Immunity can be innate or acquired. Innate immunity is resistance from birth based on genetic and physiological factors, while acquired immunity develops from exposure to pathogens through natural infection or vaccination. Acquired immunity can be active, developing from direct exposure, or passive, developing from transfer of antibodies. Both humoral immunity from antibodies and cell-mediated immunity from T-cells contribute to acquired immunity. A combination of innate barriers and immune responses work together to provide host defense against infection.

1. By – Annu Maurya

2.  The term 'immunity' is defined as resistance exhibited
by the host against any foreign antigen including
microorganisms. This resistance plays a major role in
prevention of infectious diseases. Immunity may be
innate or acquired.

3.  It is the resistance which individual possesses by birth.
 It is by virtue of his genetic and constitutional makeup.
 It does not depend on prior contact with foreign
 antigen. It may be nonspecific, when there is resistance
to infections in general, or specific when resistance to a
particular pathogen is concerned.

5.  It refers to the resistance to a pathogen, shown by all
members of a particular species e.g. B. anthracis infects
human beings but not chickens. The mechanism of
such type of immunity is not clearly understood. The
physiological and biochemical differences between
tissues of different host species may be responsible for
species specific resistance.

6.  Within one species, different races may exhibit
differences in susceptibility or resistance to infections.
This is termed as racial immunity. Algerian sheep is
highly resistant to anthrax which is a common disease
of other races of sheep.

7.  Resistance to infection varies with different individuals
of same race and species. This is known as individual
immunity. The genetic basis of individual immunity is
evident from the observation that homozygous twins
exhibit similar degrees of susceptibility or resistance to
tuberculosis.

9.  The two extremes of life (foetus and old persons) carry
higher susceptibility to various infections. In foetus, the
immune system is immature whereas in old age there is
gradual waning of immune responses. The foetus in-
utero is generally protected from maternal infections
through the placental barrier.

10.  In some diseases such as chickenpox and poliomyelitis,
the clinical illness is more severe in adults than in
young children. This may be due to more active
immune response which causes greater tissue damage.

11. Certain hormonal disorders such as diabetes mellitus,
adrenal dysfunction and hypothyroidism enhance
susceptibility to infections. Staphylococcal sepsis is
more common in diabetes which may be associated
with increased level of carbohydrates in tissues .
Corticosteroids depress the host resistance by its
antiinflammatory, antiphagocytic effects and by
inhibiting antibody formation.

12.  The elevated steroid level during pregnancy may be
related to higher susceptibility of pregnant women to
many infections.

13. Malnutrition predisposes to bacterial infections. Both
humoral and cell mediated immune responses are
reduced in malnutrition.

15.  (i) Skin- It not only acts as a mechanical barrier to
microorganisms but also provides bactericidal
secretions. The high concentration of salt in drying
sweat, the sebaceous secretions and long chain fatty
acids contribute to activity.

16. (ii) Respiratory tract
 The inhaled particles are arrested in the nasal passages
on the moist mucous membrane surfaces. The mucous
secretions of respiratory tract act as a trapping
mechanism and hair like cilia propels the particles
towards the pharynx where it is swallowed or coughed
out. The cough reflex acts as an important defence
mechanism. Some particles which manage to reach
alveoli are ingested by phagocytes present there.

17.  (iii) Intestinal tract -The mouth possesses saliva which
has an inhibitory effect on many microorganisms.
Some bacteria may be swallowed and are destroyed by
acidic pH of gastric juices. The normal bacterial flora
of intestine further exert a protective effect by
preventing colonisation of pathogenic bacteria.

18. (iv) Conjunctiva
 Tears have a major role by flushing away bacteria and
other dust particles. In addition, lysozyme present in
tears has a bactericidal action.

19. (v) The Genitourinary tract
 The flushing action of urine eliminates bacteria from
the urethra. The acidic pH of vaginal secretions in
female, due to the fermentation of glycogen by
lactobacillus (normal flora) , renders vagina free of
many pathogens. In males, semen is believed to contain
some antibacterial substances.

20.  Besides specific antibody formation , there are number
of nonspecific antibacterial substances present in blood
and tissues. These substances are properdin,
complement, lysozyme, betalysin , basic polypeptides
(leukins from leucocytes, plakins from platelets) and
interferons which possess antiviral activity. The
complement system plays an important role in the
destruction of pathogenic microorganisms that invade
the blood and tissues

21.  Once the infective agent has crossed the barrier of
epithelial surfaces, the tissue factors come into play for
defence.

22. Inflammation occurs as a result of tissue injury or
irritation, initiated by the entry of pathogens or other
irritants. It is an important non-specific defence
mechanism. Inflammation leads to vascidilation,
increased vascular permeability and cellular infiltration.
Microorganisms are phagocytosed and destroyed

23. A rise in temperature following infection is a natural
defence mechanism. It destroys the infecting
organisms.
 Fever also stimulates the production of interferon
which helps in recovery from viral infections.

24.  Following infection or injury, there is a sudden increase
in plasma concentrations of certain proteins,
collectively called acute phase proteins. These include
C reactive protein (CRP), mannose binding proteins
and many others. CRP and some other acute phase
proteins activate the alternative pathway of
complement.

26.  The resistance acquired by an individual during life is
known as acquired immunity. It is of two types,
 active
 passive.

27.  Active immunity is subdivided into two types : Natural
and Artificial.
 Natural- Through clinical or subclinical infection
 Artificial- Induced by vaccination
 It is the resistance developed by an individual as a
result of contact with an antigen. This contact may be
in the form of natural infection or by vaccination.

28.  Active immune response stimulates both humoral and
cell mediated immunity usually in parallel.

29.  It is antibody mediated immunity. It depends on the
synthesis of antibodies by plasma cells. These cells
produce specific circulating antibody which combines
specifically with the antigens and modify their activity.
This modified activity may be in the form of lysis of
antigen molecules; their toxin may be neutralised, or in
the form of removal of antigen by phagocytosis.

30.  It depends on T-lymphocytes developed against certain
antigens. Antibody synthesis also occurs in response to
these antigens, but their role is limited. The cell
mediated immunity by sensitised T-lymphocytes is
important in resistance to chronic bacterial infections.

31.  (i) Natural active immunity
 It is acquired by natural subclinical or clinical
infections. Such immunity is long lasting. Persons
recovering from smallpox infection develop natural
active immunity.

32.  (ii) Artificial active immunity
 It ,is produced by vaccination. The vaccines are
prepared from live, attenuated or killed
microorganisms, or their antigens or toxoids. In killed
vaccines the organisms are killed by heat, formalin,
phenol and alcohol.

33.  Passive immunity is subdivided into two types : Natural
and Artificial.
 Natural- Through transplacental maternal IgG
antibodies.
 Artificial-Through antiserum injection.
 Passive immunity is induced in an individual by
preformed antibodies (generally in the form of
antiserum) against infective agent or toxin. This
antiserum is prepared by injecting infective agent or
toxin in another host.

34.  It is transferred from the mother to foetus or infant.
Transfer of maternal antibodies to foetus
transplacentally and to infant through milk (colostrum)
protects them till their own immune system matures to
function.

35. It is through parenteral administration of antibodies. The
agents used for artificial passive immunity are
hyperimmune sera of animal or human origin,
convalescent sera and pooled human gammaglobulin.

37. A combination of active and passive immunisation is
employed simultaneously which is known as combined
immunisation. Passive immunity provides the
protection necessary till the active immunity becomes
effective.

38.  Injection of immunologically competent lymphocytes
is known as adoptive immunity. Instead of whole
lymphocytes, an extract of immunologically competent
lymphocytes can be used. This is being attempted in
therapy of lepromatous leprosy. Such lymphocytes are
collectively known as transfer factor.

39.  Natural infection or the live viral vaccine administered
orally or intranasally provides local immunity at the
site of entry such as gut mucosa and nasal mucosa
respectively. A special class of immunoglobulins (IgA)
plays an important role in local immunity. O ne type of
. ' ' IgA called secretory IgA is produced locally by
plasma cells.

40.  It refers to the overall resistance in a community. When
herd immunity is low, chances of epidemics increase on
introduction of a suitable pathogen. Eradication of any
communicable disease depends on development of a
high level of herd immunity rather than of immunity in
individuals.