These X4 strains kind of lay low in the lymphoid tissues, and steadily destroy of CD4 T cells, since about 90% of T cells are found in lymphoid tissue.
Below the nasopharynx and closer to the mouth is the second division of the pharynx, the oropharynx. The palatine tonsils, two rounded massed of lymphatic tissue, are located in the oropharynx.
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Gowan's breakthrough was to show these cells do not vanish but are absorbed into the lymph system, then circulate through the tissues and lymph nodes before returning into the blood.
Some lymphoid organs are in the form of diffuse lymphoid tissue, where they are just a loose arrangement of lymphoid cells and protein, this is typical in the lining of the gastrointestinal and respiratory tract.