We will cover T cell maturation in the thymus later. The thymus functions maximally during fetal development and in childhood. Whereas it is a relatively large organ at the time of birth with an average size of 70g, by age 60 its size shrinks to about 3 g. These are all characterized by the presence of follicles. Primary follicles are a small network of follicular dendritic cells and non-activated B cells.
Secondary follicles are the result of antigen recognition by B cells in a primary follicle.
The secondary follicle contains dividing B lymphocytes, memory B cells, plasma cells, macrophages and central follicular dendritic cells. T helper cells are nearby. Naive B cells from the bone marrow migrate through the blood and lymphatic system, filtering through the secondary lymphoid organs and tissues. The lymph nodes collect cells, antigens, and fluids from sites of internal infection via the lymphatic vessels that drain into them. The spleen filters cells and antigens from the blood and is the site of generation of immune responses to blood infections.