Thymus

The thymus extends from below the thyroid in the neck into the thoracic cavity. As mentioned in chapter 11, this organ grows during childhood but gradually regresses after puberty. Lymphocytes from the fetal liver and spleen, and from the bone marrow postnatally, seed the thymus and become transformed into T cells. These lymphocytes, in turn, enter the blood and seed lymph nodes and other organs, where they divide to produce new T cells when stimulated by antigens.

Small T lymphocytes that have not yet been stimulated by antigens have very long life spans—months or perhaps years. Still, new T cells must be continuously produced to provide efficient cell-mediated immunity. This is particularly important following cancer chemotherapy and during HIV infection (in AIDS),

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The Immune System

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Table 15.4 Comparison of B and T Lymphocytes

Characteristic

B Lymphocytes

T Lymphocytes

Site where processed

Bone marrow

Thymus

Type of immunity

Humoral (secretes antibodies)

Cell-mediated

Subpopulations

Memory cells and plasma cells

Cytotoxic (killer) T cells, helper cells, suppressor cells

Presence of surface antibodies

Yes—IgM or IgD

Not detectable

Receptors for antigens

Present—are surface antibodies

Present—are related to immunoglobulins

Life span

Short

Long

Tissue distribution

High in spleen, low in blood

High in blood and lymph

Percentage of blood lymphocytes

10%—15%

75%—80%

Transformed by antigens to

Plasma cells

Activated lymphocytes

Secretory product

Antibodies

Lymphokines

Immunity to viral infections

Enteroviruses, poliomyelitis

Most others

Immunity to bacterial infections

Streptococcus, Staphylococcus, many others

Tuberculosis, leprosy

Immunity to fungal infections

None known

Many

Immunity to parasitic infections

Trypanosomiasis, maybe to malaria

Most others

Table 15.5 Summary of Events in a Local Inflammation

Category

Events

Innate (Nonspecific) Immunity

Bacteria enter a break in the skin.

Resident phagocytic cells—neutrophils and macrophages—engulf the bacteria. Nonspecific activation of complement proteins occurs.

Adaptive (Specific) Immunity

B cells are stimulated to produce specific antibodies.

Phagocytosis is enhanced by antibodies attached to bacterial surface antigens (opsonization).

Specific activation of complement proteins occurs, which stimulates phagocytosis, chemotaxis of new phagocytes to the infected area, and secretion of histamine from tissue mast cells. Extravasation (diapedesis) allows new phagocytic leukocytes (neutrophils and monocytes) to invade the infected area. Vasodilation and increased capillary permeability (as a result of histamine secretion) produce redness and edema.

when the population of T lymphocytes has been depleted. Under these conditions, the thymus can replenish the T lymphocyte population through late childhood. Repopulation of T lymphocytes occurs more slowly in adulthood, and appears to be accomplished mostly by production of T lymphocytes in the secondary lym-phoid organs rather than in the thymus. This is because the thymus of adults becomes more of a fatty organ, although production of lymphocytes in the thymus has been demonstrated to occur to some extent even in people over the age of 70.

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