I really wish I could enjoy this beautiful day at the beach but I feel really sick!
It seems to me like you could have a virus!
I am sweating more than usual, I have tears in my eyes, and I feel congested.
These are all chemical barriers on epithelium. Sweating, tears and mucus are examples of skin & mucous membrane secretions. This is a part of the first line of defense!
Next, patrolling cells and proteins will start attacking invaders that penetrate your body's outer barriers. This broad range of patrolling is a part of the second line of defense. This includes leukocytes, phagocytic white blood cells, that enter infected tissue and engulf and ingest microbes. They are attracted by chemical signals released by damaged cells.
Oh no! What happens next?
The chemical signals that are released are histamines and prostaglandins. A histamine is a substance produced by bysophils in blood and mast cells in connective tissue, that causes capillaries to dilate. A prostaglandin is a hormone that has various and powerful local effects.
What kind of chemical signals?
Well that is not all. There is the Third line of defense that can also bring protection towards you and trying to fight the virus off. It responds to antigens, that are specific pathogens, specific toxins and also abnormal body cells also known as cancer. Lymphocytes are apart of the Third line of defense they consist of B cells that mature in bone marrow and learn to distinguish themselves from "self and "non self" antigens.
Is that all?
You can receive vaccinations that expose the system to harmless version of pathogen. This triggers active immunity and stimulates the immune system to produce antibodies against invaders.