CYTOKINES
Cytokines are the hormone-like
small proteins acting as intercellular messengers (cell signaling molecules) by
binding to specific receptors of target cells. These non-antibody proteins are
secreted by WBCs and some other types of cells. Their major function is the
activation and regulation of general immune system of the body. Cytokines are
distinct from the other cell-signaling molecules such as growth factors and hormones.
TYPES OF CYTOKINES
Depending upon the source
of secretion and effects, cytokines are classified into several types:
1. Interleukins
2. Interferons
3. Tumor necrosis
factors
4. Chemokines
5. Defensins
6. Cathelicidins
7. Platelet-activating
factor.
1. Interleukins
Interleukins (IL) are the
polypeptide cytokines which are produced mainly by the leukocytes and act on
other leukocytes.
Types of interleukins
So far, about 16 types
of interleukins are identified. IL-1, IL-2, IL-3, IL-4, IL-5, IL-6 and IL-8
play important role in the process of immunity. Recently IL-12 (otherwise called
natural killer cell stimulatory factor) and IL-11 are also considered as
important cytokines.
2. Interferons
Interferons (IFN) are
the glycoprotein molecules. These cytokines are considered as antiviral agents.
Types of interferons
Interferons are of
three types namely, INF-α, INF-β and INF-γ.
3. Tumor Necrosis
Factors
Tumor necrosis factors
(TNF) are of three types, TNF-α (cachectin), TNF-β (lymphotoxin) and TNF-γ.
4. Chemokines
Cytokines having chemoattractant
action are called chemokines.
5. Defensins
Defensins are the
antimicrobial peptides.
Types of defensins
Two types of defensins
are identified in human:
i. α-defensins, secreted
by neutrophils, macrophages and paneth cells in small intestine.
ii. β-defensins, secreted
by airway epithelial cells (respiratory tract), salivary glands and cutaneous cells.
6. Cathelicidins
Cathelicidins are also
the antimicrobial peptides which play an important role in a wide range of
antimicrobial activity in air passage and lungs
7. Platelet-activating
Factor
Platelet-activating
factor (PAF) accelerates agglutination and aggregation of platelets.
NATURAL KILLER CELL
Natural killer (NK)
cell is a large granular cell that plays an important role in defense mechanism
of the body. It has an indented nucleus. Considered as the third type of lymphocyte,
it is often called the non-T, non-B cell. It is derived from bone marrow. NK
cell is said to be the first line of defense in specific immunity, particularly
against viruses. NK cell kills the invading organisms or the cells of the body
without prior sensitization. It is not a phagocytic cell but its granules
contain hydrolytic enzymes such as
perforins and granzymes.
These hydrolytic enzymes play an important role in the lysis of cells of
invading
organisms.
Functions of Natural
Killer (NK) Cell
Natural killer cell:
1. Destroys the
viruses
2. Destroys the viral
infected or damaged cells, which might form tumors
3. Destroys the
malignant cells and prevents development of cancerous tumors
4. Secretes cytokines
such as interleukin-2, interferons, colony stimulating factor (GM-CSF) and
tumor
necrosis factor-α.
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