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Is a foreign molecule that evokes a specific response by a lymphocyte.

Explanation:

Antigens can be defined as molecules that can bind to antibodies, triggering immune responses(by the action of lymphocytes).

The immune system's basic function is to discriminate between proper antigens (autogenous) and non-proper antigens (heterogeneous), in order to prevent attacks by the immune system to molecules in the body itself.

Antigen can also be defined as any substance foreign to the organism that triggers the production of antibodies.

They are usually proteins or polysaccharides that can be found in the envelopes of viruses, bacteria, fungi, protozoa and parasitic worms.