Cells of the Immune System

The immune defense mechanisms of the body involve the actions ofwhiteblood cells, or leukocytes.Leukocytes include neutrophils,eosinophils, basophils, and monocytes, all of which are phagocytic and are involved in the second line of defense, as well as two types of lymphocytes (T cells and B cells), which are not phagocytic but are critical to the specific immune response .T cells direct the cell-mediated response,B cells the humoral response.
After their origin in the bone marrow, T cells migrate to the thymus ,hence the designation“T”, a gland just above the heart.There they develop the ability to identify microorganisms and viruses by the antigens exposed on their surfaces. Tens of millions of different T cells are made, each specializing in
the recognition of one particular antigen. 

No invader can escape being recognized by at leasta few T cells. There are four principal kinds ofT cells: inducer T cells oversee the developmentof T cells in the thymus; helper T cells (often symbolized TH) initiate the immune response;cytotoxic (“cell-poisoning”) T cells (often symbolized TC) lyse cells that have been infected by viruses; and suppressor T cells terminate the immune response.

B cells do not travel to the thymus; they complete their maturation in the
bone marrow. B cells are so named because they were originally characterized in a region of chickens called the bursa.From the bone marrow, B cells are released to circulate in the blood and lymph. Individual B cells, like T cells, are
specialized to recognize particular foreign antigens.
When a B cell encounters the antigen to which it is targeted, it begins to divide rapidly, and its progeny differentiate into plasma cells and memory cells. Each plasma cell is a miniature factory producing antibodies that stick like flags to that antigen wherever it occurs in the body, marking any cell bearing the antigen for destruction. The immunity that Pasteur observed resulted from such antibodies and from the continued presence of the B cells that produced them.

Helper T cell:Commander of the immune response; detects infection and sounds the alarm, initiating both T cell and B cell responses

 

Inducer T cell:Not involved in the immediate response to infection; mediates the maturation of other T cells in the thymus

Cytotoxic T cell:Detects and kills infected body cells; recruited by helper T cells

Suppressor T cell:Dampens the activity of T and B cells, scaling back the defense after the infection has been checked

B cell;Precursor of plasma cell; specialized to recognize specific foreign antigens

 

Plasma cell:Biochemical factory devoted to the production of antibodies directed against specific foreign antigens

Mast cell:Initiator of the inflammatory response, which aids the arrival of leukocytes at a site of infection; secretes histamine and is important in allergic responses

Monocyte:Precursor of macrophage

Macrophage:The body’s first cellular line of defense; also serves as antigen-presenting cell to B and T cells and engulfs antibodycovered cells.

Natural killer cell:Recognizes and kills infected body cells; natural killer (NK) cell detects and kills cells infected by a broad range of invaders; killer (K) cell attacks only antibody-coated cells