Step 1: Understanding the Concept:
The immune system is the body's primary defense against infections. It uses specialized cells to target foreign invaders.
Step 2: Detailed Explanation:
Leucocytes are the technical name for White Blood Cells (WBCs). They are responsible for protecting the body.
Antigens are the foreign substances (like viruses or bacteria) that trigger an immune response when they enter the body.
Antibodies are proteins produced by the immune system to neutralize antigens, but the question asks what the foreign bodies themselves are called.
Therefore, the first blank is Leucocytes and the second is Antigens.
Step 3: Final Answer:
The correct pair is Leucocytes/Antigens.