IMMUNOLOGY

Detailed explanation-1: -Primary lymphoid organs: These organs include the bone marrow and the thymus. They create special immune system cells called lymphocytes. Secondary lymphoid organs: These organs include the lymph nodes, the spleen, the tonsils and certain tissue in various mucous membrane layers in the body (for instance in the bowel).

Detailed explanation-2: -The primary lymphoid organs are the red bone marrow, in which blood and immune cells are produced, and the thymus, where T-lymphocytes mature. The lymph nodes and spleen are the major secondary lymphoid organs; they filter out pathogens and maintain the population of mature lymphocytes.

Detailed explanation-3: -Since the function of the lymphoid organs is to filter and trap invading pathogens and present them to immune competent cells, the high levels of viremia seen following primary infection presumably lead to an efficient infection of the lymphoid tissue with HIV or SIV.