Natural killer (NK) cells and phagocytes are part of the innate immune system's defense against microbes that penetrate the skin or bypass blood defenses. NK cells make up 5-10% of lymphocytes and are present in the blood, spleen, bone marrow, and lymph nodes. They kill infected cells and some tumor cells by releasing toxic granules that induce cell lysis or apoptosis. Phagocytes, such as neutrophils and macrophages, ingest and destroy microbes and cellular debris. Macrophages are fixed in tissues or migrate from the blood to sites of infection.

1. -BY NEHAR PATWARDHAN

2.  WHEN MICROBES PENETRATE THE SKIN AND MUCOUS
MEMBRANE OR BYPASS THE ANTIMICROBIAL
SUBSTANCES IN THE BLOOD THE NEXT NON-SPECIFIC
DEFENSE CONSISTS OF NATURAL KILLER CELLS AND
PHAGOCYTES.
 ABOUT 5-10% OF LYMPHOCYTES IN THE BLOOD ARE
NATURAL KILLER (NK) CELLS.
 THEY ARE ALSO PRESENT IN THE SPLEEN,RED BONE
MARROW AND LYMPH NODES.
 NK CELLS LACK THE MEMBRANE MOLECULES THAT
IDENTIFY B AND T CELLS, BUT HAVE THE ABILITY TO
KILL A WIDE VARIETY OF INFECTED BODY CELLS AND
CERTAIN TUMOR CELLS.

3.  NK CELLS ATTACK ANY BODY CELLS THAT DISPLAY
ABNORMAL OR ANUSUAL PLASMA MEMBRANE
PROTEIN.
 THE BINDING OF NK CELLS,SUCH AS AN INFECTED
HUMAN CELL,CAUSES THE RELEASE OF GRANULES
CONTAINING TOXIC SUBSTANCES FROM NK CELLS.
 SOME GRANULES CONTAINS A PROTEIN CALLED
PERFORIN THAT INSERTS INTO THE PLASMA
MEMBRANE OF TARGET CELL AND CREATE
CHANNELS (PERFORATIONS) IN THE MEMBRANE.

4.  AS A RESULT ,EXTRACELLUAR FLUID FLOWS INTO THE
TARGET CELL AND THE CELL BURSTS, A PROCESS
CALLED CYTOLYSIS (CYTO=CELL,
LYSIS=LOOSENNING)
 OTHER GRANULES OF NK CELLS RELEASE GRANZYMES,
WHICH ARE PROTIEN-DIGESTING ENZYMES THAT
INDUCE THE TARGET CELL UNDERGO
APOPTOSIS, OR SELF-DESTRUCTION.
 THIS TYPE OF ATTACK KILLS INFECTED CELLS,THE
RELEASED MICROBES,WHICH MAY NOT BE INTACT,CAN BE
DESTORY BY PHOGOCYTES.

6.  PHOGOCYTES (PHOGO=EAT,CYTES=CELLS) ARE SPECIALIZED
CELLS THAT PERFORM PHOGOCYTOSIS,THE INGESTION OF
MICROBES OR OTHER PACTRICLES SUCH AS CELLULAR
DEBRIS.
 THE TWO MAJOR TYPES OF PHOGOCYTES ARE
NEUTROPHILESAND MACROPHAGES.
 WHEN AN INFECTION OCCURS,NEUTROPHILESAND
MONOCYTES MIGRATE TO THE INFECTED AREA.
 DURING THIS MIGRATION,ENLARGEAND DEVELOPED INTO
ACTIVELY PHAGOCYTIC MACROPHAGE CALLED WANDARING
MARCROPHAGE.
 OTHER MAROPHAGE ,CALLED FIXED MACROPHAGE STAND
GUARD IN SPECIFIC TISSUES.

7.  AMONG THE FIXED MACROPHAGES ARE HISTIOCYTES STELLATE
RETICULOENDOTHELIAL CELLS IN THE LIVER, ALVEOLAR
MACROPHAGES IN THE LUNGS, MICROGLIA IN THE NERVOUS
SYSTEM, AND TISSUE MACROPHAGES IN THE SPLEEN, LYMPH
NODES, AND RED BONE MARROW.
 IN ADDITION TO BEING AN INNATE DEFENSE MECHANISM,
PHAGOCYTOSIS PLAYS A VITAL ROLE IN ADAPTIVE IMMUNITY.