Glossary try the best to avoid the use of medical terms that could make it more difficult to understand the information on this website. Still, there are a number of terms that can't be avoided and that are useful to know because they are so often used by the doctors, nurses, and other medical professionals with whom you might speak. The list below includes the terms used on for which we have provided definitions.

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Term Definition

A large white blood cell (WBC) found in connective tissue, lymph nodes, spleen, bone marrow and other tissues; it is an important part of the body's immune system and helps fight infections by surrounding and ingesting disease-causing microorganisms.


Harmful and potentially fatal


Examination of the breast by X-ray

Mast cell

A type of tissue cell found throughout the body but especially in connective tissue such as the skin, lining of the intestine and air passages as well as in the bone marrow. Mast cells contain granules that store chemicals. These chemicals are released as part of the body's normal response to injury but also may be released as part of an allergic response to exposure to an allergen. The chemicals that are released can cause the allergic signs and symptoms.


Abnormal accumulation of mast cells within one or more organs. Mast cells are a type of tissue cell found throughout the body that release chemicals as part of the body's normal response to injury but sometimes as part of an allergic response. Cutaneous mastocytosis is a benign disease of the skin, usually affecting children. Systemic mastocytosis affects mostly adults, who may experience signs and symptoms related to the organs affected such as skin rashes or characteristic red, blistering lesions, peptic ulcers, chronic diarrhea, joint pain or enlargement of the liver, spleen, or lymph nodes. Systemic mastocytosis may progress slowly or may be aggressive, causing organ dysfunction and, in rare cases, causing a form of leukemia.

Meconium ileus

A condition in newborn infants characterized by no stools in the first 24 to 48 hours of life


Large cells in the bone marrow that produce blood platelets


Layers of tissue that surround the brain and spinal cord


Rare cancer of the membranes that cover the outside of internal organs and line body cavities, including the chest (pleural mesothelioma), abdominal cavity (peritoneal mesothelioma), and the heart (pericardial mesothelioma)


Chemical reactions that occur in living organisms to convert one substance into another or produce energy


Product of chemical or biological processes in the body


Spread of cancer from its site of origin to distant sites


Living or occurring in a reduced-oxygen environment


a birth defect in which a baby’s head is much smaller than expected and the brain is underdeveloped; this condition can occur because a baby's brain stops developing during pregnancy or stops growing after birth. A more serious, extreme form called severe microcephaly can occur during pregnancy when a baby's brain does not develop properly or begins to develop but then stops. Babies born with microcephaly can have several other issues, such as developmental delays, intellectual disabilities, hearing loss, problems with vision, and seizures.


Smaller than normal red blood cells


Also known as: Microbe

Life form that is not visible to the naked eye such as bacteria, fungi, parasites, and viruses


a growth on the skin comprised of a cluster of skin cells that produce the skin pigment melanin (melanocytes); a mole is often raised and dark in color.

Monoclonal antibody

Antibody produced by or derived from a single type (clone) of plasma cell

Monoclonal cells

Group of cells derived from a single type (clone) of cell; the cells formed are identical.

Monoclonal gammopathy

Leukocyte (white blood cell) that functions in the ingestion of bacteria and other foreign particles. Monocytes make up 5-10% of the total white blood cell count.


Pertaining to system of bones, muscles, joints, and associated tissues (e.g., ligaments and tendons) of the body involved in the maintenance of body form and movement


Change in the genetic structure (DNA); it may occur spontaneously or be induced (e.g., by radiation, drugs, or certain mutagenic chemicals).


A diverse group of rod-shaped bacteria that include Mycobacterium tuberculosis (which causes tuberculosis) and Mycobacterium leprae (which causes leprosy) and more than 100 different species found in the environment; the environmental mycobacteria may be referred to as nontuberculous mycobacteria (NTM), mycobacteria other than tuberculosis (MOTT), and/or atypical mycobacteria.


Also known as: Myelin sheath

The fatty covering that insulates nerve fibers

Myelin sheath

Also known as: Myelin

The fatty covering that insulates nerve fibers