The immune system has evolved to protect the host from a universe of pathogenic microbes that are themselves constantly evolving. The immune system also helps the host eliminate toxic or allergenic substances that enter our body. It is a host defence system comprising many biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue. The host uses both innate and adaptive mechanisms to detect and eliminate pathogenic foreign bodies. Both of these mechanisms include self-nonself discrimination. The main parts of the immune system are: • White Blood Cells • Antibodies • Complement System • Lymphatic System • Spleen • Bone Marrow • Thymus.

1. IMMUNE RESPONSES
19th October, 2020
Submitted By,
SHRYLI K S
7th Semester,
Molecular Biology,
Yuvaraja’s College (Autonomous),
Mysuru.
YMB17118
Guided By,
Mr B Sadashiviah
Assistant Professor
Dept. Molecular Biology,
Yuvaraja’s College
(Autonomous),
Mysuru.

2. Contents
▸ Introduction
▸ The Immune System
▸ Types of Immune Systems
▸ Terminologies
▸ Immune Responses
Type of Response
▸ Innate Immune Response
▸ Adaptive Immune Response
Period of Response
▸ Primary Response
▸ Secondary Response
▸ Conclusion
▸ References
▸ Acknowledgements

3. Introduction
The immune system has evolved to protect the host from a universe of pathogenic microbes that
are themselves constantly evolving. The immune system also helps the host eliminate toxic or
allergenic substances that enter our body. It is a host defence system comprising many biological
structures and processes within an organism that protects against disease. To function properly,
an immune system must detect a wide variety of agents, known as pathogens, from viruses to
parasitic worms, and distinguish them from the organism's own healthy tissue. The host uses both
innate and adaptive mechanisms to detect and eliminate pathogenic foreign bodies. Both of these
mechanisms include self-nonself discrimination.
The main parts of the immune system are:
 White Blood Cells
 Antibodies
 Complement System
 Lymphatic System
 Spleen
 Bone Marrow
 Thymus.
White blood cells: White blood cells a re the key players in your immune system. They are made
in your bone marrow and are part of the lymphatic system.
White blood cells move through blood and tissue throughout your body, looking for foreign
invaders (microbes) such as bacteria, viruses, parasites and fungi. When they find them, they
launch an immune attack. White blood cells include lymphocytes (such as B-cells, T-cells and
natural killer cells), and many other types of immune cells.
Antibodies: Antibodies help the body to fight microbes or the toxins (poisons) they produce. They
do this by recognising substances called antigens on the surface of the microbe, or in the chemicals
they produce, which mark the microbe or toxin as being foreign. The antibodies then mark these
antigens for destruction. There are many cells, proteins and chemicals involved in this attack.
Complement system: The complement system is made up of proteins whose actions complement
the work done by antibodies.
Lymphatic system: The lymphatic system is a network of delicate tubes throughout the body. The
main roles of the lymphatic system are to:
 Manage the fluid levels in the body
 React to bacteria
 Deal with cancer cells
 Deal with cell products that otherwise would result in disease or disorders
 Absorb some of the fats in our diet from the intestine.
The lymphatic system is made up of:
Lymph nodes (also called lymph glands): which trap microbes.
Lymph vessels: tubes that carry lymph, the colourless fluid that bathes your body's tissues and
contains infection-fighting white blood cells.
White blood cells (lymphocytes).

4. Spleen: The spleen is a blood-filtering organ that removes microbes and destroys old or damaged
red blood cells. It also makes disease-fighting components of the immune system (including
antibodies and lymphocytes).
Bone marrow: Bone marrow is the spongy tissue found inside your bones. It produces the red
blood cells our bodies need to carry oxygen, the white blood cells we use to fight infection, and the
platelets we need to help our blood clot.
Thymus: The thymus filters and monitors your blood content. It produces the white blood cells
called T-lymphocytes.
As well as the immune system, the body has several other ways to defend itself against microbes,
including:
Skin - a waterproof barrier that secretes oil with bacteria-killing properties. Lungs - mucous in the
lungs (phlegm) traps foreign particles, and small hairs (cilia) wave the mucous upwards so it can
be coughed out. Digestive tract - the mucous lining contains antibodies, and the acid in the
stomach can kill most microbes. Other defences - body fluids like skin oil, saliva and tears contain
anti-bacterial enzymes that help reduce the risk of infection. The constant flushing of the urinary
tract and the bowel also helps.
An immune response is a reaction which occurs within an organism for the purpose of
defending against foreign invaders. These invaders include a wide variety of different
microorganisms including viruses, bacteria, parasites, and fungi which could cause serious
problems to the health of the host organism if not cleared from the body.

5. Types of Immune System
There are two distinct aspects of the immune response, the innate and the adaptive, which work
together to protect against pathogens. The innate branch—the body's first reaction to an invader—
is known to be a non-specific and quick response to any sort of pathogen. Components of the
innate immune response include physical barriers like the skin and mucous membranes, immune
cells such as neutrophils, macrophages, and monocytes, and soluble factors
including cytokines and complement.
On the other hand, the adaptive branch is the body's immune response which is catered against
specific antigens and thus, it takes longer to activate the components involved. The adaptive
branch include cells such as dendritic cells, T cell, and B cells as well as antibodies—also known as
immunoglobulins—which directly interact with antigen and are a very important component for a
strong response against an invader.

6. Immune Response
The Innate Response
The innate immune response is an organism's first response to foreign invaders. This immune
response is evolutionary conserved across many different species with all multi-cellular organisms
having some sort of variation of an innate response. The innate immune system consists of
physical barriers such as skin (natural barrier), cough and sneeze reflex (to push out the foreign
particles that enter our tract), tears (tear enzymes and other defence molecules), stomach acids
(HCl), and mucous membranes (antimicrobial peptides and flushed out), various cell types
like neutrophils, macrophages, and monocytes, NK cells, Dendritic cells and soluble factors
including cytokines and complement. In contrast to the adaptive immune response, the innate
response is not specific to any one foreign invader and as a result, works quickly to rid the body of
pathogens.
Pathogens are recognized and detected via pattern recognition receptors (PRR). These receptors
are structures on the surface of macrophages which are capable of binding foreign invaders and
thus initiating cell signalling within the immune cell. Specifically, the PRRs identify pathogen-
associated molecular patterns (PAMPs) which are integral structural components of pathogens.
Examples of PAMPs include the peptidoglycan cell wall or LPS, both of which are essential
components of bacteria and are therefore evolutionarily conserved across many different bacterial
species.
When a foreign pathogen bypasses the physical barriers and enters an organism, the PRRs on
macrophages will recognize and bind to specific PAMPs. This binding results in the activation of a
signalling pathway which allows for the transcription factor NF-κB to enter the nucleus of the
macrophage and initiate the transcription and eventual secretion of various cytokines such as Il-
8, Il-1, and TNFα. Release of these cytokines is necessary for the entry of neutrophils from the
blood vessels to the infected tissue. Once neutrophils enter the tissue, like macrophages, they are
able to phagocytize and kill any pathogens or microbes.
IMMUNE RESPONSES
Type of response Period of response
Innate Adaptive Primary Secondary

7. Adaptive Immune Response
The adaptive immune response is the body's second line of defense. The cells of the adaptive
immune system are extremely specific because during early developmental stages the B and T
cells develop antigen receptors that are specific to only certain antigens. This is extremely
important for B and T cell activation. B and T cells are extremely dangerous cells, and if they are
able to attack without undergoing a rigorous process of activation, a faulty B or T cell can begin
exterminating the host's own healthy cells.APC Activation of naïve helper T cells occurs
when antigen-presenting cells (APCs) present foreign antigen via MHC class II molecules on their
cell surface. These APCs include dendritic cells, B cells, and macrophages which are specially
equipped not only with MHC class II but also with co-stimulatory ligands which are recognized
by co-stimulatory receptors on helper T cells. Without the co-stimulatory molecules, the adaptive
immune response would be inefficient and T cells would become anergic. Several T cell subgroups
can be activated by professional APCs, and each T cell is specially equipped to deal with each
unique microbial pathogen. The type of T cell activated and the type of response generated
depends, in part, on the context in which the APC first encountered the antigen. Once helper T
cells are activated, they are able to activate naïve B cells in the lymph node. However, B cell
activation is a two-step process. Firstly, B cell receptors, which are just IgM and IgD antibodies
specific to the particular B cell, must bind to the antigen which then results in internal processing
so that it is presented on the MHC class II molecules of the B cell. Once this happens a T helper
cell which is able to identify the antigen bound to the MHC interacts with its co-stimulatory
molecule and activates the B cell. As a result, the B cell becomes a plasma cell which secretes
antibodies that act as an opsonin against invaders. Specificity in the adaptive branch is due to the
fact that every B and T cell is different, this is because they have a specific type of antibody and
receptors. Thus there is a diverse community of cells ready to recognize and attack a full range of
invaders. The trade-off, however, is that the adaptive immune response is much slower than the
body's innate response because its cells are extremely specific and activation is required before it is
able to actually act. In addition to specificity, the adaptive immune response is also known
for immunological memory. After encountering an antigen, the immune system produces memory
T and B cells which allow for a speedier, more robust immune response in the case that the
organism ever encounters the same antigen again.

8. Primary Immune Response
During a first encounter with foreign antigen, the immune system undergoes what is termed 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 cells incorporate messages
received from the innate players into their tailored response to the specific pathogen.
 The antigen is encountered by the immune system for the first time.
 The innate immunity acts on it.
 As part of the innate immune system, the phagocytic cell releases cytokines and
chemokines that attract other white blood cells to the site of infection, initiating
inflammation.
 Phagocytosis occurs and APCs are produced.
 That phagocytic cell may then travel to a local lymph node, the tissue where antigen and
lymphocytes meet, carrying bacterial antigens to B and T lymphocytes.
 These are recognised by helper T-cells and trough their receptors. This takes a while
because T cells are antigen specific and only a few kinds of T cell receptors can recognise a
particular kind of antigen.
 To activate the B and T cells there is a time gap thus there is a lag phase when a graph is
plotted with antibody concentration against time taken.
 Once the T cells get activated they start to proliferate.
 The proliferated T cells divide their functions into two:
a) A large number of them become short lived effector T cells and produce cytokine and
chemokines and trigger the increase production of B cells. Expansion of lymphocytes
occurs in lymph nodes.
b) A small group of them become long lived memory T cells for secondary infection.
 Of the number of B cells produced, which also have antibodies bound to their membranes,
only some are specific to the antigen.
 These B cells recognise the antigen and the antibody on them binds to the antigen at the
antibody binding sites.
 These antigens are nothing but immunoglobulins.
 The antibodies that are mainly involved here are IgM and IgG.
 These antibodies bind to the epitope region on the pathogen. And the B cell gets activated.
 The activated B cell has 2 functions:
a) Becomes a APC for further recognition
b) Undergoes repeated proliferation to produce more B cells with the antigen specific
antibody.
 The proliferation of T and B cells increases the antibody level in great folds and the graph
peaks to the log phase.
 These proliferated B cells then divide the functions among themselves.
a) Most of them become short effector B cells i.e they mature to form plasma cells.
b) A small group of them become long lived memory cells.
 The plasma cells release the antibodies to the bloodstream where they go and bind to the
antigens by the process called opsonisation and the pathogen bound molecules are now
called opsonins.
 The phagocytes recognise these opsonins and attack the pathogens.
 At this point the concentration of antigens formed and being degraded are at equilibrium
and the graph is in a stationary phase.
 After this the concentration of antigen decreases as the pathogens are eliminated and thus
the antibody production also starts decreasing, ultimately the graph declines to almost null.

9. Secondary Immune Response
All subsequent encounters with the same antigen or pathogen are referred to as the secondary
response. During a secondary response, memory cells, kin of the final and most efficient B and T
lymphocytes trained during the primary response, are re-enlisted to fight again. These cells begin
almost immediately and pick up right where they left off, continuing to learn and improve their
eradication strategy during each subsequent encounter with the same antigen. Depending on the
antigen in question, memory cells can remain for decades after the conclusion of the primary
response. Memory lymphocytes provide the means for subsequent responses that are so rapid,
antigen-specific, and effective that when the same pathogen infects the body a second time,
dispatch of the off ending organism often occurs without symptoms. It is the remarkable property
of memory that prevents us from catching many diseases a second time. Immunologic memory
harboured by residual B and T lymphocytes is the foundation for vaccination, which uses crippled
or killed pathogens as a safe way to “educate” the immune system to prepare it for later attacks by
life-threatening pathogens.
Secondary encounter with the same antigen and is acted on by innate Immune system. The same
process occurs as in primary immune response but the reaction time in the secondary immune
response is highly decreased and is short. i.e the rate of antibody synthesis, peak of the graph and
antibody persistence greatly increases. This is because of the presence of memory lymphocytes
which were formed as a result of the primary infection. The response is almost immediate thus
there is no lag phase and the log phase is much higher than the primary response. Stationary
phase is shorter since the cells already know how to eliminate the pathogen. The decline phase is
slower. However, it should be noted that new memory cells are produced in secondary immune
response too.

10. Conclusion
The coronavirus COVID-19 pandemic is the defining global health crisis of our time and the
greatest challenge we have faced since World War Two. Since its emergence in Asia late last year,
the virus has spread to every continent except Antarctica.
Coronaviruses (CoV) are a large family of viruses that cause illness ranging from the common cold
to more severe diseases such as Middle East Respiratory Syndrome (MERS-CoV) and Severe
Acute Respiratory Syndrome (SARS-CoV). A novel coronavirus (nCoV) is a new strain that has
not been previously identified in humans. Coronaviruses are zoonotic, meaning they are
transmitted between animals and people. Detailed investigations found that SARS-CoV was
transmitted from civet cats to humans and MERS-CoV from dromedary camels to humans.
Several known coronaviruses are circulating in animals that have not yet infected humans.We
have now reached the tragic milestone of one million deaths, and the human family is suffering
under an almost intolerable burden of loss.
Out of 40 million total infected patients, 30 million have recovered. How? This is where
immunity, the immune system and immune responses play a crucial role. There is still a lot of
research going on in this particular area to identify why the adaptive immune system is
particularly failing in many patients but a lot of vaccines are making their progress to the
community to boost our adaptive secondary responses.
CoVID- 19 isn’t the only disease that our immune system is failing to tackle but there are many
more such as HIV AIDS, Cystic fibrosis etc. But the field of research is always open for us.
Immunology is one vast and very interesting field that we have in the arena of biology. A lot more
can be shared and even more is waiting to be explored.

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