It is a chapter in microbiology. Please read it.

1. IMMUNITY
BY PIYUSH
BSC.(HONS) NURSING

2. DEFINITION
◦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. INNATE IMMUNITY
◦ It is an antigen- non specific defense mechanisms that host uses immediately or within several hours after
exposure to almost any microbe.
◦ It is the resistance which individual possess by birth.
◦ It is by virtue of his genetic and constitutional make up.
◦ It does not depend on prior contact with foreign antigen.
 Immediate innate immunity: it begins 0-4 hours exposure to an infectious agent and involves the action of
soluble performed antimicrobial molecules that circulate in the blood, found in extracellular tissue fluids and
are secreted by epithelial cells.
 Early induced innate immunity: It begins 4-96 hours after exposure to an infectious agent and involves the
recruitment of defense cells as a result of pathogen- associated molecular patterns or PAMPS binding to
pattern- recognition receptors or PRRs. These recruited defense cells include: phagocytic cells, natural killer
cells.
 Example of Innate Immunity: Anatomical barriers, mechanical removal, bacterial antagonism , phagocytosis.

4. MECHANISMS OF INNATE IMMUNITY
Epithelial Surfaces
• 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 bactericidal
activity.
• Respiratory Activity: 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.
• Intestinal tract: The mouth possess saliva which has an inhibitory effect on many micro- organisms. Some bacteria may
be swallowed and are destroyed by acidic Ph of the gastric juices.
• Conjunctiva: Tears have a major role by flushing away bacteria and other dust particles. In addition, lysozyme present in
tears has a bactericidal action.
• The Genitourinary tract: The flushing action of urine eliminates bacteria from the urethra. The acidic pH of vaginal
secretions in female, due to the fermentations of glycogen by lactobacillus, renders vagina free of many pathogens.

5. ◦ Antibacterial Substance: Besides specific antibody formation, there are number of non specific antibacterial
substances present in blood and tissues. These substances are properdin, complement, lysozyme etc.
◦ Cellular Factors: Once the infective agent has crossed the barrier of epithelial surfaces, the tissue factors
come into play for defence.
When an infective agent invades tissue, an exudative inflammatory reaction occurs by accumulation of
phagocytes at the site of infection.

6. ACQUIRED IMMUNITY
◦ The resistance acquired by an individual during life is known as acquired immunity.
◦ It can come from :
• A vaccine
• Exposure to an infection or diseases.
• Another person’s antibodies.
oWhen pathogens( germs) are introduced into your body from a vaccine or a disease, your body learns to
target those germs in the future by making new antibodies.
oAcquired immunity is different from innate immunity.
oIt is of two types:
 Active immunity
 Passive immunity

7. Active
immunity
• Natural Immunity:
Through clinical or
subclinical infection.
• Artificial: Induced by
vaccination.
Passive
immunity
• Natural: Through trans
placental maternal IgG
antibodies.
• Artificial: Through
antiserum injection.

8. ACTIVE IMMUNITY
◦ Active immunity is subdivided into two types: Natural and Artificial.
 Mechanism: Active immune response stimulates both humoral and cell mediated immunity usually in
parallel.
 Humoral Immunity: 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.
 Cell mediated immunity: It depends on T- lymphocytes developed against certain antigens. The cell
mediated immunity by sensitized T- lymphocytes is important in resistance to chronic bacterial infections.
TYPES:
 Natural active immunity: It is acquired by natural subclinical or clinical infections. Such immunity is long
lasting. Example: Persons recovering from smallpox infection develop natural active immunity.

9.  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.
Toxoids are prepared from bacterial exotoxins inactivated by formalin or by alum.
Toxoids are immunogenic but not toxigenic.
VACCINES:
o Live Vaccines:
• BCG for tuberculosis
• Sabin vaccine for poliomyelitis
• MMR vaccine for measles, mumps, rubella
o Killed Vaccines:
• TAB for enteric fever
• Neural and non- neural vaccines for rabies.
• Hepatitis B vaccine
• Salk vaccine for poliomyelitis
• Killed cholera vaccine

10. PASSIVE IMMUNITY
◦ Passive immunity is the transfer of active humoral immunity of ready- made antibodies. Passive immunity
can occur naturally as well as artificial.
◦ The immune system has no active role in passive immunity. Protection starts immediately after transfer of
immune serum.
◦ Passive immunity is short lasting but is useful when immunity is required immediately. Passive immunity is
of two types: Natural and Artificial.
 Natural Passive Immunity: It is transferred from the mother to fetus or infant. Transfer of maternal
antibodies to fetus transplacentally and to infant through milk protects them till their own immune system
matures to function.
 Artificial Passive Immunity: It is through parenteral administration of antibodies. The agents used for
artificial passive immunity are hyper immune sera of animal or human origin, convalescent sera and pooled
human gammaglobulin.

11. USES OF PASSIVE IMMUNIZATION
◦ To provide immediate short term protection in a non immune host, faced with the threat of a
serious infection.
◦ For suppression of active immunity which may be injurious. Example: Use of immunoglobulins
during delivery to prevent immune response to Rh factor in Rh negative mothers with Rh positive
babies.
◦ For treatment of serious infections.

12. Difference Between Active and Passive
Immunity
ACTIVE IMMUNITY
◦ Produced actively by immune system.
◦ Induced by infection or by contact with
immunogens.
◦ Long lasting and effective protection.
◦ Immunity is effective only after a lag period i.e.
time required for generation of antibodies.
◦ Immunological memory present.
◦ Not applicable in immune deficient persons.
PASSIVE IMMUNITY
◦ Received passively by the host.
◦ Conferred by administration of ready- made
antibodies.
◦ Protection short lived and less effective.
◦ Immunity is effective immediately.
◦ No immunological memory present.
◦ Applicable in immunodeficient persons.

13. MISCELLANEOUS
Combined Immunity: A combination of active and passive immunization is employed simultaneously which is
known as combined immunization.
Local Immunity: Natural infection or the live viral vaccine administered orally or intranasal provide local
immunity at the site of entry such as the gut mucosa and nasal mucosa respectively. A special class of
immunoglobulins(IgA) plays an important role in local immunity.
Herd Immunity: It refers to the overall resistance in a community. When herd immunity is low, chances of
epidemics increases on suitable pathogen. Eradication of any communicable disease depends on development
of high level of herd immunity rather than of immunity in individual.