Essentials of Anatomy and Physiology

phagocytize pathogens and old RBCs; bilirubin

is formed and sent to the liver for excretion in

bile

• stores platelets and destroys damaged platelets

Thymus—inferior to the thyroid gland; in
the fetus and infant the thymus is large (see
Fig. 14–5); with age the thymus shrinks

1. Produces T lymphocytes (T cells).


2. Produces thymic hormones that make T cells
   immunologically competent, that is, able to recog-
   nize foreign antigens and provide immunity.

Immunity—the ability to destroy foreign
antigens and prevent future cases of certain
infectious diseases

1. Antigens are chemical markers that identify cells.
   Human cells have “self ” antigens—the HLA types.


2. Foreign antigens stimulate antibody production
   or other immune responses, and include bacteria,
   viruses, fungi, protozoa, and malignant cells.

Innate Immunity (see Fig. 14–6)

1. Is nonspecific, responses are always the same, does
   not create memory, and does not become more
   efficient. Consists of barriers, defensive cells, and
   chemical defenses.


2. Barriers
   
   
   • Unbroken stratum corneum and sebum; living
     epidermal cells secrete defensins
   
   
   • Subcutaneous tissue with WBCs
   
   
   • Mucous membranes and areolar CT with WBCs;
     upper respiratory epithelium is ciliated
   
   
   • HCl in gastric juice
   
   
   • Lysozyme in saliva and tears
   
   
   
   


3. Defensive cells
   
   
   • Phagocytes—macrophages, neutrophils, eosino-
     phils; macrophages also activate the lymphocytes
     of adaptive immunity
   
   
   • Langerhans cells and other dendritic cells—acti-
     vate lymphocytes
   
   
   • Natural killer cells—destroy foreign cells by rup-
     turing their cell membranes
   
   
   • Basophils and mast cells—produce histamine and
     leukotrienes (inflammation)
   
   
   
   


4. Chemical defenses
   
   
   • Interferon blocks viral reproduction
     
     
     • Complement proteins lyse foreign cells, attract
       WBCs, and contribute to inflammation
     
     
     • Inflammation—the response to any kind of dam-
       age; vasodilation and increased capillary perme-
       ability bring tissue fluid and WBCs to the area.
       Purpose: to contain the damage, eliminate the
       cause, and make tissue repair possible.
       Signs: redness, heat, swelling, and pain
     
     
     
     
   
   
   
   

Adaptive Immunity (see Fig. 14–7)

1. Is very specific, may involve antibodies, does create
   memory, and responses become more efficient.
   Consists of cell-mediated and antibody-mediated
   immunity; is carried out by T cells, B cells, and
   macrophages.


2. T lymphocytes (T cells)—in the embryo are pro-
   duced in the thymus and RBM; they require the
   hormones of the thymus for maturation; migrate to
   the spleen, lymph nodes, and nodules.


3. B lymphocytes (B cells)—in the embryo are pro-
   duced in the RBM; migrate to the spleen, lymph
   nodes, and nodules.


4. The antigen must first be recognized as foreign;
   this is accomplished by B cells or by helper T cells
   that compare the foreign antigen to “self ” antigens
   present on macrophages.


5. Helper T cells strongly initiate one or both of the
   immune mechanisms: cell-mediated immunity and
   antibody-mediated immunity.

Cell-Mediated (cellular) Immunity (see Fig.

14–7)

1. Does not involve antibodies; is effective against
   intracellular pathogens, malignant cells, and grafts
   of foreign tissue.


2. Helper T cells recognize the foreign antigen, are
   antigen specific, and begin to divide to form differ-
   ent groups of T cells.


3. Memory T cells will remember the specific foreign
   antigen.


4. Cytotoxic (killer) T cells chemically destroy for-
   eign cells and produce cytokines to attract macro-
   phages.

Antibody-Mediated (Humoral) Immunity

(see Fig. 14–7)

1. Does involve antibody production; is effective
   against pathogens and foreign cells.

The Lymphatic System and Immunity 339