A normal appendix is a thin, finger-shaped pouch of tissue that extends from the large intestine. It typically measures 6 to 10 centimeters long (roughly the length of your pinky finger) and less than 6 millimeters in diameter. On the outside, it looks like a small, pinkish-gray worm-like tube with a smooth, glistening surface. On imaging, it appears as a narrow, thin-walled structure with no surrounding inflammation.
External Appearance and Size
If you were looking at a healthy appendix during surgery, you’d see a slender, tubular pouch dangling from the cecum, which is the first part of the large intestine. Its outer surface is smooth and shiny, covered by a thin membrane called the serosa that gives it a slightly glossy look. The color ranges from pinkish to grayish-pink, similar to the rest of the intestine. Blood vessels run along its surface, visible as fine reddish lines.
The appendix has a blind end, meaning it’s closed off at the tip like a deflated balloon. Where it connects to the cecum, there’s a small opening that allows intestinal contents to flow in and out. The walls feel soft and pliable when healthy, not swollen or rigid. An inflamed appendix, by contrast, becomes swollen, red, and firm, sometimes doubling or tripling in diameter.
Where It Sits in the Abdomen
Most people picture the appendix hanging straight down in the lower right abdomen, but its position varies quite a bit. A CT study of over 1,500 patients found that only about 25% of people have the most commonly described position, called retrocecal, where the appendix tucks behind the cecum. Another 20% have it sitting below the cecum, about 19% have it behind part of the small intestine, and roughly 17% have a pelvic position where it dips down toward the pelvis.
This variability explains why appendicitis pain doesn’t always show up in the “textbook” lower right spot. If your appendix points toward your pelvis, for example, the pain might feel lower or more central. If it sits behind the cecum, the discomfort might seem deeper or more toward the back.
What It Looks Like on a CT Scan
On a CT scan, a normal appendix appears as a small, round or oval tube in cross-section, with thin walls and a diameter under 6 millimeters. That 6 mm threshold is one of the key markers radiologists use to distinguish a healthy appendix from one that’s inflamed. When the diameter exceeds 6 mm, wall thickening is visible, or fat around the appendix appears hazy and streaky, appendicitis becomes a concern.
A normal appendix isn’t always easy to spot on imaging. Studies show that a standard CT scan successfully visualizes the appendix in about 77% of cases. The remaining times, it can be too small, tucked into an unusual position, or obscured by surrounding bowel loops. Not seeing the appendix on a scan doesn’t necessarily mean something is wrong.
Internal Structure Under a Microscope
The appendix has the same four-layer structure as the rest of the intestine, but with one major difference: it’s packed with immune tissue. Its inner lining is covered with glandular cells that produce mucus, similar to the colon. Just beneath that lining sits a layer rich in white blood cells called lymphocytes, sometimes so densely packed that they blur the boundaries between tissue layers.
The next layer, the submucosa, is where the appendix really stands out. It’s almost entirely filled with clumps of immune cells organized into structures called lymphoid follicles. These appear as round clusters of tightly packed cells, often with lighter-staining centers where immune cells are actively dividing and maturing. This dense concentration of immune tissue is why many researchers believe the appendix plays a role in immune surveillance, essentially sampling bacteria and other material that passes through the gut.
Surrounding the immune-rich interior are two layers of muscle, one running in circles and one running lengthwise, that give the appendix its ability to contract. The outermost coating is a thin layer of connective tissue containing blood vessels and nerves, topped by a delicate sheet of flat cells that forms the appendix’s smooth exterior surface.
How It Changes With Age
The appendix doesn’t look the same throughout your life. In children and young adults, the lymphoid tissue is at its peak, making the appendix relatively plump and immunologically active. This is also the age range when appendicitis is most common, partly because the lumen (the hollow center) can be narrowed by all that immune tissue, making blockages more likely.
As you age, the immune tissue gradually shrinks in a process called involution. By older adulthood, much of the lymphoid tissue has been replaced by fibrous scar-like tissue, and the appendix becomes thinner and less prominent. In people over 69, the appendix may be noticeably atrophied. This shrinkage is one reason acute appendicitis becomes less frequent in older adults, though when it does occur in this age group, symptoms are often more subtle and harder to recognize because the diminished immune tissue produces a weaker inflammatory response.