Adaptive or acquired immunity develops after exposure to an antigen from a microbe. It is long-lasting and can be either active or passive. Active immunity develops when the immune system is stimulated by an infection or vaccine, causing antibody and T-cell responses. It is slow to develop but provides lasting protection. Passive immunity transfers pre-formed antibodies from another source, providing rapid but temporary protection. Both humoral and cell-mediated responses contribute to active immunity.
2. Adaptive (Acquired) Immunity
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is also called acquired immunity, since the potency of immune
response is acquired by experience only. Differences between innace
and acquired immunity are summarized
3. Types of acquired immunity
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Acquired immunity against a microbe may be induced by the host's
response to the microbe or by transfer of antibodies or lymphocytes
specific for the microbes. It is of two types:
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active imwnity
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passive immunity.
4. Active immunity
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The immunity induced by exposure to a foreign antigen is called
activimmity. Aceive immunity is the resistance developed by an
individual after contact with foreign antigens, e ... microorganisme.
This contact may be in the form of clinical or immunization
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This contact may be in the form of
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1_clinical or subclinical infection
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2_immunization with live or killed infectious agents or their antigens,
or
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3_exposure to microbial products, such as toxins and coroids. In all
these stimulated
5. Active immunity
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In all these circumstances, the immune systems of the host is
stimulated to elicit an immune response consisting of antibod ies and
activated helper T (Th) cells and cytotoxic T lymphocytes/cells (CTI)
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Active immunity develops after a late period, during which inmunity
of the host is geared up to act against microorganism . Hence it is
slow in onset , especially during this primary response . However,
once the active immunity develops is long lasting and this is the major
advantage of the active immunity.
6. The active immunity is of two types :
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natural active immunity
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artificial active immunity
7. •
• Natural active immunity: It is acquired by natural clinical or
subclinical infections. Such natural immunity is long lasting . For
example , individuals suffering from smallpox become immune to
second attack of the disease .
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Artificial active immunity : It is induced in individuals by vaccines .
There is a wide range of vaccines available against many microbial
pathogens . These may be live vaccines killed vaccines , or vaccines
containing bacterial products
8. •
Active immunity is mediated by humoral immunity and cell - mediated immunity .
These two
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Humoral immunity: It is mediated by molecules in the blood and mucosal
secretions called antibodies. The anti- bodies are secreted by a subset of
lymphocytes known as B cells. The antibodies recognize microbial antigens,
combine specifically with the antigens, neutralize the infectivity of microbes, and
target microbes for elimina-tion by various effector mechanisms. Humoral
immunity is the principal defense mechanism against extracellular microbes. -
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Cell-mediated immunity: It is mediated by both activated T, cells and CTLS.
Cytokines secreted by T, cells activate various phagocytic cells, enabling them to
phagocytose and kill microorganisms. This type of cell-mediated immune
response is especially important against a host of bacte- rial and protozoal
pathogens. CTLS play an important role in killing virus-infected cells and tumor
cells. They act by killing altered self-cells.
Mediators of active immunity
10. Antigen recognition
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Antigens , which are generally very large and complex , are not recognized
in their entirety by lympho cytes . Instead , both B and T lymphocytes
recognize discrete sites on the antigens called antigenic determinants , or
epitopes . Epitopes are the immunologically active regions on a com plex
antigen , the regions that actually bind to B - cell or T - cell receptors .
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B cells and T cells differ in their mechanisms of antigen recognition . While
B cells recognize the antigen by interacting with the epitope on their own,
T cells recognize the antigen only when the epitope is “presented” by one
of the specialized antigen - presenting cells. Once the antigen has been
recognized, these cells then go on to diversify by several intricate mecha
nisms. This diversification helps in conferring the specificity, one of the
cardinal characteristics of the immune system.
11. Major histocompatibility complex (MHC
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It is a large genetic complex with multiple loci . The MHC loci encode
two major classes of membrane-bound glycoproteins: class I and class
II MHC molecules. Class II molecules present antigens to the T cells,
while class I molecules do the same for CTLs. In order for a foreign
protein antigen to be recognized by a T cell, it must be degraded into
small antigenic peptides that form complexes with class I or class II
MHC molecules. This con version of proteins into MHC - associated
peptide fragments is called antigen processing and presentation.
12. Passive immunity
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When immunity is conferred by transfer of serum or lympho cytes
from a specifically immunized individual , it is known as passive
immunity . This is a useful method for conferring resis tance rapidly ,
i.e. , without waiting for the development of an active immune
response
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Passive immunity may be natural or artificial
13. Passive
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Natural passive immunity : It is observed when IgG is passed from
mother to fetus during pregnancy . This forms the basis of prevention
of neonatal centimes in neonates by active immunization of pregnant
mothers . It is achieved by administering tetanus toxoid to pregnant
mothers during the last trimester of pregnancy . This induces
production of high level of antibody les in mother against tetanus
toxin , which are subsequently transmitted from mother to fetus
through placenta . The anti bodies subsequently protect neonates
after birth against the risk of tetanus . Natural passive immunity is
also observed by passage of IgA from mother to newborn during
breastfeeding
14. Passive
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Artificial passive immunity : It is induced in an individual by administration
of preformed antibodies . generally in the form of antiserum , raised
against an infecting agent . Administration of these antisera makes large
amounts of antibodies available in the recipient host to neutralize the
action of toxins . The preformed antibodies against rabies and hepatitis A
and B viruses , etc. given during incubation period prevent replication of
virus , and hence alter the course of infection Immediate availability of
large amount of antibodies is the main advantage of passive immunity .
However , short lifes pan of these antibodies and the possibility of
hypersensitity reaction , if antibodies prepared in other animal species are
given to individuals who are hypersensitive to these animal globulins ( eg ,
serum sickness )
16. Local Immunity
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The immunity at a particular site , generally at the site of invasion and
multiplication of a pathogen , is referred to as local immunity . Local
immunity is conferred by secretory IgA antibodies in various body
cretions . These antibodies are produced locally by plasma cells
present on mucosal surfaces or in secretory glands . Natural infection
or attenuated live viral vaccines given orally or intranasally induces
local immunity at gut mucosa and nasal mucosa , respectively
17. Herd Immunity
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Herd immunity refers to an overall level of immunity in a commu nity
. Eradication of an infectious disease depends on the development of
a high level of herd immunity against the pathogen. Epidemic of a
disease is likely to occur when herd immunity against that disease is
very low indicating the presence of a larger number of susceptible
people in the community
