The Lymphoid System and Immunity

These flashcards cover the anatomical structures, physiological functions, and immune defense mechanisms of the human lymphoid system as described in Chapter 22.

Immunity

Resistance to damage or disease caused by pathogens such as bacteria, viruses, fungi, and parasites.

Lymphoid System

The main organ system defending against disease, consisting of lymph plasma, lymphatic vessels, lymphoid tissues, and red bone marrow.

Lymphatic Capillaries

Vessels closed at one end with overlapping endothelial cells that permit interstitial fluid entry but not exit, ranging from $10 - 60\,\mu\text{m}$ in diameter.

Lacteals

Specialized lymphatic capillaries in the small intestines that carry absorbed dietary lipids.

Chyle

Creamy white, lipid-rich lymph collected from the digestive tract.

Thoracic Duct

The largest lymph vessel in the body; it begins at the cisterna chyli and drains lymph from most of the body into the left subclavian and internal jugular veins.

Primary Lymphoid Organs

Sites where stem cells divide and develop immunocompetence, specifically the red bone marrow and the thymus.

Secondary Lymphoid Organs

Sites where immunocompetent cells perform defensive functions, including lymph nodes and the spleen.

Thymus

A bilobed organ in the mediastinum where T lymphocytes become immunocompetent; it consists of an outer cortex with immature T cells and an inner medulla with mature T cells.

Spleen

The largest mass of lymphoid tissue; it contains white pulp for blood screening and red pulp for removing damaged blood cells and storing platelets.

Mucosa-associated lymphoid tissue (MALT)

Clusters of immune cells called lymphoid nodules located in the lamina propria of the GI, urinary, respiratory, and reproductive tracts.

Innate Immunity

Non-specific defenses that provide immediate, general protection against a wide range of pathogens with the same efficacy every time.

Adaptive Immunity

Specific defenses characterized by specificity and memory that must be activated by exposure to particular pathogens.

Lysozyme

An enzyme found in saliva, sweat, and tears that disrupts bacterial cell walls.

Interferons

Proteins released by virally-infected cells that induce antiviral responses in neighboring cells.

Complement Proteins

Proteins that assemble to form a membrane attack complex for cytolysis, promote phagocytosis, and contribute to inflammation.

Natural Killer (NK) Cells

Lymphocytes that release perforins (creating pores) and granzymes (inducing apoptosis) to kill infected or tumor cells.

Inflammation

A nonspecific response to tissue damage characterized by five signs: pain, redness, immobility, swelling, and heat.

Antigen

An "antibody generating" molecule recognized as foreign that evokes an immune response.

Epitope

The specific part of an antigen that is recognized by and binds to B or T cell receptors.

Positive Selection

A process in the thymus where only T cells capable of reacting survive.

Clonal Selection

The process where an activated lymphocyte proliferates and differentiates into effector cells and memory cells.

Cell-mediated Immunity

A branch of adaptive immunity involving T cells that is effective against intracellular pathogens, cancer cells, and foreign tissue transplants.

Antibody-mediated (Humoral) Immunity

A branch of adaptive immunity involving B cells and antibodies that is effective against extracellular pathogens in body fluids.

MHC-I Proteins

Plasma membrane proteins expressed by all nucleated cells that present endogenous antigens from within the cell.

MHC-II Proteins

Proteins expressed only by antigen-presenting cells (APCs) that present exogenous antigens processed from outside the cell.

Plasma Cells

Effector B cells that secrete antibodies into the blood and lymph plasma.

IgG

The most abundant antibody in blood and lymph; it is the only class that can cross the placenta from mother to fetus.

IgE

Immunoglobulins involved in allergic reactions and protection against parasitic worms; found on basophils and mast cells.

Active Immunity

Long-term immunity where the body's own system produces antibodies and memory cells in response to an antigen.

Passive Immunity

Temporary immunity lasting weeks to months, acquired by receiving pre-made antibodies without forming memory cells.

Autoimmune Disorder

A homeostatic imbalance where the immune system attacks the body's own tissues, such as in rheumatoid arthritis or type 1 diabetes mellitus.