Body has its own system to counter the entry and effect of pathogen which operates as :
Non specific defence (Innate immunity) : 1st and 2nd line of defence, includes anatomical, physiological, phagocytic and inflammatory barriers.
Specific defence: 3rd line of defence (or Immune system) also called as adaptive or acquired immunity.
First line of defence is provided by
Physical barriers : The outermost layer, stratum corneum, is keratinized tough and resistant hence prevents the entry of any substance or pathogen into the body. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.
Physiological barriers : Sweat and sebum (oily) make skin surface acidic and unworthy for growth of pathogen. Lysozyme in external secretions like in sweat, tear, saliva is antibacterial enzyme. Acid in the stomach, saliva in the mouth, tears from eyes — all prevent microbial growth.
Cellular barriers : Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.
Cytokine barriers : Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.
Second line of defence comes into action in case of cut, injury or abrasion of skin.
This includes activities of WBC, histiocytes, mast cells, etc. causing
Inflammation and dilation of blood vessels and increase in its permeability.
Secretion of histamine, pyrogen, peroxidase, toxins and antitoxins.
Phagocytosis and mobility.