Biomedical Engineering Reference
In-Depth Information
cytokines, receptors on the cell surface, antigen
dose, nature of the antigen and direct cell to cell
interaction with the antigen presenting cell
(Constant and Bottomly
1997
; Kidd
2003
) . The
nature of the innate immune response to an anti-
gen or pathogen dictates whether the subsequent
adaptive CD4
+
T-cell response will be mainly
Th1 or Th2 (Janeway and Medzhitov
2002
) .
These two distinct subsets of T helper cells are
believed to be responsible for different functions
of host defence and are distinguished by the spec-
trum of cytokines secreted by them. The key
cytokines are interferon-g (IFN- g) and IL-4,
which are central to stimulatory and inhibitory
roles of a Th subset (Coffman
2006
) . Thus, IFN- g
and IL-4 inhibit differentiated Th1 or Th2 cells
by blocking the differentiation of these subsets
from naïve precursors. IFN-g has been found to
inhibit Th2, whereas IL-4 and IL-10 inhibit Th1
(Coffman
2006
). The IL-12 favours Th1 and has
no effect on Th2 (Constant and Bottomly
1997
) .
None of the cytokines specific to one particular
subset are exclusive products of Th cells because
other leucocytes can contribute to Th1- or Th2-
type responses (Mosmann and Sad
1996
) .
An enhanced immune reactivity may occur
due to immunological memory or repeated host
encounters with the same antigen (Janeway et al.
2001
; Sordillo and Streicher
2002
) . Both innate
and acquired immune responses have the ability
to recognise conserved components of pathogens
called pathogen-associated molecular patterns
(PAMPs) such as lipopolysaccharide (LPS),
peptidoglycan and bacterial DNA (Hornef et al.
2002
). The host cell recognition of pathogen-
associated molecules relies on a number of
membrane receptors, that is, the toll-like recep-
tors (TLRs), which provide cellular signalling
during the initiation of the immune response
(Medzhitov et al.
1997
; Janeway et al.
2001
) .
Livestock species like cattle and buffaloes
have a large population of lymphocytes in their
blood and account more than 50% of leucocytes
population in a healthy animal. Lymphocytes
consist of two different subsets, namely, T- and
B-cells, which differ in functions and molecule
secretions. T-lymphocytes recognise antigens
through membrane receptors and are responsible
2
Innate and Adaptive Immunity
The responses of the immune system are of innate
and adaptive type. Innate immunity is the predo-
minant defence during early stages of infections
or a challenge and is activated by antigens, but
the response is not amplified by repeated expo-
sure to the same antigen (Tizard
2000
) . Innate
immunity includes physical barriers such as
the skin, mucosal secretions, tears, urine, acid
in stomach as well as complement and antigen-
nonspecific cellular components, and is consi-
dered to be the first line of defence against
pathogens, namely, bacterial, viral, protozoal or
fungal. Beneficial microorganisms in the intes-
tine and respiratory tract which compete against
invading pathogens are also an important part of
innate immunity. The innate system provides the
opportunity and time required by the acquired
immune system to develop an antibody response
against a specific pathogen, usually several days
to several weeks. Innate immunity, although
always present to some degree, is regulated and
may be strengthened or weakened by many fac-
tors, like wounds, dehydration, nutritional status,
genetics and stress. The phagocytic cells are the
major players of innate immunity, and they also
serve as the connection between innate and adap-
tive immunity (Tizard
2000
) .
Adaptive immunity is an antigen-specific
immune response that evokes over time and is
more complex response. Specialised cells like
macrophages and dendritic cells initiate adaptive
immune responses by presenting antigen to naïve
lymphocytes to initiate a cell-mediated or humoral
response. More specifically, these antigen presen-
ting cells present antigen to activate naïve T-cells
carrying receptors for a specific antigen, thus
initiate T-cell immunity. Activated CD4
+
T-cells
commit early to a pathway of differentiation that
results in the formation of two distinct subsets
called T helper 1 (Th1) or T helper 2 (Th2) cells
(Mosmann and Coffman
1989
) . Differentiation
of either of these subset is established during
priming of naïve CD4
+
T-cells, and this differen-
tiation process in the early phase of the immune
process may be influenced by factors like
