Lymphocytes: infected cells or tumour cells. Both types

Lymphocytes:

Lymphocytes
can be broadly divided into three subtypes-B cells, T cells and natural killer
cells on the basis of function and cell membrane components.

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B
lymphocytes or B cells are produced in bone marrow and circulate in blood. The molecules
expressed on mature B cells are Fc receptors, Class II MHC molecules, CD 21,
B7-1 CD40. B cells when encounter with antigen differentiated into plasma cells
and memory cells. Plasma cells secretes antibody and will remove antigens from
the body.

T
lymphocytes or T cells mature in thymus. Like that of B cells these are also
having membrane receptors for antigens. T cells recognize only processed
antigens, which are bound with major histocompatibility complex (MHC)
molecules. T cells also expresses distinctive membrane molecules, complex
polypeptide CD3 and most can be distinguished by the presence of one or other
molecules like CD 4 or CD 8. CD4+ T cells generally function as T
helper (TH) cells and are class II MHC restricted and CD+8
cells function as T cytotoxic (TC) cells and are class I MHC restricted.
T helper cells secrete various cytokines which helps in the activation of B
cells, T cells and other cells that participate in immune response. T cytotoxic
cells acts against altered self cells like virus infected cells or tumour cells.
Both types of T cells can be found throughout the body. They are activated in the
lymph nodes and spleen, but they are also found in other tissues of the body,
most conspicuously the liver, lung, blood, and intestinal and reproductive
tracts. Another subpopulation of T cells are T suppressor (TS) cells,
which may be involved in suppression of humoral or cell mediated immune
response.

Natural killer
cells or null cells do not express immunoglobulin receptor like that of B cells
ant t cell receptors present of t cells. They express the CD2 marker, CD 16, the
IL-2 receptor and elaborate Tissue Necrosis Factor (TNF). It functions as the non-specific killer toward the virus infected
cells and tumour cells.

 

Granulocytes
or Polymorphonuclear (PMN) Leukocytes:

 Granulocytes are classified as neutrophils, eosinophils
and basophils, based on their cellular morphology and staining characteristics.
Neutrophils are the first to arrive at the site of infection and are phagocytic
cells. Eosinophils are motile phagocytic cells and are mostly active against
parasitic infection. Basophils are non-phagocytic granulocytes that function in
allergic responses by releasing pharmacologically active substances.

Peripheral blood mononuclear
cells: The human body is nourished by a dynamic
circulatory system composed of cellular components of which have a relatively
rapid
turnover rate (Vlata et al., 2006). PBMC are classified as a fluid
connective tissue, which can be termed as cells suspended in a fluid matrix
functioning to connect the entire biological system at the physiological level.
Blood cells also involve in the first line of the immune
defense system, using an arsenal of neutrophils, eosinophils, basophils, B
cells, T cells and monocytes to defend against foreign substances, injury and
provide a protective barrier between the external and internal (Liew et al.,
2006). These peripheral blood mononuclear cells play crucial role in the immune
defense during the pathological conditions by stimulating the process of
activation, cell division and differentiation to generate a large pool of
activated effector T-cells which react to the antigen (Khanduja et al.,
2006; Winkler et al., 2005).

Macrophages

These are scavengers or
antigen-presenting cells (APC) because they pick up and ingest foreign
materials and present these antigens to T cells and B cells. Macrophages are
important in the regulation of immune responses. This is one of the important
first steps in the initiation of an immune response. Stimulated macrophages
exhibit increased levels of phagocytosis and are also secretory.

Dendritic Cells:

Dendritic
cells, which also originate in the bone marrow, function as antigen presenting
cells (APC). Most dendritic cells process and present antigen to TH
cells. They express high levels of class II MHC molecules and B7 molecules,
hence they are more efficient APCs than macrophages. These cells are usually found
in thymus, lymph nodes and spleen. However, they are also found in the
bloodstream and other tissues of the body. They capture antigen, migrate to
blood or lymph and circulate to various lymphoid organs where an immune
response is initiated by presenting to
TH cells.

Phagocytosis:

Macrophages
are capable of ingesting and digesting exogenous antigens, such as whole
microorganisms and insoluble particles and endogenous antigens such as injured
or dead host cells, cellular debris etc. A number of antimicrobial and cytotoxic
substances produced by activated macrophages can destroy phagocytosed
microorganisms. It involves oxygen dependent and oxygen independent killing mechanisms.

Oxygen
dependent killing involves number of reactive oxygen intermediates and reactive
nitrogen intermediates which have potent antimicrobial activity.
During phagocytosis metabolic process called oxidative burst occurs in
macrophages. This results in activation of membrane bound oxidase that catalyses
reduction of oxygen to superoxide anion, a reactive oxygen species which is extremely
toxic to microorganisms. Superoxide anion also generates hydroxyl radicals and hydrogen
peroxide. When macrophages ae activated with bacterial cell wall component such
as lipopolysaccharides together with T cell activated cytokines IFN-? they
begin to express high level of nitric oxide synthase (NOS), an enzyme that
oxidizes L-arginine to L-citulline and nitric oxide (NO) gas.

L-arginine
+  O2 + NADPH                        NO + L-citrulline + NADP

Nitric oxide has potent
antimicrobial activity, it can also combine with superoxide anion to form more
potent antimicrobial substances. Much of the antimicrobial activity of
macrophages against pathogens is due to nitric oxide and substances derived
from it.

Oxygen independent
killing involves action of defensins, tumour necrosis factor ? (TNF- ?)
lysozyme and hydrolytic enzymes.