immunity chapter Immunity System and types of immune systems

Immune systems types of immune system First line of defense second line of defense

1. Chapter 13
IMMUNITY
The ability of body to resist microorganisms, their toxins if any , foreign
cells and abnormal cells of body is termed as immunity.
Study of immune system is known as immunology.
Immune system
A complex network of cells, tissues, organs, and the substances they
make helps the body fight infections and other diseases.
Immune system is divided into two functional divisions
Innate immune system
It is part of immune system which provide us natural immunity.
It is non specific and works against any pathogen which tries to enter body.
It consists of two lines of defenses:
1. First line of defense
First line of defense is considered to be non specific in nature since it
prevents the entry of all kinds of pathogens into body.
It is manifested by physical and chemical barriers .
The physical barriers are skin, mucous membrane and ciliated epithelium.
Chemical barriers consist of saliva, tears, mucous, gastric juice.

2. A. Skin
Skin is the one of largest organ of our body.
It performs different functions such as maintaining temperature, synthesis
Of vitamin D but primary function is to act as a barrier.
Composed of three main layers: epidermis, dermis, hypodermis
Epidermis
It is outer most layer of skin lack blood supply and is dependent upon
dermis for supply of nutrients.
Made up of closely packed dead cells with keratin which make the skin
surface mechanically tough and resistant to degradation by bacterial enzyme.
Fatty acids on the skin surface create a dry, salty, and acidic environment
that inhibit growth of some microbes.
Dead cells of epidermis are frequently shed along with microbes that may
Be clinging to them.
Wandering among the three layers of skin, there is variety of immune cells
such as Langerhans cells (skin resident macrophages) and T cells for the
elimination of intruding microbes.

3. Dermis
It lies below the epidermis and consisting of living cells, blood vessels and
nerves.
It contain sweat gland (sudoriferous gland ) and sebaceous gland which
secretes sweatandsebum (oil).
Sebaceous gland opens into hair follicle to lubricate the hair.
Sebum contains some acids which make a thin film on the skin surface and
serves as chemical barrier on skin.
Sweat gland secrete variety of polypeptides for the inhibition of growth of
microorganisms on the skin surface.
Hypodermis
It is innermost layer of skin mainly contain fat storing adipose tissues.
It connect the skin with underlying muscles and serves as insulating layer.

6. B. Digestive system
Alimentary canal is lined bymucosa and in addition, our stomach has
gastric gland which secrete Hcland variety of enzymes to destroy microbes.
The internal lining of digestive canal, respiratory passage and reproductive
structurecontain mucosa (mucous membrane) which secrete mucous.
These linings are made up of epithelial cells.
It serves as non specificphysical and chemicalbarrier.
In addition to mucosa and mucous, digestive tract also contain lymphoid
tissues such as tonsils in pharynx, Peyer’s patches in small intestine and
appendix.
Lymphoid tissues are rich in macrophagesand lymphocyteswhich kill the
germs.
Another sector of lymphoid tissue are WBCs(plasma cells and lymphocytes)
wandering in basement membrane of small intestine.

7. C. Air passage way
Mucosa lines the air passage way also help in defense mechanism by
trapping germs and dust particles.
The ciliated epithelium in lining of air passage way remove contaminated
mucous out of tract and serves as first line of defense.
Mucus membrane of respiratory system is consisting of ciliated columnar
epithelium.
The ciliaand mucusof epithelium propel the germs out of tract.

8. Immune system
A. Innate immune system
First line of defense
It is non specific and consisting of physical and chemical barriers
Physical barriers include skin, mucus membrane and ciliated epithelium.
Chemical barriers include saliva, mucus, gastric juice, tears and spermine in
semen.
Second line of defense
It is also non specific and consisting of different cells and molecules.
It includes phagocytes, natural killer cells, inflammatory response,
antimicrobial proteins, fever.
B. Adaptive immune system
Third line of defense
It is specific in nature.
It is consisting of two parts
1. Cell mediated immunity
2. Antibody mediated immunity or humoral immunity

9. 2. Second line of defense
The pathogens may enter our body when it is broken either accidently like
cut, burntetc or either through insect bite.
Like first line of defense, it is also non specific in nature and comprised of
wide variety of WBCs and proteins.
It is accomplished by major responses such as phagocytosis, cytotoxicity,
inflammation, fever.
Phagocytosis
It is a process through which some specific type of WBCs take in and digest
micro-organisms.
Among the WBCs neutrophils, macrophages (monocytes), dendritic cells
perform phagocytosis.
Neutrophils
These are most abundant WBCs (50 to 70% of all leucocytes) and are
specially adapted to phagocytosis of bacteria.
The nature neutrophils have multilobed (3-5 lobed), non spherical nucleus.
They are usually short lived and upon maturation they migrate from blood
to tissues.

10. Through specific receptors on their surface, neutrophils recognize various
bacterial molecules such as peptidoglycans, flagellin protein of flagella,
lipopolysaccharide, lipopeptideetc.
Upon recognition, intruders (germs) are engulfed by neutrophils and
digested.
Monocytes
They have single bean shaped nucleus and cytoplasm is non granulated.
They constitute about 2-8% of total WBCs.
Like granulocyte they originate from myeloid stem cells.
Average life span is about 2 to 5 days in blood circulation.
They migrate from blood to tissue where they grow in size and become
either macrophages dendritic cells and live for months to years.

11. Macrophages
The leucocytes other than neutrophils involved in phagocytosis of foreign
cells, worn out cells are macrophages.
They are derived from monocytes and become macrophages when enter
lymphoid and non lymphoid tissues.
They have horse shoe shaped nucleus and are largest among all leucocytes.
Macrophages have ability to adapt the specific tissue conditions so they are
also named such as alveolar macrophages in alveoli of lungs, Kupffer cells in
liver, microgliain nervous system.
Macrophages not only phagocytize germs but also remove and present
their antigen to T-cellshence they are also termed as antigen presenting cells.
They release specific signaling molecules called cytokinesto activate other
immune cells.
Macrophages also initiate the process of inflammationand secrete nitric
oxidewhich kills phagocytized germs.

12. Dendritic Cells
Dendritic cells are another crucial component of the myeloid lineage.
They are specialized antigen-presenting cells that bridge the gap between
the innate and adaptive immune systems.
Dendritic cellscapture and process antigensfrom pathogens, and then
present these antigens to T cells, initiating adaptive immune responses.
Upon activation through specific inflammatory cytokines secreted by
macrophages , dendritic cells migrate to secondary lymphoid tissues such as
tonsils, Peyer’s patches, spleen, tonsils.
As soon as their task of activation of T cells achieved, unlike macrophages
they die.

13. Cytotoxicity
It refers to ability of a substance or certain cells to damage or cause death
of target cell.
It works against virally infected cells, cancer tumor cells.
Process of cytoxicity is carried by Natural Killer cells or Cytotoxic T cell.
Although both NK cells as well as Tc cells are lymphoid in cell lineage and
originate in the bone marrow yet they are different in mode of activation
against their target cell.
NK cells
Type of lymphocyte involved in detecting and eliminating virally infected
cells as well as cancer cells.
In such cases, unlike Tc cells, they do not need any prior activation by
antigen presenting cells hence named as natural killer cells.
NK cells check on the cell surface, a conjugate molecule called Major
Histocompatibility complex-I(MHC-I) of own body cell for identification.
In presence of MHC-I, the NK remain inactive while in absence of MHC-I on
target cell, it is activated to destroy the target cell.
The lysis of target cell is brought about by releasing cytotoxic granules
having perforinsand granzyme.

14. Porins forms pores in target cell while granzyme enter the target cell and
cause its lysis.
Cytotoxic T Cells
It is also a type of lymphocyte produced by bone marrow and matures in
thymus and then released into blood.
During their maturation process, T cells which develop CD8 receptor along
with T cell receptor on surface are turned into cytotoxic cells.
Through their receptors CD8 and TCR, the cytotoxic T cells identify Major
Histocompatibility complex-I present on all of their own nucleated cells of our
body except macrophages because they have Major histocompatibility
complex-II.
For activation of cytotoxic T cell, several chemical signals are required from
T cells and dendritic cells.
When cytotoxic T cell encounter virally infected cell, they recognize them
immediately through the viral antigens expressed on MHC-I receptor of
infected cell.
Tumor Necrosis Factor-alpha and interferon gamma released by infected
cell activate the macrophages and dentritic cells.

15. Like NK cells, they release Perforin proteins and apoptotic protease known
asgranzyme which cause lysis of cell.
Unlike NK cells they proceed toward other infected cells and cause lysis.

18. Protective proteins (compliment protein)
In our body fluids, different proteins circulates and protect us from germs.
Upon recognition of bacteria, compliment proteins are activated.
Some of them damage the plasma membrane of target cells while others
attract different cells of immune system such as mast cell, neutrophilsand
macrophagesto enhance the immune responses.
Inflammatory response
Inflammation (to set on fire) is one of non specific immune responses
which is usually a consequence of infection.
However, it may be due to other factors like tissue damage, irritant, auto
immune disorders.
It is categorized as two types, acute and chronic.
The acute type is short term inflammation and is characterized by redness,
hotness, pain, swelling and loss of function of affected organ.
Chronic inflammation is long term in duration and occurs due to any disease
like allergy, cardiovascular disorder.
Whenever damage to our tissues takes place either through an accident or
a pin prick to skin, bacteria enters our tissues.

19. As a result damaged tissues release different proteins such as histamineand
prostaglandinswhich dilate near by blood vessels and attract neutrophils and
macrophagesto the site of infection.
A war is set on that area where our own cells may also die forming a thick
paste like, yellow , greenish pus.
Fever(pyrexia)
When body temperature rises above the normal, it is termed as fever.
It could be a sign of infection, heat stroke, brain tumors, toxins, some
medicines.
The substances which induce fever are termed as pyrogens.
These may be endogenous(own cellular secretions) or exogenous(external
source).
The important pyrogens released from actively macrophages in response to
bacteria and viruses are interleukin-1and interleukin-6, tumor necrosis factor
and alpha interferons.
Pyrogens are transported to hypothalamus through blood.

20. Hypothalamus produces prostaglandins to increase thermogenesis and
decrease heat loss and cause rise in body temperature.
High temperature reduce the multiplication of bacteria and fungi so that
immune system cells can easily kill them.
Bacteria produces some toxins (exogenous pyrogens)which are detected by
brain as pyrogens and induce fever.
Antipyreticssuch as aspirin, paracetamol, ibuprofen inhibit prostaglandin
secretion by hypothalamus to reduce pain.
3. Third line of defense
The non specific second line of defense is supported by third line of
defense.
It is complex and specific in nature.
There are two types of WBCs involved in third line of defense; T cells and B
cells.
B lymphocyte mature in bone marrow while T lymphopcytes are released in
immature form from bone marrow and mature in thymus and again released
into blood.
B cells were named as they were discovered in glandular sac opening called
Bursa of Fabricus into cloaca of chicken.

21. This part of immune system is termed as Acquired or adaptive immune
systemand its responses vary from person to person and is most powerful.
The cells which contain MHC-I receptor are considered as selfand immune
system remain inactive.
When any antigen is found on MHC-1 receptor, immune system become
active and attack on infected cells to destroy germs.
MHC-I in human are also termed as Human Leukocyte antigen.
Adaptive immune system provide immunity two ways
A. Cell mediated Immunity through T lymphocytes
B. Antibody mediated immunity through B lymphocytes

22. 1. Humoral immunity
It is provided by antibodies secreted by B cells in the circulatory system.
This is helpful in bacterial infection.
Each B cell has specific type of antibodies on surface which serves as
antigenic receptor.
When infection occurs, the antibodies on surface of B cell will bind to
antigens on the surface of microorganism.
Antigen-antibody complex cause B cells to divide rapidly to give rise large
effector cells called plasma cells.
Plasma cells secrete antibodies into circulation that combine with germs.
Microorganisms bearing such antibodies are identified, engulfed or
lysed.
Antibodies also neutralize toxin produced by bacteria and cause
agglutination of microorganisms.
Some of B cells do not secrete antibodies and become B memory cells and
help in future re-infection.

23. 2. Cell mediated immunity
This type of immunity is offered by T lymphocytes which do not secrete
antibodies.
They mediate immunity by killing infected cells and help in inflammation.
This is important in defense against virus and some parasites that hide
within the host cells and tumor cells.
Several type of T cells work in cell mediated immunity.
A. Cytotoxic T cells
The T cells having CD8 receptors and T cell receptor identify the specific
antigen on virally infected target cell and cause lysis.
B. Helper T cells
These cells upon activation through CD4 receptors on their plasma
membrane by APC (antigen presenting cells) release cytokines like interleukins,
interferons to activateB cells, Tc cells and other macrophages to destroy
specific antigen carrying target cells.

24. C. Suppressor T cells (Ts) or regulatory cells
These cells suppress the activity of activated Tc cells, helper T cells, B cells
after completions of their work.
If their activity is not suppressed, they will harm own body cells.
They also posses CD4, CD8 and TCR receptor on plasma membrane
D. T Memory cells
These cells are capable of surviving for years even after the elimination of
infectious organisms.
T memory cells rapidly proliferate and become effector T cells and play
important role in re-infection.

25. Active immunity
Immunity acquired by own immune response is called active immunity.
If immune system is activated naturally by infection, it is said to be natural
active immunity.
If immune system is activated by giving vaccine, it is said to be artificial
active immunity.
Passive immunity
A type of immunity acquired by antibodies transported from another
person or even an animal to develop immunity in body.
If antibodies are transferred to one person from same species, it is said be
natural passive immunity.
For example: A pregnant mother passes some of her antibodies into her fetus
through placenta.
The first breast feeding of mother, the colostrum also contain some antibodies.
Such immunity is short lived and provide temporary protection.
If antibodies are transferred from animal or human (who is already immune to
that disease have antibodies) to human, it is said to be artificial passive
immunity.

26. Primary immune response
The first exposure of germs to immune system.
It takes long time about 5 to 10 days to develop clone effector cells which
work in future memory in case of re-infection.
Secondary immune response is quicker than primary response.
It develops to its maximum in 3 to 5 days.
This quicker response is possible due to immunological memory.
It provide life long protection against some diseases such as small pox.