The document discusses immunity and the immune system. It defines immunity as the natural or acquired resistance of an individual to pathogens. The immune system is a complex system that distinguishes self from non-self and provides defense against infectious agents. Immunology is the study of the immune system and immune responses. The immune system includes innate immunity, which provides a natural defense, and acquired immunity, which develops from exposure to pathogens. Key components of the immune system are antigens, antibodies, B lymphocytes, T lymphocytes, and lymphoid organs that help the immune system function.

2. Immunity is a natural or acquired resistance of an individual to
the development of pathological condition even after having received infective
dose of virulent pathogen, its toxin or an allergen.
IMMUNE SYSTEM is a complex system of the body including
cellular and molecular components which has the primary function of
distinguishing self from nonself and defence against infectious agents, foreign
substances and cancer.
IMMUNOLOGY is the branch of biology that deals with the study of immune
system and immune responses. Study of structure and function of immune
system is called basic immunology. Other branches are Clinical immunology,
laboratory immunology, serology and immunochemistry.
Edward Jenner is known as father of immunology.
FUNCTIONS OF IMMUNE SYSTEM
 React to foreign dangerous agents.
 Immunological surveillance.
 Defence against pathogens
Virus, fungi, bacteria, protozoa,
parasites.
 Detect and remove abnormal cells.
e.g.: - tumour, damaged cells.
 Anti-allergen action
 Distinguish self from foreign.

3. INNATE IMMUNITY
 It is a natural genotype based system of defence elements with
which an individual is born and which are always available to
the living body for providing protection against various
infections.
 Also known as INBORN IMMUNITY, GENETIC IMMUNITY,
FAMILIAL IMMUNITY.
 There is no antigenic recognition or development of specific
immune response. It is therefore called nonspecific immunity.
 It is the major source of body defence against pathogens in most
organisms.

4.  Psychological barriers: -
Operate at biochemical and functional levels. They
include friendly microbes, body temperature, ph and
body secretions.
1: - Friendly microorganisms: - They occur over the
skin, nasal chambers, intestine and vagina. They
produce secretions harmful to pathogens.
2: - Body temperature: - Temperature rises in response
to toxins produced. by pathogens. Fever or rise in
temperature inhibits growth of many pathogens.
3: - Body secretions: - Acid(HCl) in stomach, Oil,
Sweat, Cerumen(earwax), lysozyme (present in tears,
saliva, mucus, sweat) also inhibit growth of pathogens.
 Physical barriers: -
They are barriers which do not allow the entry of
pathogens and other foreign agents into the body.
It includes skin, hair, mucous membranes, mucus, cilia
and friendly microorganisms.
1: - Skin: - It is covered by a horny layer of dead keratinized cells
which does not allow entry of foreign agents.
2: - Cilia: - They occur in nasal tract. The particles and microbes
trapped in mucus are pushed outwardly by cilia for throwing out.
3: - Mucous membranes: - The membranes lie in all internal tracts
i.e. digestive, respiratory and urinogenital. They prevent the entry
of invading pathogenic organisms.

5.  Cellular barriers: -
They are of two types, phagocytic barriers and
natural killer cells.
 Phagocytic barriers: - They bring about
phagocytosis of invading microorganisms and
foreign particles. Phagocytic barriers are, therefore,
an important component of innate immunity. There
are two types of phagocytes, leucocytes and
macrophages.
Phagocytic leucocytes or WBC's are of two types
neutrophils and monocytes.
1.Neutrophils: -
Also known as PMNL (polymorphonuclear
leucocytes)
come out of blood capillaries and reach the site
of infection
Most abundant phagocytic leucocytes.
2.Macrophages: -
Components of reticuloendothelial system
Large, Irregular phagocytic cells present in both
fixed and wandering states
Attack, engulf, eliminate microbes and foreign
particles whenevertheyhappen toenterthe body.

6.  NKC's (Natural killer cells): -
Small lymphocytes which originate from bone marrow
Specialized to perform cytotoxic activity without prior
sensitization
have interferon augmented activity against virus infected
cells
 Take part in APOPTOSIS.
 Cytokine barriers: -
Nonantibody proteins which are released by cells after coming in contact
with antigen. Intercellular mediators that produce immune response.
 Interferons: -They are glycoproteins released by living cells in
response to viral attack. Interferons. They make the surrounding cells
resistant to viral infection by inhibiting multiplication of viral particles.

7.  Inflammatory barriers: - A localised immune response appears at
the site of infection or tissue injury which is manifested as redness,
swelling, pain and heat.
The damaged mast cells of connective tissue produce alarm signals in the
form of histamine and prostaglandins.
 Complement system: -
Contains 30 serum proteins which have a
cascade like effect resulting in lysis of
microbes.
TYPES
1.Classic pathway: - Functions
in acquired immunity.
2. Alternative pathway: -
Functions in innate immunity.
Alternate pathway: -Also known as
PROPERDIN SYSTEM. It is activated
directly by presence of bacterial endotoxins, microbial polysaccharides,
microbe cell wall and other components of invading microorganisms.
Certain proteins of system undergo cleavage and form
i) membrane attack complex (Lytic complex)
ii) biologically active fragments. Protein components of membrane attack
complex get embedded in the plasma membrane of microbe and form pores.
As a result, water enters the microbe and it bursts.

8. ACQUIRED IMMUNITY
 It is immunity or occurrence of resistance to a disease which develops during
life time of an individual by obtaining or producing antibodies and cells
against the specific microorganisms.
 Also known as adaptive immunity and specific immunity.
 Occurs only in vertebrates.
 Develops only on exposure to concerned microorganism
Primary response: -The first encounter with the pathogen produces
a low intensity response called primary response.
 Effected by IgM, NK cells and alternate pathway of immune system.
 Slow and feeble response.
Secondary response: -Subsequent encounter with the same
pathogen produces a highly intensified response called secondary response or
anamnestic response.
 Heightened and quick response, lasts longer
 Effected by lymphocytes.

9. Features of acquired immunity: -
1. Specificity: - It is specific for each type of pathogen. Therefore, it has the
ability to distinguish among various types of foreign molecule.
2.Diversity: - It can develop against all diverse types of pathogens, their
toxins and other molecules.
3.Discrimination between Self and nonself: - It can differentiate foreign and
body cells and molecules.
4.Memory: - The first encounter between the specific foreign agent or
microbe and the body's immune system produces both immune response
and memory. Because of it a second encounter with the same pathogen
TYPES OF ACQUIRED IMMUNITY: -
Acquiredactiveimmunity:-Immunityacquiredbytheindividualeitherduring
vaccination or previous contraction of a disease.
Period required for developing it is long
May last a few months (e.g.: -typhoid vaccination), to life long
(e.g.: -chicken pox)
Acquiredpassiveimmunity: -Immunityto diseaseisacquired duetoobtaining
antibodies from outside.
Foetus obtains antibody against diseases from mother through placenta
(IgG) and Colostrum(IgA) which is a thick yellow colored early milk
Develops quickly comparatively to active immunity.

10. IMMUNE SYSTEM
 It is a specialized system of the body that recognizes foreign
antigens, responds to them for their elimination and keeps a
memory of the same.
 Important role in allergies, autoimmunity and organ implantation.
 Consists of antibodies, cells, tissues and lymphoid organs.
 Working of immune system is based on two components, humoral
and cell mediated.
HUMORAL IMMUNE SYSTEM OR ANTIBODY
MEDIATED IMMUNE SYSTEM(AMIS): -
 It consists of different types of antibodies that
occur in body humors or fluids like lymph and
blood plasma.
 Antibodies are formed by plasma cells which
are in turn formed by B lymphocytes.
 AMIS occurs against pathogens that enter body
fluids.
CELL MEDIATED IMMUNE SYSTEM(AMIS): -
 It is a component of immune system which consists of T lymphocytes.
 Cell mediated immune response or Cell mediated
immunity(CMI) functions against pathogens
which pass into host cells.
 Also operates against cancer cells and
transplants.

11. B-LYMPHOCYTES
 Some lymphocytes are processed in liver during embryo stage
and bone marrow after birth called B-lymphocytes.
 Short lived, only for few days.
 Component of Antibody Mediated Immune System(AMIS)
 Produce specific plasma cells which secrete antibodies
FUNCTIONS: -B-cells are sensitized both directly by
antigens as well as by helper T-cells. An activated
B-lymphocyte enlarges and undergoes division to form PLASMA
CELLS. PLASMA CELLS have abundant endoplasmic reticulum.
They secrete antibodies. One type of plasma cell secretes only a
particular type of antibody.
T-LYMPHOCYTES
 Some lymphocytes pass to thymus for preprocessing called T-
lymphocytes or THYMUS PREPROCESSED LYMPHOCYTES.
 They live for 4-5 years. Some live throughout life.
 Component of Cell Mediated Immune System(CMIS)
TYPES OF T-LYMPHOCYTES AND THEIR FUNCTIONS: -
On coming in contact with antigens, a T-lymphocyte produces a clone of
lymphocytes.
Clone has four types of cells
Helper T-cells(TH): -
 Form 75% of the total lymphocytes of a clone.
 Secrete Lymphokines for performing several kinds of functions like
proliferation of other T-cells, stimulation of B-lymphocytes, attraction
of macrophages and feedback.

12. Killer or cytotoxic T-cells(TC): -Cytotoxic T-cells reach the site of infection, come
in contact with microbes and secrete perforins. Perforins form holes. It is followed by
secretion of toxic materials into the microbes for killing the same. Soon after a killer
T-cell separate and attacks another pathogen.
TC cells also attack cancer cells, cells of transplanted organs and TH cells invaded by
HIV
Memory T-cells: -They are those cells which previously sensitized and retain the
sensitization for future.
CLONAL SELECTION: -
Body has numerous types of both B and T cells, each with a specific its surface. The
number lymphocytes carrying specific receptor is therefore, small. In case of B-
lymphocyte, the receptor is antibody being produced by it. If a receptor carrying
lymphocyte happens to come in contact with antigenic determinant specific to it, the same
becomes activated. The activated lymphocyte specific for the antigenic determinant now
begins to divide rapidly and produce a large clone of cells. The formation of a clone of B-
Day and T-lymphocytes against a particular antigenic determinant or antigen is called
clonal selection. All of them are derived from a single parent cell and exhibit the same
specificity. The cells then differentiate into different types, mainly effector and memory
cells. While the effector cells take part in elimination of antigen containing foreign agent,
the memory cells are long lived lymphocytes which keep memory of contact between
antibody determinant and lymphocyte receptor.
.

13. LYMPHOID ORGANSthey are those organs which function as site
of formation, multiplication and maturation of lymphocytes.
 Primary lymphoid organs: -
These are the organs where B-lymphocytes and T-lymphocytes are formed,
mature and acquire their antigen specific receptors
 Bone marrow and thymus are primary lymphoid organs.
 Bone marrow is the organ where all types of blood cells including
lymphocytes are formed.
 B-lymphocytes mature in bone marrow while T-lymphocytes mature
in thymus.
 They provide microenvironment for development and maturation of T-
lymphocytes. Thymus is lobed endocrine gland situated below sternum
near the heart. It is quite large at birth and keeps on reducing with age
so that at puberty it is quite small.

14.  Secondary lymphoid organs: -
These are the sites of proliferation and differentiation. These organs where
lymphocytes reside after maturation are called secondary lymphoid organs.
Lymph nodes, spleen and lymphoid tissues of respiratory tract, digestive tract
(Peyer’s patches, appendix, tonsils) and other mucosal surfaces.
 MALT: - Lymphoid tissue is located in lining of different tracts of the body (e.g.
respiratory, digestive and urogenital) is uncapsulated and called Mucosal
Associated Lymphoid Tissue(MALT). it constitutes nearly 50% of lymphoid
tissue of the body. Both types of lymphocytes reside in the same secondary
lymphoidorgan, of course, in fairly distinctareas. Whenever aforeign agententers
the body, it is trapped by secondary lymphoid organ present near the portal of
entry and mounts an immune response with the help of its T-cells and B-cells.
 Spleen: - It is a large bean-shaped vascular organ which is popularly called blood
bank and grave yard of RBCs. It is present on left side nearly in contact with
stomach. Spleen filters and traps blood borne microbes. Antibodies produced by
lymphocytes take part in eliminating the microbes through phagocytes.
 Lymph nodes: -These are small oval solid swellings present on lymph vessels
at several places but more abundant in neck, chest, armpits, groins, tonsils, sub
maxillary area. Each lymph nodes are partitioned internally into channels. It also
possesses follicles having lymphocytes. Lymph nodes filter out microbes and
other antigens. The trapped antigens are acted upon by lymphocytes and
macrophages.

15. ANTIGENS Antigen is any foreign substance, toxin or particle or pathogen
which induces the immune system of the body to produce cells and antibodies to
dispose the same.
An antigen often has higher molecular mass of 8000 daltons or more. Antigenic
substances are generally proteins and polysaccharides. Pathogens function as antigens
because they possess either antigenic material on the surface or produce antigenic
toxins. Sites present over surface of antigen that are recognized by T- and B- cells and
antibodies are called ANTIGENIC DETERMINENTS OR EPITOPES. The regions
of antibodies and lymphocytes that function as receptors for epitopes are called
paratopes.
ANTIBODIES
They are glycoproteins, called immunoglobulins which have specific amino acid
sequences by which they can interact with specific antigens. Antibodies form 20%
plasma proteins.
STRUCTURE: -
Each antibody has a combination of atleast 2 light(L) and 2 heavy(H) polypeptide
chains(H2L2). The heavy chain has a large number of amino acids while a lighter
chain has smaller number of them. Usually, the polypeptide forms a Y-shaped
configuration. The stem of Y is exclusively formed by heavy chains. In the arms of
Y, both light and heavy chains occur parallel to each other except for antigen
binding sites. Attachments and bending occur by means of disulphide bonds( S S ).
In certain immunoglobulins the number of chain pairs can be 10. An antibody has
a variable portion in the arms. It is called the V-region or antigen binding fragment
Fab. The remainder of the antibody is called constant portion or crystalline fragment
Fc.

16. TYPES OF ANTIBODIES: -
1.IgA: -It is present in all body secretions including mother’s milk and colostrum.
Colostrum is thin, yellow protein and mineral rich early milk of the mother.
 IgA is also called secretory immunoglobulin.
 Take part in activating alternate pathway.
 Form first line of defence against inhaled
and ingested pathogens.
 Enables a person to fight against pathogens even before becoming sensitized.
 Effective against a number of antigens, especially the ones containing
polysaccharides.
2. IgD:-It occurs in small quantities in serum tissue.
 It is effective against toxins and allergens.
 Along with IgM it appears on the surface of B-cells as
antigenic receptor.
 IgD activates B-cells.
3.IgE: -The antibody is concentrated in mucous membranes, skin and lungs. It is
mediator in allergic response.
IgE triggers off immediate hypersensitive reactions on contact
with antigens. For this it attaches to basophils and mast cells.
The cells are induced to release histamine and other inflammatory
substances.
4.IgM: - It is largest of the antibodies but its small in number.
 First to reach the site of infection.
 It is pentamer, having ten binding
sites so it is highly effective.

17. 5.IgG: -
 Constitute 75%.
 Can pass through placenta and has is present
in mother’s milk.
 Abundant in blood, lymph and intestine.
AUTOIMMUNITY
Memory based acquired immunity evolved in higher vertebrates based on the
ability to differentiate foreign organisms from self cells. Higher organisms can
distinguish foreign molecules as well as organisms. But sometimes due to genetic
and other unknown reasons, the body attack self cells. This results in damage to
the body and is called auto immune disease. RHEUMATOID ARTHRITIS is an
example.