Final answer:
In the early stages of an infection, antigens would likely be found in lymph draining into a lymph node. Lymphatic vessels carry lymph and antigens to the lymph nodes where the immune response begins with the presentation of antigens to lymphocytes, eventually leading to antibody production.
Step-by-step explanation:
Early on in an initial infection, antigens might be found in lymph draining into a lymph node. Antigens are molecules that are recognized by the receptors of B and T lymphocytes; these molecules could include parts of pathogens, such as bacteria or viruses, that have entered the body. As the afferent lymphatic vessels lead into a lymph node, they bring lymph along with these antigens from the surrounding tissues. This lymphatic fluid is then filtered in the lymph nodes, where antigen-presenting cells (APCs), such as macrophages and dendritic cells, capture and process these antigens. Subsequently, a response from the immune system is initiated in which lymphocytes are informed about the potential pathogens, leading to the production of antibodies — specific proteins secreted by plasma cells that can inhibit infection by preventing the antigen from binding its target, tagging a pathogen for destruction, or activating the complement cascade, all crucial for defending against the invading pathogens.