Antigens are
a. specific molecules, or parts of molecules, that the body recognizes as foreign
b. proteins the body produces against invading substances
c. cells that protect the body against invaders
d. proteins on the surface of our cells to which pathogens attach
e. enzymes secreted to destroy a pathogen's cell wall

The correct answer is option a. "specific molecules, or parts of molecules, that the body recognizes as foreign".

Explanation:

Antigens are defined as specific molecules, or parts of molecules, that the body recognizes as foreign. Antigens may be parts of a pathogen or a molecule that is just foreign, and are crucial to trigger the immune system because they are recognized by B cells that produce antibodies. During the immune response antigens are attacked by antigen-specific antibodies (Ab) or B cell antigen receptors (BCR).

The correct answer is: A. specific molecules, or parts of molecules, that the body recognizes as foreign.

Explanation:

An antigen is any specific substance that triggers an immune response. For every antigen, there is a specific antibody that is used to "attack" it. An antigen could be any molecule that the body recognizes as foreign, from pathogens to the own body cells in the cases of autoimmune disorders.

The proteins that the body produces against invading substances are called antibodies.

The cells that protect the body against invaders are the white blood cells, also called leukocytes.

The proteins on the surface of the cells to which pathogens attach are receptors.

The enzymes secreted to destroy a pathogen's cell wall are perforins.

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