T-helper cell

T helper cells (also known as effector T cells or Th cells) are a sub-group of lymphocytes (a type of white blood cell or leukocyte) that plays an important role in establishing and maximizing the capabilities of the immune system. These cells are unusual in that they have no cytotoxic or phagocytic activity; they cannot kill infected host (also known as somatic) cells or pathogens, and without other immune cells they would usually be considered useless against an infection. Th cells are involved in activating and directing other immune cells, and are particularly important in the immune system. They are essential in determining B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages. It is this diversity in function and their role in influencing other cells that gives T helper cells their name.
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Immune cells that are triggered by antibodies to seek and attack invading organisms. Cells called macrophanges summon T-helper cells to the site of the infection and present a protruding antigen onto which the T-helper cell locks, thus "recognizing" the invading substance. The T4-helper cell then reproduces and secretes its potent lymphokine hormones that stimulate B-cell production of antibodies; signal "natural killer" or cytotoxic (cell-killing) T-cells; and summon more macrophanges to the site of the infection. T-helper cells are also called T4 cells and are normally twice as common as T8 cells. If a person has AIDS, the proportion of T4 to T8 cells is often reversed.