It is true that the body begins to manfacture antibodies upon being infected. You need to wait to test so that there are enough antibodies for the tests to detect. A conclusive test result is 3 months post exposure.
Antibody generation mechanism becomes active as soon as it detects any virus. The system produces mirror images of the invading organism and traps and kills. HIV is smarter in two ways, first, it mutates rapidly and second, it enters the defending cells (CD4) and converts them into virus generating factories, thus the whole self defense mechanism eventually collapses.
Since antibodies are however produced regardless of type of virus invaded, sometime the symptoms are misleading and we assume this all due to HIV, whereas our system was not fighting against HIV at that time, but against some other infection for instance, and we are negative when tested. Therefore its advised by all knowledgeable to always believe on test not on lymph nodes enlargement