BACTERICIDAL: Capable of killing bacteria.

BASELINE: 1. Information gathered at the beginning of a study from which variations found in the study are measured. 2. A known value or quantity with which an unknown is compared when measured or assessed.

BASOPHIL: A type of white blood cell, also called a granular leukocyte, filled with granules of toxic chemicals that can digest microorganisms. Basophils, as well as other types of white blood cells, are responsible for the symptoms of allergy. The granules stain blue when exposed to a basic dye for microscopic examination.

B CELLS: See B Lymphocytes.

BINDING ANTIBODY: As related to HIV infection: An antibody that attaches to some part of the HIV virus. Binding antibodies may or may not adversely affect the virus.

BIOPSY: The surgical removal of a piece of tissue from a living subject for microscopic examination to make a diagnosis (for example, to determine whether abnormal cells such as cancer cells are present).

BIOTECHNOLOGY: 1. The use of living organisms or their products to make or modify a substance. These include recombinant DNA techniques (also referred to as genetic engineering) and hybridoma technology. 2. The industrial application of the results of biological research, particularly in fields such as recombinant DNA or gene splicing, which permits the production of synthetic hormones or enzymes by combining genetic material from different species.

BLINDED STUDY: A clinical trial in which participants are unaware as to whether or not they are in the experimental or control arm of the study.

BLOOD BRAIN BARRIER: The barrier between brain blood vessels and brain tissues whose effect is to restrict what may pass from the blood into the brain.

B LYMPHOCYTES (B CELLS): One of the two major classes of lymphocytes. During infections, these cells are transformed into plasma cells that produce large quantities of antibody directed at specific pathogens. This transformation occurs through interactions with various types of T cells and other components of the immune system. In persons with AIDS, the functional ability of both the B and the T lymphocytes is damaged, with the T lymphocytes being the principal site of infection by the HIV virus. See also Lymphocyte; T Cells.

BODY FLUIDS: Any fluid in the human body, such as blood, urine, saliva, sputum (spit), tears, semen, mother's milk or vaginal secretions. Only blood, semen, mother's milk and vaginal secretions have been linked directly to the transmission of the HIV virus.

BONE MARROW: Soft tissue located in the cavities of the bones where blood cells such as erythrocytes, leukocytes and platelets are formed. See also Erythrocytes; Leukocytes; Platelets.

BOOSTER: A second or later dose of a vaccine given to increase the immune response to the original dose. See also Vaccine.