GENERAL HISTOLOGY

Detailed explanation-1: -The third line of defense is specific resistance. This system relies on antigens, which are specific substances found in foreign microbes. Most antigens are proteins that serve as the stimulus to produce an immune response.

Detailed explanation-2: -The natural killer cells are the third major part of the innate immune system. They specialize in identifying cells that are infected by a virus or that have become tumorous. To do this, they search for cells that have changes in their surface, and then destroy the cell surface using cell toxins.

Detailed explanation-3: -The first line of defence is made up of barriers to prevent infection. The second line of defence is made up of responses to infection that apply generally to all pathogens. The third line of defence provides immunity against specific pathogens.