Have you ever wondered:
- How does our body overcomes common cold on its own?
- Why humans do not get infected by plant diseases?
- Why after getting vaccinated, our body becomes immune to that specific disease such as polio?
The answer to the above questions lies in the fact that our body has a
defense mechanism called as IMMUNE SYSTEM which recognizes the foreign
pathogens, fights against them and destroys them.
In this article we will discuss about the different types of
immunity.
Topics Covered:
3)
Defense mechanism of human body
- Local defense system (Physical and chemical barriers)
- Immune System (Antibodies, Antitoxins)
What is Immunity?
Immunity means body's defense against a disease. It is the state of
protection against foreign pathogens or substances called as
Antigens (example: viruses, bacteria, fungi, parasites, toxins,
etc).
The disease causing organisms are called as pathogens and the
process by which they induce illness is called pathogenesis.
How does pathogens enter human body?
The pathogen or harmful foreign substances called as Antigen, can enter
human body through various channels such as:
1. Skin
2. Mucous membrane of eyes, nose, urinary or genital tracts
3. Food, Water, Air
Defense Mechanism of the human body
The defense mechanism of the body works in following steps:
- Prevent entry of pathogen (Local Defense System)
- If pathogen enters body, then immune system of our body recognizes pathogens, fights and destroys them.
1) Prevent entry of pathogen (Local Defense System)
The body first tries to prevent the entry of the pathogen in the body by
putting up various physical and chemical barriers such as:
- Mechanical barriers like skin, hair and mucus.
- If pathogen enters the body, it is thrown out of the body via sneezing, coughing or vomiting
- Germ-killing secretions such as tears, saliva and Hydrochloric acid (HCl) in stomach
- White Blood Cells (WBCs) which engulf bacteria and destroys them via phagocytosis
2) If the pathogen is able to dodge Local Defense System of the body and
is able enter deeper tissues and organs, then IMMUNE System
gets activated which:
a. Recognize pathogen
b. Fights pathogen
c. Destroys pathogen
- Antibodies and antitoxins fight against pathogens at this stage.
Types of Immunity
There are basically two types of immunity:
- Innate Immunity
- Adaptive Immunity
a) INNATE Immunity
1) Innate immunity is the first line of defense against the
foreign pathogen and it act as the local defense system of the
body.
2) Innate Immunity is also called as natural or native immunity as it is already
present in our body by birth (inborn)
or naturally without any external stimulation or a previous infection.
3) Innate immunity includes built-in molecular and cellular mechanisms that are encoded in the germline and are evolutionary more primitive, aimed at preventing infection or quickly eliminating common invaders.
4) Key Elements of Innate Immunity are
Physical and Chemical barriers
that prevent infection such as:
- epithelial cell layers of the skin,
- mucosal tissues (e.g., gastrointestinal, respiratory, and urogenital tracts),
- glandular tissues (e.g., salivary, lacrimal, and mammary glands)
5)
Innate immunity is faster but less specific than adaptive responses, which takes several days but are highly
antigen specific.
-
Innate immunity is less specific as it is unable to distinguish between small differences in foreign antigens.
6) Outcome of innate immunity is:
- Rapid recognition and
- Phagocytosis or destruction of the pathogen
7) How does innate immunity works?
The series of pre-existing serum proteins collectively called as
complement bind to pathogen-associated structures and initiate
a cascade of labeling and destructive events.
b) ADAPTIVE or Acquired Immunity
1) Definition:
The immunity or resistance to a disease
developed during an individual's lifetime
after confronting the disease once or by getting vaccinated is called
Adaptive or Acquired Immunity.
2)
Adaptive immunity is slower but
more antigen specific
when compared to Innate immunity.
-
Adaptive immunity is slower partly because fewer cells possess the perfect receptor for the job: the antigen-specific, randomly generated receptors found on B and T cells.
-
It is also slower because parts of the adaptive response rely on prior encounter and “categorizing” of antigens undertaken by innate processes.
3) Key elements of Adaptive immunity are:
- B and T lymphocytes which undergo selection and proliferation after encountering an antigen.
4) TYPES of adaptive immunity:
The adaptive immunity is of two types:
- Actively acquired immunity
- Passively acquired immunity
(i) Actively acquired immunity
- Actively acquired immunity is the resistance developed in the body due to a previous infection.
Example:
If a person has already suffered from a disease such as
chicken pox, it usually does not suffer from it again in his
lifetime.
- Type of Actively acquired immunity
- Natural (via Infection)
- Artificially (via vaccination)
Working of Actively Acquired Immunity
Once T and B lymphocytes encounter antigen, they undergo selection and
proliferation (as per clonal selection theory of antigen
specificity).
B and T lymphocytes have antigen-specific membrane receptors.
Once antigen binds to the membrane receptor of B or T cell, it activates
the cell and causes its proliferation i.e forming many copies of the
daughter cell and increasing in number.
- In B-cells, the antigen binds to B-cell receptor causing its differentiation into antigen-selected antibody secreting B-cell, which produce antibodies which binds to foreign proteins or infection causing agent and destroys them.
- In T-cells, the antigen binds to T-cell receptor causing its differentiation into Antigen-selected T cells which:
a) either
produces soluble messenger called Cytokines that helps other cells of
immune system to destroy antigen
b) or
produces Antigen-selected T cells which directly kills the infected cells.
(ii) Passively acquired immunity
Passively acquired immunity is the resistance developed in the body when
the ready-made antibodies are supplied from the outside.
Example:
- Types of Passively acquired immunity
Passively acquired immunity is of two types:
- Natural (via Maternal antibodies)
2. Artificially (via Monoclonal antibodies)
- readymade antibodies produced in other
animals)
Immunologic memory of Adaptive Immunity (Secondary Memory)
Immunologic memory is the ability of the immune system to respond much
more swiftly and with
greater efficiency during a second exposure
to the same pathogen.
During a first encounter with foreign antigen, adaptive immunity
undergoes a primary response, during which the key lymphocytes
that will be used to eradicate the pathogen are clonally selected,
honed, and enlisted to resolve the infection. These lymphocyte cells
incorporate messages received from the innate players into their
tailored response to the specific pathogen. All subsequent encounters
with the same antigen or pathogen are referred to as the
secondary response.
How does Innate and Adaptive immunity communicate with each other?
For innate and adaptive immunity to work together, these two systems must
be able to communicate with one another. This communication is achieved
by:
- cell-cell contact
- soluble messengers such as cytokines, chemokines
Difference between Innate and Adaptive Immunity
The key differences between Innate and Adaptive Immunity are as
follows:
|
|
Innate |
Adaptive |
|
Response Time |
Minutes to hours |
Days |
|
Specificity |
Limited and fixed |
Highly diverse;
Adapts to improve during the course of immune response |
|
Response to repeat infection |
Same each time |
More rapid and effective with each subsequent exposure |
|
Major components |
Barriers (e.g., skin);
Phagocytes;
Pattern recognition molecules |
T and B lymphocytes;
Antigen-specific receptors;
Antibodies |
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