Initial host-defense mechanisms against invasion by a pathogen include:

(a) anatomical skin and mucosal integrity, (b) physiological milieu of the tissues, e.g. acidic gastric pH, (c) mechanical clearance of cell debris, e.g. mucociliary movements, and (d) normal microbial colonizing flora that competitively oppose invading pathogens. Once these barriers are breached, second line of defense is presented by immunological system, which is a complex interplay of various components, acting in unison to eliminate the invading pathogen or even an aberrant host-antigen, e.g. tumor cells.

Mukesh Agrawal

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