State the number of polypeptide chains in $H_{2}L_{2}$ (an antibody molecule).

The 'first line of defence' is known as:

Which of the following components is $NOT$ associated with the immune system?

In higher vertebrates, the immune system can distinguish between self and non-self cells. Due to genetic or other unknown reasons, the immune system sometimes loses this property and starts attacking self-cells, resulting in:

Which immunoglobulin is present in abundant amounts in the mother's initial milk secretion (colostrum)?

Antibodies are produced by