Haptens are molecules that are not fully functional antigens on their own but can elicit an immune response when they bind to larger carrier molecules. Option 1, pseudoantigens, is incorrect because haptens are not false or fake antigens. Option 3, antibodies, is incorrect because haptens are not antibodies themselves, but rather the molecules that bind to antibodies. Option 4, isoantigens, is incorrect because isoantigens refer to antigens that are found in individuals of the same species but may differ between individuals. Therefore, the correct answer is option 2, incomplete antigens, as haptens are considered to be "incomplete" because they require a carrier molecule to stimulate an immune response.