periarteriolar lymphoid sheaths periarteriolar lymphoid sheaths periarteriolar lymphoid sheaths (or periarterial lymphatic sheaths, or pals) are a portion of the white pulp of the spleen. they are populated largely by t cells and surround central arteries within the spleen; the pals t-cells are presented with blood borne antigens via myeloid dendritic cells. in contrast, the lymphoid portions of the white pulp are dominated by b cells.