What are 2 ways that our body protects us against foreign invaders? Can you explain these examples using new metaphors? What’s the difference between an antigen and an antibody?

The Immune System and COVID-19

What are 2 ways that our body protects us against foreign invaders? Can you explain these examples using new metaphors? (e.g., white blood cells are like an army of soldiers)
What’s the difference between an antigen and an antibody?
Sometimes the body can make antibodies against antigens that are already in the body. Usually, people who experience this are said to have an “autoimmune disease”. What do you think this means? What kind of effects do you think this could have on a person’s body?
Knowing that a blood test might be able to reveal how severe a person’s COVID-19 infection will be (based on their antibodies), how do you think this information could be helpful?
Knowing now how the SARS-COV-2 virus affects the alveolar type II cells and subsequently other systems in the body, what other questions do you still have?