The immune system consists of lymphatic organs, antibodies, and regulatory agents that work together to defend the body against diseases. The primary lymphatic organs are the red bone marrow and thymus gland, where lymphocytes originate and mature. Lymphocytes then congregate in secondary lymphatic organs like the spleen, lymph nodes, and tonsils. When tissue rejection occurs, the immune system recognizes and destroys cells bearing foreign MHC antigens. Diseases of the immune system include autoimmune disorders, where the immune system mistakenly attacks the body's own cells, and immune deficiencies, where the immune system is unable to adequately protect against disease.

1. F. Javier Vallés F.
Human Biology 12
Mrs. Ramirez
Defenses Against Diseases
1
The immune system, which plays an important role in keeping us healthy, consists of a
network of lymphatic organs, antibodies and regulatory agents. Antibodies are proteins
produced by the body in response to the presence of a formolecule that can be
recognized by lymphocytes because it binds to an antigen receptor. Immunity is the
ability to react to antigens so that the body remains free of diseases. Disease, a state of
homeostatic imbalance, can be due to infection by foreign agents or to failure of
immune system to function properly.
Lymphatic organs contain large numbers of lymphocytes, the type of white blood cells
that plays a pivotal role in immunity. The primary lymphatic organs are the red bone
marrow and the thymus gland. Lymphocytes originate and/or mature in these organs.
The red bone marrow is the site of stem cells that are ever capable of diving and
producing blood cells. Some of these cells become the various types of white blood
cells: neutrophils, eosinophils, basophils, monocytes, and the vertebral column.
The red bone marrow consists of a network of connective tissue fibers, which
support the stem cells and their progeny. Lymphocytes differentiate into the B
lymphocytes and the T lymphocytes. Bone marrow is no only the source of B
lymphocytes, but also the place where B lymphocytes mature.
The soft, thymus gland is located anterior to the ascending aorta and posterior to the
sternum in the upper thoracic cavity. Connective tissue divides the thymus into lobules,
which are filled with lymphocytes. The thymus gland produces thymic hormones, such
as thymosin, that are thought to aid in maturation of T lymphocytes. Thymosin may also
have other functions in immunity.
Immature T lymphocytes migrate from the bone marrow through the bloodstream to
the thymus, where they mature. The thymus is absolutely critical to immunity; without a
thymus, a person does not reject foreign tissues, blood lymphocyte level are drastically
reduced, and the body’s response to most antigens is poor or absent.

2. F. Javier Vallés F.
Human Biology 12
Mrs. Ramirez
Defenses Against Diseases
2
The spleen is located in the upper left region of the abdominal cavity posterior to the
stomach. Connective tissue divides the spleen into partial compartments, each of which
contains tissue known as white pulp and red pulp. The white pulp contains a connection
of lymphocytes; the red pulp, which surrounds venous sinuses, is involved in filtering the
blood. Blood entering the spleen must pass through the sinuses before exiting.
Lymphocytes and microphages react to pathogens, and macrophages engulf debris and
remove any worn out red blood cells.
Lymph nodes, which are small structures, occur along lymphatic vessels. Connective
tissue forms the capsule of a lymph node and also divides the organ into compartments.
Each compartment contains a nodule packed with B lymphocytes and a sinus that
increases in size toward the center of the node. As lymph courses through the sinuses,
it is filtered by macrophages, which engulf pathogens and debris. T lymphocytes, also
present in sinuses, fight infections and attack cancer cells.
The tonsils are patches of lymphatic tissue locatedin a ring about the pharynx. The
tonsils perform the same function as lymph nodes, but because of their location, they
are the first to encounter pathogens and antigens that enter the body by way of the
nose and mouth.
Peyer’s patches located in the intestinal wall and the vermiform appendix attached to
the cecum, a blind pouch of the large intestine, encounter pathogens that enter the body
by way f intestinal tract.
Allergies are hypersensitive to substances. The response of these antigens
calledallergens, usually includes some degree of tissue damage. An immediate allergic
response can occur within seconds of contact with the antigen. The response is caused
by antibodies known as IgE, which are attached to the plasma membrane of mast cells
in the tissues and also to basophils in the blood. When an allergen attaches to the IgE
antibodies on these cells, mast cells release histamine and other substances that bring
the allergic symptoms.

3. F. Javier Vallés F.
Human Biology 12
Mrs. Ramirez
Defenses Against Diseases
3
If a person has asthma, the airways leading to the lungs constrict, resulting in difficult
breathing accompanied by wheezing. Anaphylactic shock is an immediate allergic
response that occurs because the allergen has entered the bloodstream. Anaphylactic
shock is characterized by a sudden and lifethreatening drop in blood pressure due to
increased permeability to the capillaries by histamine. Delayed allergic response is
initiated by memory. T cells at the site of allergen contact in the body. The allergic
response is regulated by the cytokines secreted by both T cells and macrophages.
Rejection of transplanted tissue results because the recipient’s immune system
recognizes that transplanted tissue is not “self”. Cytotoxic T cells respond by causing
disintegration of the transplanted tissue.
Organ rejection can be controlled by carefully selecting the organ to be transplanted
and administering immunosuppressive drugs. It is best if the transplanted organ has the
same type of MHC antigens as those of the recipient, because cytotoxic T cells
recognize foreign MHC antigens. Two well-known immunosuppressive drugs,
cyclosporine and tacrolimus, both act by inhibiting the response of T cells to cytokines.
When a person has an autoimmune disease, cytotoxic T cells or antibodies
mistakenly attack the body’s own cells as if they bear foreign antigens. When a person
has an immune deficiency, the immune system is unable to protect the body against
disease. AIDS is an example of an acquired immune deficiency.
In the autoimmune disease myasthenia gravis, neuromuscular junctions do not work
properly, and muscular weakness results. “Myasthenia gravis is an autoimmune
disorder that affects the neuromuscular junction at the postsynaptic level. Although the
cause of the disorder is unknown, the role of immune responses in its pathogenesis is
well established. The disorder is characterized by fluctuating, fatigable weakness of
muscles under voluntary control”. (Milind J. Kothari, DO)
In multiple sclerosis, the myelin sheath of nerve fiber breaks down, causing various
neuromuscular symptoms.“Multiple sclerosisis a disease of the central nervous system,

4. F. Javier Vallés F.
Human Biology 12
Mrs. Ramirez
Defenses Against Diseases
4
the main network of nerves that carry electrical impulses throughout the body. The
central nervous system is made up of the brain and spinal cord. Both have nerve fibers
that transport electrical messages from the brain to the rest of the body. The nerve
fibers are wrapped in a fatty tissue called myelin, which helps transmit the messages”.
(Ed Zimney, MD)
A person with systemic lupusevrythematosus has varioussymptoms prior to death
due to kidney damage. Systemic lupus erythematosus is a chronic autoimmune disease
that can affect almost any organ system; thus, its presentation and course are highly
variable, ranging from indolent to fulminant. (Christie M Bartels, MD)
In rheumatoid arthritis, the joints are affected. Rheumatoid arthritis is a chronic
systemic inflammatory disease of unknown cause. An external trigger that triggers an
autoimmune reaction, leading to synovial hypertrophy and chronic joint inflammation
along with the potential for extra-articular manifestations, is theorized to occur in
genetically susceptible individuals.(Katherine K Temprano, MD)
In conclusion, lymphocytes and other with blood cells are produced by the red bone
marrow, which is a primary lymphatic organ along with the thymus, where T
lymphocytes mature. Lymphocytes congregate in the secondary lymphocytic organs
such as the spleen, the lymph nodes and the tonsils. When tissue rejection occurs, the
immune system has recognized and destroyed cells that bear MHC antigens. Finally,
Diseases of the immune system are autoimmune or immune deficiency disorders.

5. F. Javier Vallés F.
Human Biology 12
Mrs. Ramirez
Defenses Against Diseases
5
Appendix

6. F. Javier Vallés F.
Human Biology 12
Mrs. Ramirez
Defenses Against Diseases
6
References
Medscape Reference- Drugs, Diseases and Procedures, Rheumatoid Arthritis
(Katherine K Temprano, MD)
The Journal of the American Osteopathic Association, Myasthenia Gravis (Milind J.
Kothari, DO)
What Is Multiple Sclerosis?Understanding how different types of multiple sclerosis affect
the body (Ed Zimney, MD)
Human Biology-Defenses Against Diseases-Organs, Tissues and Cells of the Immune
System-Immunity Side Effects (Sylvia S. Mader)
Medscape Reference- Drugs, Diseases and Procedures-Systemic Lupus
Erythematosus (SLE) (Christie M Bartels, MD)