A monocyte that has left the circulation and settled and matured in a tissue. Because of their placement in the lymphoid tissues, macrophages serve as the major scavenger of the blood, clearing it of abnormal or old cells and cellular debris as well as pathogenic organisms.

A group of genes that code for cell-surface histocompatibility antigens. These antigens are the primary reason why organ and tissue transplants from incompatible donors fail.

See Surface marker.

A large tissue cell that does not circulate in the blood. They are also important in producing the signs and symptoms of hypersensitivity reaction, such as those of an insect sting, and certain forms of asthma.

A product in the male organism of a gene from the X chromosome.

A process where two successive cells divide and produce cells, eggs, or sperm that contain half the number of chromosomes in the somatic cells. During fertilization, the nuclei of the sperm and ovum fuse and produce a zygote with the full chromosome complements.

A cell that produces the dark pigment melanin; responsible for the pigmentation of skin and hair.

The ability of antigen-specific T or B cells to"recall" prior exposure to an antigen and respond quickly without the need to be activated again by CD4 helper T cells.

A subset of antigen-specific T or B cells that "recall" prior exposure to an antigen and respond quickly without the need to be activated again by CD4 helper T cells.

Cells from the immature embryonic connective tissue. A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce cartilage.

The middle layer of the embryonic disk, which consists of a group of cells derived from the inner cell mass of the blastocyst. This middle germ layer is known as gastrulation and is the precursor to bone, muscle, and connective tissue.

A stage of mitosis where chromosomes are firmly attached to the mitotic spindle at its equator but have not yet segregated toward opposite poles.

An elongated, hollow tubular structure present in the cell. Microtubules help certain cells maintain their rigidity, convert chemical energy into work, and provide a means of transportation of substances in different directions within a cell.

From a single cell.

An exceptionally pure and specific antibody derived from hybridoma cells. Because each of the clones is derived from a single B cell, all of the antibody molecules it makes are identical.