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.


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 (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


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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