Innate immunity cellular factors.

  C. Cellular Factors 

Natural immunity is provided by the following cellular factors: 

1. Phagocytosis

 2. Natural killer cells (NK cells).

 1  .Phagocytosis 

Phagocytosis is a process of "cell eating" (GK - phagia- eating, cytos - cel .

The phagocytic cells were discovered by Metchnik - off (1838). 

They are two types, namely microphages and macrophages.

 These are the profession phagocytes.  On some occasions, other cells also perform the f ction phagocytosis. 

 The microphages are the polymorphonuclear leukocytes (PMN).

 It include neutrophils,basophil and eosinophils.  These are the dominant white cell the blood stream. These cells possess multilobed nuclei. They do not divide are short lived. 

(Semen - The secretion of the testis, consisting largely of  sperms.)  

The macrophages are the cells of the "mononuclear phagocytic system" which was originally called the "reticulo - endothelial system 

'These cells are derived from the bone marrow promonocytes which later differentiate into monocytes of the blood (2-10% of the total white blood corpuscles of man). 

These monocytes are the blood representatives of the mononuclear phagocytic system. Monocytes remain in the blood for a short time (about 24 hours mean half life). Then they migrate into the tissues to become macrophages. 

Some of the macrophages are free while others settle down in certain tissues and get fixed there.

•  Promonocytes (in bone marrow)
•  Monocytes (in blood)
•  Macrophages

 1.Free macrophages Eg. Alveolar, peritoneal, free macrophages in lymph and other tissues. 

2.Fixed macrophages Eg. Kupffer cells (liver) . osteoblasis (bone), fixed macrophages of lymphoid tissues, fixed macrophages in most tissues (histiocytes).  

 The macro phages are long lived. They are usually found engulfing bacteria, viruses and protozoans.

 Process of Phagocytosis

 The process of phagocytosis includes the following stages: Chemotaxis, attachment, ingestion, intracellular killing and digestion. Chemotaxis involves the movement of the phagocytes to the site of infection or inflammation in response to the chemotactic factors produced by the foreign particles (microbial organisms) or damaged or dead tissues. The next stage is the attachment of phagocytes to the foreign particles. The attachment is promoted by opsonins (like antibodies or activated complement factors) which form a coating around the particles. Once the attachment is made, the process of ingestion starts. The cell mbrane of the phagocytes produce pseudopodia around the particle thus the particle is completely enclosed in the vacuole which is known as a phagosome.  The phagosome soon fuses with the lysosome of the phagocytic 'secondary lysosome ". Cell and the fusion results in the formation of a phagolysosome' also known as secondary lysosome.

The next stage is the intracellular killing. This is done by the antimicrobial hydrogen peroxide and myeloperoxide.  produced by the lysosomes. These substance include lysozyme, which also have an antibacterial action. 

In addition, polymorphs (PMN) have cationic proteins and lactoferritin 

Finally the killed organisms and cells are digested by the hydrolytic enzymes exterior. of the lysosomes followed by the elimination.  of the products of digestion to the 

Phagocytes can also kill neoplastic cells and other cells directly without ingesting them but just by membrane contact which is known as' contactual cell injury.

 

2. Natural Killer Cells (NK Cells)

 These  are non - phagocytic lymphocytes having large granules.  Hence, these cells are also known as large granular lymphocytes.