Armpits contain a special kind of sweat gland called an apocrine gland.
The fluid those glands release is not usually very smelly by itself.
The odor appears after skin bacteria start breaking it down.
Two types of sweat glands matter
Most of your body uses eccrine sweat glands.
They produce the watery sweat that helps cool you.
Armpits also contain many apocrine glands.
Their secretions contain more fats and proteins.
Those compounds give bacteria more material to work with.
Bacteria create the odor
Your skin naturally carries bacteria.
When those bacteria break down compounds in apocrine sweat, they produce smaller molecules with much stronger smells.
That is why sweat itself does not always smell strongly.
The bacteria are doing much of the finishing work.
Why do armpits smell stronger than your forearm?
Apocrine glands are concentrated in areas such as the armpits and groin.
Those areas are also warm, moist, and often covered.
That creates a good environment for bacterial activity.
Why does puberty make body odor stronger?
Apocrine glands become more active around puberty.
That is one reason body odor often becomes much more noticeable during adolescence.
How does deodorant help?
Deodorants mainly target odor.
Some reduce bacterial growth.
Others add fragrance.
Antiperspirants work differently by reducing how much sweat reaches the skin surface.
Why do some people smell stronger than others?
Genetics, skin bacteria, hormones, diet, clothing, hygiene, medications, and activity all influence odor.
There is no single “normal” amount of body odor.
Fresh sweat is not the main villain.
The smell usually appears after bacteria start processing it.
And that’s the short explanation.
Related explanations: Why does sweat smell?, Why do feet smell?, and Why do we sweat?