How do the body's defenders recognize an enemy? what is an antigen?

Immune system comprises of all the cells and processes responsible for the defense system in vertebrates. Skin form the first line of defense system that does not allow the penetration of pathogens inside the body. Inside the body there are certain cells called lymphocytes that play role in the defense mechanism. B lymphocytes form the antibodies against the antigens or foreign objects. Similarly T lymphocytes also helps in killing and recognizing the foreign particles.

An antigen is a foreign object that are targeted by antibodies. Antigens are recognized by antibodies present in the body.Antibodies get bind to the antigens at specific sites and triger the process of defense system.

Ithink the dna sequences may vary but the amino acid sequences are identical. 

Photo means light and synthesis means combination so combination of light