Immunity

SHORT NOTES ON IMMUNITY

1. IMMUNITY



Innate immunity:
It is also known as native immunity. It is the resistance which individual possesses by
birth. It is due to his genetic and constitutional make up. This type of immunity is present
for life. It is non-specific type.

Types of innate immunity:
Species immunity: It refers to the resistance to a pathogen shown by all members of a
particular species. eg. B. anthracis infects human beings not chickens, birds are immune
to tetanus. The physiological and biochemical differences between tissues of different
host species may be responsible for species specific resistance.

Racial immunity: Within same species, different races may exhibit different type of
resistant to infection. Eg: Algerian race sheep is immune to anthrax which is a common
Immunity
Acquired
(Specific)
Active
Natural
(exposure to
disease)
Artificial
(Vaccination)
Passive
Natural
(From mother to
fetus)
Artificial
(Injection of Ab)
Innate
(Natural)

2. disease of other races of sheep. In humans, White Americans are more resistance to
tuberculosis than that of American negroes.

Individual immunity: Resistant to infection varies with different individual of same race
and species. This is known as individual immunity.

Mechanism of Innate Immunity:
Innate immunity is the resistant which individual possesses by birth.
The body can mobilize many factors which act non-specifically to the (invasion) entry of
foreign organisms. These includes:
- External defences mechanism:
- Internal defences mechanism:


External Defences Mechanism:
Skin:
Skin acts as mechanical barrier to invasion of m/o to the body. It also provides
bactericidal secretions, like high concentration of salt in drying sweat. The sebaceous
secretion containing unsaturated fatty acids and free saturated fatty acids have
bactericidal and fungicidal activity.

Mucous membrane:
The mucous secretion by the mucous membrane of respiratory tract, digestive tract,
urogenital tract and other tissues also form a protective covering. They collect and hold
many m/o until they can be disposed off or close their infectivity and are expelled out by
the peristalisis movement of mucous membrane.

Conjunctivae:
Tears (lacrymal secrection) have a major role by flushing away bacteria and other dust
particles. In addition, lysozyme present in tears has a bactericidal action.

3. Genitourinary tract:
The flushing action of urine eliminates bacteria from urethra. The acidic pH of vaginal
secretions in female, due to the fermentation of glycogen by lactobacillus (normal flora)
makes vagina free of many pathogens. In males, it is believe that semen contains some
antibacterial substances.

Intestinal tract:
Mouth possesses saliva which has an inhibitory effect on many m/o. Some bacteria may
be swallowed and are destroyed by acidic pH of gastric juice.

Other barriers:
1) Lysozymes: It is an enzyme found in many body fluids and secretions except CSF,
urine, sweat, which has an effective antimicrobial action and can lyse certain bacteria.

2) Coughing: It is also thought to be an one of the important non-specific defence
mechanism. The inhaled particles in respiratory tract are arrested in the nasal passage by
moist mucous membrane. The mucous secretions of respiratory tract act as a trapping
mechanism and hair like cilia propels the particles towards pharynx where it is coughed
out or swallowed to the intestine.

3) Perspiring: The term perspiring means secretion of body fluid through the pores of
skin or simply sweating. The sweat in drying condition contains high concentration of
salt as well as the secretion also contains long chain fatty acids which also has
bactericidal activity.

4. Internal Defence Mechanism:
Phagocytosis:
(Phagocytosis is defined as the process in which phagocyte cells, recognize, ingest,
intracellular killing or lysis of foreign particle or organisms)
Phagocytosis is defined as the process of engulfment of solid particulate materials by
cells (cell-eating). The cells performing such functions are called phagocytes.
The process includes recognization of foreign particles or organism by the phagocytic
cells, followed by their ingestion, killing or lysis and the neutralized products are released
in the blood circulation.
There are 2 main types of phagocytic cells.
i) Polymorphonuclear neutrophils (PMNs):
Appears in acute inflammatory response also called microphages.
ii) Circulating monocytes and fixed tissue mononuclear phagocytes called as
macrophages.

The mechanism of phagocytosis involves in following steps:
i) Chemotaxis: Phagocytes reach to the site of infection or to the site of entry of
microbes.
ii) Attachments: (Adherence) Microbes adhere to the surface of the phagocytic cells by
electrostatic force. Such attachment is brought by the presence of opsonin’s ,
complements, IgG . (Opsonization: It is the process in which m/o or other particles are
coated by serum components and preparing them for recognization and ingestion by
phagocytic cells)
iii) Ingestion: The microbes are ingested into the phagocytic cells by the extension
pseudopodia around the microbe. The phagocyte engulfs the microbe into a vacuole
called phagosome fuses with a lysosome forming a complex called phagolysosome.
Lysosome contains hydrolytic enzymes and other bactericidal substances.
iv) Intracellular killing and Digestion: The hydrolytic enzymes of the lysosome granules
are discharged into the phagolysosome leading to intracellular killing in few minutes,
although complete digestion takes few hours.

5. Microbes that resist killing and digestion by phagocytes get adapted to grow within the
phagocytes producing diseases.
Fig: Phagocytosis

6. Complement System:
The complement system is a non-specific protective system common to all vertebrates. It
consists of a set of around 20 immunologically and chemically distinct different soluble
glycoprotein (serum proteins) which are produced by hepatocytes and are present in
blood and other body fluids.
Complement system acts only on m/o sensitized by specific Ab. They possesses
bactericidal activity. Proteins of complement are independent and work in a cascade.
Complement are synthesised rapidly during acute inflammatory response. Thus, also
known as acute phase protein.

General properties of complement:
 Present in all normal serum.
 Does not increase upon immunization.
 They are non-specific serologic reagent.
 Complements are activated by antigen-antibody complex.
 IgM, IgG1, IgG2 and IgG3 react with complement.
 They are heat labile (inactivated at 56
0
C for 30 min)

Complement generally works in 2 pathways:
i) Classical pathway:
The classical pathway includes 9 proteins. The classical pathway is initiated by the
binding of complement to the Ag-Ab complex. The component responsible for classical
pathways are grouped under 3 fractional unit.
C1 – the recognization unit
C4 –C2 –C3 – the activation unit
C5 - C6 - C7 - C9 – the membrane attack unit

Fig:

8. ii) Alternate pathway:
The alternate pathway includes 13 proteins. The alternate pathway starts from C3
complement. Alternate pathway generally activated by non-immunological means, such
as bacterial cell surface and their components like enzymes, endotoxins and microbial
polysaccharides.
Interferons: (IFN’s)
Interferons are a group of low molecular weight regulatory cytokines produced by certain
eukaryotic cell in response to a viral infection. They are believed to be the bodies first
line of defence against viral infection.

Interferons usually are species specific but virus non-specific. Several classes of
interferons are recognized.
i) IFN’s – α: synthesized by virus infected leucocytes.
ii) IFN’s – β: derived from virus infected fibroblast.
iii) IFN’s – γ: produced by Ag-stimulated by T-cells and Natural killer cells.

General properties:
 They donot contain nucleic acids
 They are non-toxic and non antigenic.
 They can be inactivated by proteolytic enzyme.
 They can tolerate heating at 37
0
C for 30-60 min.


Natural killer cells:

Natural killer cells are large lymphocytes which contain azurophilic granules in the
cytoplasm. They are also known as large granular lymphocyte. These are found in spleen
and peripheral blood.

9. Natural killer cells are cytolytic for virally transformed cells, the certain tumors and also
in allograft (same species) rejection. Hence, it is believed that natural killer cells play an
important role in antiviral and in anti-tumour immunity.



Specific Defence Mechanism (Acquired Immunity):
The resistant acquired by an individual during its life is called acquired immunity or
specific immunity.
It is of two types:
i. Active
ii. Passive

i) Active immunity:
It is the resistant developed by an individual as a result of effective contact with an
antigen. The contact may be in the form of natural infection or by vaccination such
contact leads to stimulate the immune system to form antibodies or the production of
immunologically active cells. Active immunity develops slowly over a period of days to
weeks but once developed, it is long lasting.

Mechanism:
The response of active immunity stimulates both humoral and cell mediated immunity
usually in parallel.

1) Humoral immunity (Antibody Mediated Immunity, AMI): It is an antibody mediated
immunity. It depends on the synthesis of antibodies by plasma cells. These cells produce
circulating antibody which combines specifically with antigens and modify their activity.
This modify activity may be in the form of lysis of antigen molecules, neutralisation of
their toxins or removal of antigen by phagocytosis.
2) Cell Mediated Immunity (CMI): It mainly depends on the specifically developed T-
cells by certain antigens. The cell mediated immunity sensitised T-lymphocytes which is

10. important in resistance to chronic bacterial infections. In chronic infection, organism can
multiply and survive in phagolysosome (Tuberculosis, Leprosy) and in viral infections
(Herpes simplex)

Types of Active immunity:
1) Natural active immunity: It is acquired by natural infection or (repeated exposure to
small dose of the infecting organisms which were unnoticed), such immunity is long
lasting and plays important roles in preventing epidemics, eg: poliomyelitis, tuberculosis.
Some of these follow over infections eg: small-pox.

2) Artificial active immunity: It is the immunity produced by vaccination. The vaccines
are prepared from live, attenuated or killed m/o or their antigens or toxoids. (In killed
vaccines the organisms are killed by heat, formalin, phenol and alcohol). Toxoids are
prepared from bacterial exotoxins inactivated by formalin. Toxoids are immunogenic but
not toxigenic.

Eg. of vaccines.
a) live vaccines:
- BCG for Tuberculosis
- Ty 21a for Typhoid
- Sabin vaccine for poliomyelitis
- MMR vaccine for measles, mumps and rubella
- 17D vaccine for yellow fever

b) killed vaccines:
- TAB for enteric fever
- killed cholera vaccine
- Neutral and non-neutral vaccine for rabies

11. - salk vaccine for poliomyelitis
- Hepatitis B vaccine

c) Bacterial products:
- Tetanus toxoid for tetanus
- Diptheria toxoid for diphtheria

ii) Passive Immunity: It is the resistant induced in the recipient by transfer of preformed
(ready made) antibody against infective agent or toxin in another host. Such antibody or
antiserum is prepared by injecting infective agent or toxin in another host. The immune
system has no active role in passive immunity.
Protection starts immediately after transfer of immune serum (antibody). Passive
immunity is short lasting but is useful when immunity is required immediately.
It is further divided into 2 types:
a) Natural passive immunity: It is the resistance passively transferred from the
mother to foetus and infants. Eg: Transfer of maternal antibody from the mother to
foetus (IgG) through placenta and to infants through milk (colostrum), protects
them till their own immune system develops.
By active immunization of mother during pregnancy can help to improve the
quality of passive immunity in infants. (Breast fed – diphtheria, tetanus, mumps)
b) Artificial passive immunity: It is the resistance passively transferred to a recipient
by parenteral administration of antibodies. The agents used for artificial passive
immunity are hyper-immune sera of animal or human origin, covalescent sere
(sera patient recovering from infectious disease) and pooled human sera.



(Herd immunity:The overall resistance in a community is termed as Herd immunity.
Eradication of any communicable disease depends on development of high level of herd
immunity rather than the immunity in individuals.)