called immunity.
❖ Types of Immunity
There are two major types of immunity:
1. Innate Immunity or Natural or Non-specific Immunity.
2. Acquired Immunity or Adaptive Immunity

1. Innate Immunity
• This type of immunity is present in an organism by birth.
• Innate immunity includes certain barriers and defence mechanisms that keep foreign particles out of the
body.
• Our body has few natural barriers to prevent the entry of pathogens
Types of Barriers
• The four types of barriers are:
i) Physical barrier
• These include the skin, body hair, cilia, eyelashes, the respiratory tract, and the gastrointestinal
tract. These form the first line of defence.
• The skin does more than providing us with fair or dark complexions. Our skin acts as a physical
barrier to the entry of pathogens. The mucus coating in our nose and ear is a protective barrier
which traps the pathogen before it gets inside.
ii) Physiological barriers
• We know that our stomach uses hydrochloric acid to break down the food molecules. Due to such a
strongly acidic environment, most of the germs that enter our body along with the food are killed
before the further process is carried on.
• Saliva in our mouth and tears in our eyes also have the lysosome enzyme that does not allow the
growth of pathogens even though they are exposed all day.
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iii) Cellular barriers

• In spite of the physical and physiological barriers, certain pathogens manage to enter our body.
• The cells involved in this barrier are leukocytes (WBC), neutrophils, lymphocytes, basophil,
eosinophil, and monocytes.
• All these cells are all present in the blood and tissues.
• Cells Involved In Innate Immunity
i) Phagocytes: These circulate through the body and look for any foreign substance. They
engulf and destroy it defending the body against that pathogen.
ii) Macrophages: These have the ability to move across the walls of the circulatory system.
They release certain signals as cytokines to recruit other cells at the site of infections.
iii) Mast Cells: These are important for healing wounds and defence against infections.
iv) Neutrophils: These contain granules that are toxic in nature and kill any pathogen that
comes in contact.
v) Eosinophils: These contain highly toxic proteins that kill any bacteria or parasite in contact.
vi) Basophils: These attack multicellular parasites. Like the mast cells, these release histamine.
vii) Natural Killer Cells: These stop the spread of infections by destroying the infected host cells.
viii) Dendritic Cells: These are located in the tissues that are the points for initial infections.
These cells sense the infection and send the message to the rest of the immune system by
antigen presentation
iv) Cytokine barriers
• In case a cell in our body experiences a virus invasion, it automatically secretes proteins called
interferons which forms a coating around the infected cell and prevents the cells around it from
further infections.
2. Acquired Immunity
• Acquired immunity or adaptive immunity is the immunity that our body acquires or gains over time.
Unlike the innate immunity, this is not present by birth.
• The ability of the immune system to adapt itself to disease and to generate pathogen-specific
immunity is termed as acquired immunity. It is also known as adaptive immunity.
• An individual acquires the immunity after the birth, hence is called as the acquired immunity.
• It is specific and mediated by antibodies or lymphocytes which make the antigen harmless.
• The immune system of our body identifies the pathogens which have encountered in the past. Our
body starts producing antibodies to engulf the pathogen and destroy its antigen.
• When it encounters for the first time, it is called a primary response. Once a body gets used to these
pathogens, antibodies are ready to attack them for the second time and are known as naturally
acquired immunity.
• The acquired immunity in our body has certain special features
Features of Acquired Immunity
→ Specificity: Our body has the ability to differentiate between different types of pathogens,
whether it is harmful or not, and devise ways to destroy them.
→ Diversity: Our body can detect vast varieties of pathogens, ranging from protozoa to viruses.
→ Differentiate between self and non-self: Our body has the unique ability to differentiate
between its own cells and foreign cells. It immediately starts rejecting any foreign cell in the
body.
→ Memory: Once our body encounters a pathogen, it activates the immune system to destroy
it. It also remembers what antibodies were released in response to that pathogen, so that,
the next time it enters, a similar procedure is followed by the body to eliminate it.
• Cells Involved in Acquired Immunity
The acquired immunity involves two types of cells:
B-cells and T-cells
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B-cells
• They develop in the bone marrow.
• These cells are activated on their encounter with foreign agents.
• The B-cells immediately differentiate into plasma cells which produce antibodies specific to that
foreign particle or so-called antigen.
• These antibodies attach to the surface of the antigen/foreign agent.
• These antibodies detect any antigen in the body and destroy it.
• The immunity dependent on B-cells is called humoral immunity.
T-cells
• They originate in the bone marrow and develop in the thymus.
• T-cells differentiate into helper cells, cytotoxic cells, and regulatory cells. These cells are released
into the bloodstream.
• When these cells are triggered by an antigen, helper T-cells release cytokines that act as messengers.
• These cytokines initiate the differentiation of B-cells into plasma cells which release antibodies
against the antigens.
• The cytotoxic T-cells kills the cancer cells.
• Regulatory T-cells regulate immune reactions
• Types of Acquired Immune Response
1. Humoral Immune Response
• The antibodies produced by B-lymphocytes are present in the blood cells and they are transported all
over the body. This is why it is called the humoral immune response as it consists of an antibody
produced by the lymphocytes.
• When an antibody on a B-cell binds with an antigen, humoral immunity comes into play. The antigen
is internalized by the B cell and presented on the helper T cell. This activates the B-cell.The activated
B cells grow and produce plasma cells.These plasma cells release antibodies in the bloodstream.
• The memory B cells retain the information about the pathogen to prevent any disease caused by that
pathogen in the near future.
2. Cell-mediated Immune Response
• Cell-mediated immunity is initiated by the T helper cells.
• The cytotoxic T cells eliminate the infected cells from the body by releasing toxins, thereby, promoting
apoptosis or programmed cell death.
• The T helper cells help to activate other immune cells.
• The T-lymphocytes are capable of recognizing whether tissue or an organ is from our body or foreign
bodies. This is the reason why we cannot transplant and implant the organs into our body even if we
find the donor with the same blood group because our body might reject the transplanted organ. The
T-cells quickly recognize that the tissue or an organ as a foreign and do not allow it to become a part of
the body. This is why transplant receivers have to take immunosuppressant medication for the rest of
their lives.
• This response is controlled by the T-lymphocytes.
• Types of Acquired Immunity
1. Active Immunity or Adaptive immunity
• Active immunity involves the direct response to a foreign antigen within the body. In the case of the
acquired or adaptive immune system, the body remembers the pathogens it has encountered in the
past. This is a direct result of the active immune system.
• Active immunity occurs when we are in contact with the pathogen or its antigen.
• Antigens stand for antibody generator. It is with the help of antigens released by the pathogen that
our body tackles the pathogen.
• So what our body does is, it starts producing antibodies to attack the pathogen based on its antigen.
When this happens for the first time, it is called a primary response
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2. Passive Immunity
• Passive immunity involves the immune response by the antibodies attained from outside the body.
• While active immunity may protect us from a disease for a lifetime, passive immunity is the more
short term.
• Passive immunity develops immediately and our body could begin its attack on the pathogen right
away.

❖ AutoImmunity
• Sometimes the immune system attacks its own tissues and organs instead of the foreign agents. This
is called autoimmunity.
• Type I diabetes is an example of autoimmune disease
❖ Vaccines
• It is a preparation that is administered (as by injection) to stimulate the body's immune response against
a specific infectious agent or disease
• A vaccine is made up of the antigens of the pathogen that cause the disease.
• Vaccines work by exposing you to a small and safe amount of weakened, dead, inactive, or
biosynthetic bacteria or viruses. This prompts your immune system to recognize the pathogen and/or
defend you from certain diseases.
• For eg., the smallpox vaccine contains the antigens of the pathogen causing smallpox disease. When a
person is vaccinated with the smallpox vaccine the antibody-producing cells are stimulated that produce
smallpox antibodies. Thus, the body is protected against the disease occurring in future.
• Vaccinating pathogenic microbes into our body deliberately produces a similar response and is termed
as artificially acquired immunity.
❖ Types of vaccines
1. Live-attenuated vaccines
• Live vaccines contain a weakened (or attenuated) form of an infectious agent. ‘Live’ in the term ‘live
vaccine’ is a bit of a misnomer.
• Examples of active/live vaccines: Measles/mumps/rubella (MMR); chickenpox; shingles, smallpox.
2. Inactive or dead vaccines
• Inactive, dead, or inert vaccines contain pathogens that cannot multiply, and cannot cause the diseases.
However, your immune system is still able to recognize and attack these inactive pathogens to build
immunity.
• Examples of inactive/dead vaccines: Flu (shot), polio, rabies, whooping cough (pertussis)
3. Conjugate or subunit vaccines
• These biosynthetic vaccines only contain fragments of the pathogen. This helps the immune system
develop a response to a specific piece of a pathogen.
• Examples of conjugate and subunit vaccines: Human papillomavirus (HPV), pneumococcal disease,
meningococcal disease
4. DNA and mRNA vaccines
• Our clinical staff is often asked how mRNA vaccines work. DNA and mRNA vaccines are intended to give our
cells genetic instructions to create inert pieces of viruses. Our immune system can then recognize and
target these pieces to build immunity.
• There currently are no approved DNA or mRNA vaccines, though many are being studied in clinical trials.
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5. Viral vector vaccines

• These vaccines use harmless viruses to deliver genetic instructions (DNA) to cells and prompt an immune
response.
• Examples of viral vector vaccines: Hepatitis B, Ebola
6. Toxoids
• Though uncommon, these vaccines help your immune system fight the effects of toxins — not the actual
infection.
• Examples of toxoid vaccines: Tetanus, diphtheria

❖ Immunization
• Immunisation is the process of inducing or providing immunity artificially.
• This may be done by the administration of a vaccine, toxoid or externally produced antigen in order to
stimulate antibody production.
• The aim of immunisation is to reduce the incidence of, or to eliminate a particular disease.
• Immunisation has both a direct and an indirect effect.
→ Direct effect; antibody protection in the individual
→ Indirect effect; reduction of the incidence of the disease in others – so called ‘herd immunity’
Vaccine Type of agents
Diptheria Toxoid
Measles Live attenuated virus
BCG Live bacterial antigen
H.Influenza Bacterial polysaccharide conjugated to protein
Hepatitis Inactive viral antigen
Cholera Killed antigen
Botulism Antitoxin

❖ Difference between vaccination and immunisation

• Vaccination is an administration used to give vaccines i.e giving oral vaccines or injections to improve
immune system development and fight against diseases.
• Immunization is the process in which the individual immune system is improved to fight against the
infected agents.
• Types of immunisation-
Active- When vaccine stimulates the production of antibodies which actively fight the virus or the
bacteria
Passive-Passive immunity carries the antibodies from outside the body. The substance such as immune
globulin(immunoglobulins) is transfer to protect from a certain disease. This protection takes place
immediately as compared to active immunization which takes time to produce antibodies.
❖ What is immunoglobulin ?
• Immunoglobulin's are glycoprotein molecules that are produced by plasma cells in response to an
immunogen and which function as antibodies.
• The immunoglobulins derive their name from the finding that they migrate with globular proteins when
antibody-containing serum is placed in an electrical field
• Examples
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Structure of immunoglobulins

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