Imaging: Any method used to produce a picture of internal body structures. Some imaging methods used to detect cancer are x-rays (a breast x-ray is called a mammogram), magnetic resonance (MR) imaging, scintigraphy, computed tomography (CT) imaging, and ultrasonography. (See also mammogram, bone scan, computed tomography, magnetic resonance imaging, ultrasonography).

Immune system: The complex system by which the body resists infection by microbes (such as bacteria or viruses) and rejects transplanted tissues or organs. The immune system may also help the body fight some cancers. (See also antibody, antigen, lymph nodes).

Immunocytochemistry or immunohistochemistry: A laboratory test that uses antibodies to detect specific chemical antigens in cells or tissue samples viewed under a microscope. This procedure can be used to help detect and classify cancer cells. It is also one of the methods used for estrogen receptor assays and progesterone receptor assays. (See also monoclonal antibodies).

Immunology: Study of how the body resists infection and certain other diseases. Knowledge gained in this field is important to cancer treatments based on the principles of immunology. (See also immunotherapy).

Immunosuppression: A state in which the ability of the body's immune system to respond is decreased. This condition may be present at birth, may be caused by certain infections (such as human immunodeficiency virus or HIV), or by certain cancer therapies, such as cytotoxic (cancer-cell killing) drugs, radiation, and bone marrow transplant action.

Immunotherapy: Treatments that promote or support the body's immune system response to a disease, such as cancer.

Incisional biopsy: The removal of part of a lump for microscopic (pathologic) examination.  (See also biopsy).  

Infiltrating ductal carcinoma (IDC): Cancer beginning in the milk ducts of the breast and penetrating the wall of the duct, invading the fatty tissue of the breast and possibly other regions of the body. IDC is the most common type of breast cancer, accounting for 80% of breast cancer diagnoses.  Also called invasive ductal carcinoma.

Infiltrating lobular carcinoma (ILC): Cancer beginning in the milk glands (lobules) of the breast, but often spreading to other regions of the body. ILC accounts for 10% to 15% of breast cancers.   Also called invasive lobular carcinoma.

Inflammatory carcinoma: The appearance of inflamed breasts (red and warm) with dimples and/or ridges caused by the infiltration of tumor cells into the lymphatics.

Infraclavicular nodes: Lymph nodes located beneath the clavicle (collar bone).

Inframammary fold: The lower breast fold that attaches the lowest portion of the breast to the rib cage.

Infusion: The slow intravenous (through the vein) delivery of drugs or fluids.

Injection: The use of a syringe and needle to push fluids or drugs into the body.  Also called shot. 

In situ: Literally meaning, "in place."  The term, in situ, applies to cancer that is within the original tissue and has not yet broken through any boundaries between tissues. (See also ductal carcinoma in situ, lobular carcinoma in situ). 

Interferon: A protein produced by cells, interferon helps regulate the body's immune system, boosting activity when a threat, such as a virus, is detected. Scientists have learned that interferon helps fight against cancer, so it is used for immunotherapy of some types of cancer.

Internal mammary nodes: Lymph nodes beneath the breast bone on each side. Some breast cancers may spread to these nodes.

Intraductal papilloma: Small, finger-like, polyp-like, noncancerous growths in the breast ducts that may cause a bloody nipple discharge. These are most often found in women 45 to 50 years of age. When many papillomas exist, breast cancer risk is slightly increased.

Intravenous (IV): A method of supplying fluids and medications, using a needle inserted in a vein.

Invasive cancer: Cancer that has spread beyond the area it originally developed in, to involve adjacent tissues. For example, invasive breast cancers develop in milk glands (lobules) or milk passages (ducts) and spread to the adjacent fatty breast tissue. Some invasive cancers spread to distant areas of the body (metastasize), but others do not. Also called infiltrating cancer. (See also invasive ductal carcinoma, invasive lobular carcinoma).

Invasive ductal carcinoma: A cancer that originates in the milk passages (ducts) of the breast and then breaks through the duct wall, where it invades the fatty tissue of the breast. When it reaches this point, it has the potential to spread (metastasize) elsewhere in the breast, as well as to other parts of the body through the bloodstream and lymphatic system. Invasive ductal carcinoma is the most common type of breast cancer, accounting for about 80% of breast malignancies. Also known as infiltrating ductal carcinoma.

Invasive lobular carcinoma: A cancer that arises in the milk-producing glands (lobules) of the breast and then breaks through the lobule walls to involve the adjacent fatty tissue. From this site, it may then spread elsewhere in the breast. About 15% of invasive breast cancers are invasive lobular carcinomas. It is often difficult to detect by physical examination or even by mammography. Also called infiltrating lobular carcinoma.

Inverted nipple: A condition in which the nipple is tucked into the areola (pigmented region surrounding the areola).