Assessment of CD27 and CD28 on T cell populations in the appendix revealed that CD4+ Tem cells had reduced CD27 expression in all layers of the appendix combined in children with complex compared to simple appendicitis (epithelium simple appendicitis median 87%, IQR 85-91% versus complex appendicitis median 79%, IQR 77-81%; lamina propria median 86%, IQR 83-88% versus median 72%, IQR 67-82%; muscular layer median 76%, IQR 69-80% versus median 68%, IQR 65-75%). Here, CD28 is linked to appendicitis.