What is Immunity?

(N/A) The overall ability of the host to fight disease-causing organisms,conferred by the immune system,is called Immunity.
- Immunity is of two types: $(i)$ Innate immunity,$(ii)$ Acquired immunity.
$(i)$ Innate immunity: It is a non-specific type of defense that is present at the time of birth.
This is accomplished by providing different types of barriers to the entry of foreign agents into our body. Innate immunity consists of four types of barriers:
Physical barriers: Skin on our body is the main barrier which prevents the entry of microorganisms. The mucus coating of the epithelium lining the respiratory,gastrointestinal,and urogenital tracts also helps in trapping microbes entering our body.
Physiological barriers: Acid in the stomach,saliva in the mouth,and tears from the eyes all prevent microbial growth.
Cellular barriers: Certain types of leukocytes $(WBC)$ of our body like polymorpho-nuclear leukocytes ($PMNL$-neutrophils),monocytes,and natural killer cells (types 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.
$(ii)$ Acquired immunity: Acquired immunity is pathogen-specific. It is characterized by memory.
Our body,when it encounters a pathogen for the first time,produces a response called the primary response.
Primary response is of low intensity.
Subsequent encounters with the same pathogen elicit a highly intensified secondary or anamnestic response.
This is ascribed to the fact that our body appears to have memory of the first encounter.