What Is the Axilla? Anatomy, Function, and Location

The axilla is the medical term for the armpit, the pyramid-shaped space where your upper arm meets the side of your chest. Far from being just a hollow covered in skin and hair, it is one of the most densely packed regions in the body, housing major blood vessels, nerves, lymph nodes, and sweat glands within a compact area roughly the size of your fist. Because so many vital structures pass through on their way from the torso to the arm, the axilla plays an outsized role in everything from cancer staging to temperature screening to the bacteria that make you smell after a workout.

Where the Axilla Sits and What Defines Its Borders

The axilla sits just below the shoulder joint, forming a space between the upper arm and the chest wall. Anatomists describe it as a four-sided pyramid. The front wall is formed by the pectoralis major and minor muscles, the back wall by the latissimus dorsi and other shoulder-blade muscles, and the inner wall by the upper ribs covered with the serratus anterior muscle. The outer wall is narrow, formed mainly by the upper arm bone itself. The base is the skin and fascia you can pinch when you lift your arm, and the apex is a small gap between the collarbone, first rib, and shoulder blade through which all the major vessels and nerves enter the space.

This pyramidal arrangement matters because it explains why the armpit can feel surprisingly deep when a doctor presses into it during an exam. The structures inside are not sitting on the surface; they are layered within a compartment packed with fat and connective tissue. Raising your arm opens the space wider and makes its contents more accessible, which is why clinicians ask you to lift your arm during a physical examination or an ultrasound.

Major Blood Vessels and Nerves

The axillary artery is the main blood supply to the entire upper limb. It is a continuation of the subclavian artery, changing its name as it passes the outer edge of the first rib and entering the axilla. Surgeons and anatomists traditionally divide it into three segments based on its relationship to the pectoralis minor muscle: the first part sits above the muscle, the second part behind it, and the third part below it. Each segment gives off branches that feed the chest wall, shoulder, and upper arm.

The axillary artery is normally flanked by the axillary vein (which runs alongside it carrying blood back toward the heart) and the cords of the brachial plexus, the network of nerves that controls movement and sensation in the arm and hand. The brachial plexus forms three cords named for their usual position around the artery: lateral, medial, and posterior. Cadaver studies have confirmed that this classic arrangement holds in most people, though occasional variants exist where the cords shift position relative to the artery.

Variations in axillary blood-vessel anatomy are not rare. In some individuals the axillary artery splits earlier or differently than expected, dividing into deep and superficial branches rather than following the textbook pattern.

Lymph Nodes and Their Role in Cancer Staging

Tucked among the fat and connective tissue of the axilla are roughly 20 to 40 lymph nodes, though exact numbers vary from person to person. These nodes filter lymph fluid draining from the arm, the chest wall, and most of the breast. That drainage pattern is why the axillary lymph nodes are so central to breast cancer care: if cancer cells break away from a breast tumor, they often travel through the lymph system and lodge in these nodes first.

Surgeons have long used a classification system that divides axillary lymph nodes into three levels based on their position relative to the pectoralis minor muscle. Level I nodes sit below and lateral to the muscle, level II nodes are behind it, and level III nodes are above and medial to it.¹ This framework guides how extensively a surgeon needs to remove nodes and helps pathologists map the extent of spread.

For decades, axillary lymph node dissection, the removal of a large number of these nodes, was standard in breast cancer treatment. The development of sentinel lymph node biopsy changed that. In this technique, a dye or radioactive tracer is injected near the tumor and tracked to the first node that receives drainage, the “sentinel” node. If that node is cancer-free, further node removal can often be skipped. Sentinel lymph node biopsy is now considered standard of care for initial staging.² Updated guidelines have even expanded the situations in which surgeons can safely skip it altogether, particularly in older postmenopausal patients with small, low-grade, hormone-receptor-positive tumors and negative ultrasound findings.³

Sweat Glands, Bacteria, and Body Odor

The axilla contains two distinct types of sweat glands, and they work differently. Eccrine glands, found across most of the body, produce a watery sweat made primarily of water along with small amounts of minerals like sodium and potassium, plus metabolites such as lactate and urea. Their main job is cooling you down. Apocrine glands, which are concentrated in the axilla and groin, produce something different: an oily, initially odorless fluid containing proteins, lipids, and steroids.⁴ Apocrine glands become active around puberty, which is why body odor typically starts during adolescence.

The odor itself comes not from the sweat but from bacteria that break down the proteins and lipids in apocrine secretions. Research mapping the microbial communities of the armpit has found that certain bacteria, particularly Corynebacterium species, are strongly linked to body odor, while other genera like Propionibacterium are associated with less smell.⁵ The balance of these bacterial populations varies widely between individuals, which helps explain why some people develop stronger body odor than others even when they sweat similar amounts.

This bacterial ecosystem also explains why freshly applied deodorant works differently from antiperspirant. Deodorants target the bacteria or mask their byproducts. Antiperspirants use aluminum compounds to temporarily block eccrine ducts and reduce the volume of sweat reaching the skin surface. The distinction matters because it is the apocrine secretions, not the watery eccrine sweat, that feed the odor-producing bacteria.

Is Axillary Temperature Measurement Reliable?

Placing a thermometer under the arm is one of the oldest and least invasive ways to estimate body temperature. In adults, axillary readings correlate reasonably well with core body temperature. One study found that the average difference between axillary and core readings was only about 0.2°C when using a mercury thermometer, with a strong correlation between the two measurements.⁶ The researchers noted that the old rule of thumb, that axillary temperature runs a full degree Celsius below core, did not hold in practice; the gap was much smaller.

In young children and infants, the picture is less reassuring. Studies in pediatric populations have found that axillary readings do not reliably reflect rectal or oral temperatures and should be interpreted with caution.⁷ In infants specifically, one study documented a mean difference of about 0.8°C between rectal and axillary readings, with axillary screening picking up fewer than four out of five actual fevers.⁸ For quick screening in older children and adults, an armpit reading is a reasonable first step. For infants or any situation where an accurate reading matters for a clinical decision, rectal measurement is more trustworthy.

Conditions That Commonly Affect the Axilla

Because the axilla is warm, moist, and rich in hair follicles and glands, it is prone to a handful of conditions that rarely occur elsewhere on the body, or that occur elsewhere but concentrate here.

Hidradenitis suppurativa is one of the more debilitating. It starts with plugged hair follicles that progress to deep, painful nodules and abscesses, and in some cases advance to draining tunnels under the skin with permanent scarring.⁹ Despite older beliefs that it originates in the sweat glands, current evidence points to follicular blockage as the initial trigger, with immune-system overreaction driving the inflammation.¹⁰ In a subset of patients, genetic mutations affecting certain protein complexes in the skin play a role. The condition tends to flare in areas where skin rubs against skin, making the axilla one of the most commonly involved sites.

Hyperhidrosis, or excessive sweating localized to the armpits, is another condition closely associated with this region. Primary axillary hyperhidrosis involves sweating well beyond what the body needs for temperature regulation, and it can significantly affect quality of life. Treatments range from prescription-strength antiperspirants and botulinum toxin injections to newer topical medications and, in severe cases, surgery.

Simpler issues are common, too. Contact dermatitis from fragrances or preservatives in deodorant, fungal infections thriving in the warm crease, and inflamed ingrown hairs are everyday reasons people end up examining their armpits more closely than usual.

Nerve Damage Risks During Axillary Surgery

Surgery in the axilla, whether for cancer, cosmetic reasons, or other conditions, carries specific risks because of how many nerves pass through the space. Two nerves are especially vulnerable.

The long thoracic nerve runs along the chest wall on its way to the serratus anterior muscle, the muscle that holds your shoulder blade flat against your ribs. If this nerve is injured during axillary lymph node dissection, the shoulder blade can “wing” outward when you push against a wall or reach forward. In one study of patients who underwent axillary dissection for breast cancer, about 11% developed nerve injury confirmed on testing. Most of these were partial injuries, and the majority resolved within a year, but persistent injury was still present in roughly 2% of patients at the 12-month mark.¹¹

The intercostobrachial nerve, which carries sensation from the inner upper arm and armpit, is the most common source of chronic pain after mastectomy and axillary dissection. Damage to this nerve can cause persistent burning, numbness, or shooting pains in the armpit and upper arm lasting well beyond the surgical recovery period.¹² A meta-analysis of randomized trials found that deliberately preserving this nerve during surgery, rather than cutting through it, substantially reduced the risk of chronic pain and sensory problems.¹³ A separate cohort study reported that six months after surgery, about 86% of patients whose nerve was preserved were symptom-free, compared with only about 54% of those whose nerve was cut.¹⁴

These findings have shifted surgical practice. More oncologic surgeons now make deliberate efforts to identify and spare the intercostobrachial nerve when performing axillary procedures, recognizing that the long-term sensory consequences of cutting it outweigh the small amount of extra operative time required to preserve it.

Anatomical Variations That Surprise Surgeons

Not everyone’s axilla looks the same inside. One well-documented variation is the axillary arch, sometimes called Langer’s arch, a band of muscle or connective tissue that stretches across the axilla between the latissimus dorsi and the pectoralis major or nearby structures. A large cadaver study examining 400 axillae found this arch present in about 9% of individuals, roughly half of them bilaterally.¹⁵ When the arm was moved into abduction and external rotation, about two-thirds of these arches made contact with the axillary blood vessels and nerves, suggesting they could compress these structures during certain movements.

For most people who have one, an axillary arch causes no symptoms and goes unnoticed for life. But during axillary surgery, it can be mistaken for another structure or can obscure the surgeon’s view, complicating lymph node dissection or sentinel node biopsy. In rare cases, it may also contribute to vascular or neurological compression syndromes in the arm.¹⁶

Accessory Breast Tissue in the Armpit

One of the more unexpected things you can find in the axilla is breast tissue. During early embryonic development, a ridge of tissue called the mammary ridge (or milk line) extends from the armpit down to the groin on each side of the body. Normally, all of this tissue regresses except for the pair of breast buds that develop into the adult breasts.¹⁷ Sometimes the regression is incomplete, leaving behind accessory breast tissue, and the axilla is the most common place for this to happen. Estimates suggest it occurs in up to about 6% of the population.¹⁸

Accessory breast tissue in the armpit may go unnoticed or may present as a soft lump that swells and becomes tender in sync with the menstrual cycle or during pregnancy and breastfeeding. Because it is real breast tissue, it can develop any condition that affects the breast proper, including cysts, fibroadenomas, and even breast cancer. Radiologists and clinicians need to be aware of this possibility when evaluating lumps in the armpit, since an imaging finding in the axilla could represent a primary breast lesion rather than a swollen lymph node.

The Antiperspirant and Breast Cancer Question

For years, a popular concern held that aluminum compounds in antiperspirants might be absorbed through the skin of the axilla and contribute to breast cancer. The idea gained traction partly because of the axilla’s proximity to breast tissue and its dense lymph node network. Research, however, has not supported this link. A study that examined excised apocrine glands after participants had applied a specially formulated aluminum-staining antiperspirant found no aluminum compounds inside the glands, consistent with earlier work suggesting that these compounds do not penetrate into the apocrine tissue.¹⁹ While no single study can definitively close a question like this, the weight of current evidence does not support avoiding antiperspirants out of cancer fear.

That said, some people do react to antiperspirants, just not with cancer. Allergic contact dermatitis from fragrance additives or preservatives is far more common and far more evidence-based as a reason to switch products. If your armpits are chronically red, itchy, or irritated, the culprit is more likely the perfume in your stick than the aluminum.

Why the Axilla Gets Examined So Often

Given everything packed into this small space, it is no surprise that doctors check the axilla routinely. A physical exam of the armpit is part of standard breast cancer screening, because enlarged or hardened lymph nodes can be felt by hand before they show up on imaging. It is checked in patients with arm swelling to assess for lymphatic obstruction. Dermatologists examine it for hidradenitis suppurativa, fungal infections, and unusual moles (melanoma can occur in sun-protected skin, too). Vascular specialists may evaluate it when blood flow to the arm seems compromised.

Even outside formal medical encounters, the axilla is a site people monitor instinctively. A new lump under the arm, a change in odor, unusual sweating, or persistent pain are among the most common reasons people consult a doctor about this area. Most of these concerns turn out to be benign, such as a reactive lymph node swelling during an infection or an irritated hair follicle. But the axilla’s concentration of structures means that when something does go wrong, it often involves something that warrants attention rather than dismissal.