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The medical term for a foreign substance is "antigen." An antigen is any substance that the immune system identifies as foreign and triggers an immune response.
vaccineinoculantadjuvantantigen
the immune system
When a foreign substance enters the body.
Immune System
its an allergy, or an allergic reaction
an allergie
Lymphocytes
The first defense is nonspecific.
The Immune System is the defense network of the body. The immune system works with specific receptors on specialized cells - detecting foreign bodies and signaling for the immune system to destroy them.
An antigen is a foreign body. It is used as a "template" to begin an appropriate immune response.
Any sort of "foreign" substance can, and most often will, elicit an immune response under certain curcumstances--provided the immune system is healthy. This would include bacteria, viruses and fungi. Related: An unhealthy, or compromised immune system can act in all sorts of undesirable ways, ranging from low response to threat or injury to actually creating antibodies to attack its own system or other organs and tissue.