Which type of chains are present in an antibody molecule?

In the given figure,the part labeled as '$a$' represents:

Which of the following immunoglobulins constitutes the largest percentage in human milk?

The immune response provided by the body against pathogens with the help of $B$-lymphocytes is which type of immunity?

Explain the human immune system.

$B$-lymphocytes produce an army of proteins in response to pathogens entering our body to fight them. These proteins are called.........