The breasts are made up of glandular tissue organized into lobules and ducts, surrounded by fat, lymph nodes, ligaments, nerves and blood vessels. Each breast is surrounded by a large number of lymph nodes, including a number of lymph nodes in the armpit. Each lymph node houses thousands of immune system cells, and each lymph node allows for lymphatic fluid circulation throughout the body. A breast lump in the armpit often develops as a result of swollen lymph nodes and can indicate the presence of a number of disorders.
Infection or Inflammation
One possible cause of a lump under the armpit is infection or inflammation, according to the University of Maryland Medical Center. The major function of lymph node cells is to respond to and attack foreign pathogens or allergens. When the immune system detects a foreign particle or microbe within the body, immune system cells increase in number in preparation to fight an infection. As a result, the lymph nodes increase in size after infection, leading to lumps in the armpit.
A breast lump in the armpit as a result of infection generally occurs in both armpits, and patients may also notice swollen lymph nodes along the sides of the neck and in the groin region. Swollen lymph nodes as a result of infection or inflammation are temporary, and the lump will disappear after the infection has been neutralized.
Breast Cancer
Another, more serious possible cause of a breast lump in the armpit is breast cancer. Breast cancer begins in the glandular tissue of the breast--either the ducts, lobules or the nipple. Once the breast cancer begins to spread, cancer cells migrate into the lymph nodes under the arm and continue to grow. The result is swollen and enlarged lymph nodes that become matted together and stick to the skin.
A lump under the arm as a result of breast cancer is usually unilateral--the lump only occurs under one arm. Breast cancer may also cause a number of other symptoms, such as skin wrinkling and dimpling, flat or inverted nipples or abnormal nipple discharge, according to the Ohio State University Medical Center. Patients may also be able to feel a lump within the breast, in addition to the lump under the arm. A unilateral underarm lump requires medical attention to rule out the possibility of cancer.
Lymphoma
Another potential cause of a lump under the arm is lymphoma, a cancer that develops within the lymph nodes. There are a number of types of lymphoma, which can involve cancer growth in the lymph nodes, spleen, thymus and bone marrow. Since there are several lymph nodes concentrated under the arm, swollen lymph nodes as a result of lymphoma may be noticeable in the armpit.
A lump under the arm as a result of lymphoma would likely affect both armpits and would lead to swollen lymph nodes in other parts of the body, such as the neck and groin. Patients with lymphoma may also develop unexplained fever, weight loss, fatigue and may suffer from severe night sweats, according to the University of Maryland Medical Center.
