21. cells of the immune system are able to respond to the presence ofinvading organisms because they recognize the(1) antigens present on the invaders2) antibodies present in invading pathogens) dna partem in the nuclei of viruses(4) antibiotics released from microbes

1.  antigens present on the invaders

Explanation:

Through the antigens present to the invaders the Cells of the immune system can easily able to respond and recognize.  During an infection, chemical signals attract phagocytes to places where the pathogen has invaded the body. 

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