Lymphatic and Immune System Chapter 21 part 1

In 2008, over 15% of adults in certain African countries were infected with HIV, a statistic unchanged by 2012.
HIV causes Acquired Immune Deficiency Syndrome (AIDS), which critically undermines the immune system.
Initial case clusters in 1981 involved rare, severe infections in young adults, indicating a new, unknown disease linked to HIV.
Late 20th-century advancements in anti-HIV drugs transformed HIV from a fatal disease to a manageable chronic condition.

The lymphatic system facilitates the drainage of excess body fluids and filters pathogens from blood.
It consists of lymph fluid, vessels, ducts, and lymphatic organs, forming a drainage system integral to immune function.
Lymph fluid is interstitial fluid that has entered the lymphatic system and is returned to the bloodstream via ducts at the junction of the jugular and subclavian veins.
Humans have approximately 500-600 lymph nodes located throughout various regions of the body.

Lymphatic capillaries are open-ended vessels that collect interstitial fluid, functioning primarily in the soft tissues of the body.
Interstitial fluid enters lymphatic capillaries through overlapping endothelial cells that open when pressure increases.
In the small intestine, specialized lymphatic capillaries known as lacteals transport dietary lipids and lipid-soluble vitamins, forming a substance called chyle.

Lymphatic vessels resemble veins with a three-tunic structure and contain one-way valves to prevent backflow.
Superficial and deep lymphatics converge into larger lymphatic trunks that lead to the right lymphatic duct and thoracic duct.
The thoracic duct drains the majority of the body, while the right lymphatic duct serves a smaller area.

The immune system consists of barriers (skin), innate (rapid, nonspecific), and adaptive (slow, specific) immune responses.
Hematopoietic stem cells in the bone marrow differentiate into various immune cells, including phagocytic cells, lymphocytes, and granule-containing cells.

B lymphocytes produce antibodies that specifically bind to antigens from pathogens.
T lymphocytes (including cytotoxic T cells) do not produce antibodies; instead, they assist in coordinating immune responses and directly killing infected cells.
Plasma cells are activated B cells that secrete antibodies, while natural killer (NK) cells are part of the innate immune response, targeting virally infected or cancerous cells.

The bone marrow and thymus are primary organs where lymphocytes mature.
B cells mature in the bone marrow, whereas T cells migrate to the thymus for maturation.

The thymus, located between the sternum and heart, is crucial for T cell maturation, with a structure composed of lobules containing thymocytes and supportive cells.

By 2050, it's projected that 25% of the U.S. population will be over 60, with many experiencing immune system deficiencies linked to aging.
Immunosenescence describes the decline in immune function associated with aging, highlighting the need for research and understanding in aging-related therapies.### Age-Related Immune Deficiencies
Thymic involution is the gradual shrinkage of the thymus gland, starting at birth and progressing until around 35–45 years, resulting in about a one percent loss per year thereafter.
Total thymic tissue loss could hypothetically occur by the age of 120 years, suggesting a genetic limit to human lifespan.
All vertebrates with a thymus gland show signs of thymic involution, indicating a common biological process.
Studies demonstrate that thymic grafts follow the donor's age rather than the recipient's, suggesting genetic programming.
The FOXN1 gene's decreasing expression with age contributes to the loss of thymic epithelial cells vital for naïve T cell development.
Thymic involution is influenced by hormone levels; sex hormones can enhance its progression, with temporary involution observed during pregnancy.

Potential strategies to reverse or slow immunosenescence include thymic transplants from younger donors and gene therapies targeting gene expression.
Progress in immunosenescence research may lead to effective therapies, albeit likely taking decades to develop.

Lymphocytes mature in primary lymphoid organs but mount immune responses from secondary lymphoid organs like lymph nodes and spleen.
Naïve lymphocytes circulate through blood and lymph, with secondary organs serving as concentration sites.
Secondary lymphoid organs share features such as lymphoid follicles, reticular fibers, germinal centers, and specialized high endothelial venules.

Lymph nodes filter lymph, removing debris and pathogens, essential for adaptive immune responses through T cells and B cells.
These structures are encapsulated and compartmentalized by trabeculae and supported by reticular fibers.
Lymph enters via afferent lymphatic vessels and exits through efferent vessels, aiding immune surveillance.

The spleen, roughly 12 cm long, connects to the stomach and filters blood through extensive vascularization.
It hosts immune responses to blood-borne pathogens, with red pulp functioning as a filtration system and white pulp dedicated to T and B cell responses.
The spleen lacks a strong capsule and is structured into red pulp (containing red blood cells) and white pulp (lymphoid tissue).

Lymphoid nodules consist of dense lymphocyte clusters without a fibrous capsule, located in high-exposure areas like the respiratory and digestive tracts.
Tonsils, a type of lymphoid nodule, aid immunity against oral pathogens, with crypts enhancing pathogen entry for immune response.
Tonsil swelling indicates an active immune response; recurrent infections may lead to tonsil removal.

MALT comprises lymphoid follicles associated with mucous membranes in various organs, including the gastrointestinal tract and lungs.
Peyer’s patches in the small intestine have specialized M cells that transport antigens, facilitating adaptive immune responses.

The immune system comprises the innate response (rapid and non-specific) and the adaptive response (slow but highly specific).
Innate immunity directs and prepares the body for adaptive responses, contributing to pathogen destruction and influencing inflammatory processes.