Answer:
The immune system response against pathogens in order to protect our cells from the damage and prevent diseases.
There are different cells in the blood including macrophages, mast cells, and natural killer cells etcetra which recognises the specific pathogen-associated molecular patterns (PAMPs) on the pathogen’s surface. PAMPs can be classified as polypeptide, carbohydrate, and nucleic acid which are expressed by bacteria, parasites, and viruses. Immune system detects "self" from the "foreign" antigen and destroy the pathogen without damaging the host cell.
Hence, immune cells detect foreign pathogens through specific pathogen-associated molecular patterns (PAMPs) on the pathogen’s surface.