The defence mechanism of the body against different diseases is known as:

The immune system of an old person becomes weak because of . . . . . . .

An antigen is

Antigen is a substance which

Which of the following organs are primary lymphoid organs?

Choose the true statements.
$I.$ Innate immunity is accomplished by providing different types of barriers.
$II.$ Acquired immunity is present from birth and is inherited from parents.
$III.$ Sweat,tears,acid in the stomach,and saliva prevent microbial growth.