The Endocrine System

Overview of Hormones and Their Functions

The endocrine system regulates physiological processes through the secretion of hormones, which are chemical messengers released by glands and transported via the bloodstream to target organs. Hormones control growth, metabolism, reproduction, and homeostasis.

• Hormones: Chemical messengers produced by endocrine glands.

• Target Organs: Specific tissues or organs affected by hormones.

• Examples: Insulin (pancreas), Thyroid hormone (thyroid gland), Testosterone (testis).

Major Hormones, Sources, and Actions

The following table summarizes key hormones, their sources, target organs, and primary actions.

Additional info: Hormones often act via feedback mechanisms to maintain homeostasis. For example, insulin and glucagon regulate blood glucose levels.

The Lymphatic System and Lymphoid Organs

Structure and Function

The lymphatic system returns fluids leaked from blood vessels and provides the structural basis for the immune system. It consists of lymphatic vessels, lymph (fluid), and lymph nodes.

• Lymphatic Vessels: One-way drainage system, ensuring lymph flows toward the heart.

• Lymph: Fluid collected from tissues, cleansed in lymph nodes.

• Lymphoid Organs: Spleen, thymus, tonsils, lymph nodes, and other tissues house immune cells.

Lymphatic Capillaries and Vessels

• Lymphatic Capillaries: Blind-ended, highly permeable vessels that absorb large molecules (proteins, pathogens).

• Lacteals: Specialized capillaries in the intestine that absorb fats.

• Collecting Vessels: Merge into lymphatic trunks and ducts.

• Lymphatic Ducts: Right lymphatic duct (drains right upper body), thoracic duct (drains rest of body).

Lymphoid Cells and Tissues

• Lymphocytes: T cells (manage immune response, attack infected cells) and B cells (produce antibodies).

• Macrophages: Phagocytize foreign substances, activate T cells.

• Dendritic Cells: Capture antigens, deliver to lymph nodes.

• Reticular Cells: Produce stroma, a supportive network for immune cells.

Main Functions: Provide proliferation sites for lymphocytes and surveillance points for immune cells.

Types of Lymphoid Tissue

• Diffuse Lymphoid Tissue: Loose arrangement, found in most organs.

• Lymphoid Follicles (Nodules): Spherical bodies with germinal centers of B cells; found in nodes, Peyer's patches, appendix.

Primary and Secondary Lymphoid Organs

• Primary: Red bone marrow (B cell maturation), thymus (T cell maturation).

• Secondary: Lymph nodes, spleen, MALT, diffuse tissues (sites of antigen encounter and activation).

Lymph Nodes

• Functions: Cleanse lymph (macrophages remove debris), immune activation (lymphocytes mount response).

• Locations: Deep clusters, superficial regions (inguinal, axillary, cervical).

Spleen

• Largest lymphoid organ: Located in left abdominal cavity.

• Functions: Lymphocyte proliferation, immune surveillance, cleanses blood, stores platelets and monocytes.

• White pulp: Immune function (lymphocytes).

• Red pulp: Destruction of old blood cells and pathogens (macrophages).

Mucosa-Associated Lymphoid Tissue (MALT)

• Locations: Respiratory, genitourinary, digestive tracts.

• Functions: Protects against pathogens entering mucosal surfaces.

• Examples: Tonsils, Peyer's patches, appendix.

Thymus

• Site of T cell maturation: Most active in childhood, atrophies with age.

• Structure: Lobules with cortex (dividing lymphocytes) and medulla (regulatory T cells).

• Unique Features: No follicles (lacks B cells), blood-thymus barrier prevents premature activation.

The Immune System

Overview and Organization

The immune system provides resistance to disease through two main systems: innate (nonspecific) and adaptive (specific) defenses. These systems are intertwined and work together to protect the body from pathogens.

• Innate Defense: First and second lines of defense; rapid, nonspecific.

• Adaptive Defense: Third line of defense; specific, slower, has memory.

Innate (Nonspecific) Defenses

• First Line: Skin and mucous membranes (physical and chemical barriers).

• Second Line: Phagocytes, natural killer (NK) cells, inflammation, antimicrobial proteins, fever.

Key Points:

• Physical Barriers: Keratin, mucosae, acid mantle, enzymes, mucin, defensins.

• Phagocytes: Neutrophils (abundant, die fighting), macrophages (robust, develop from monocytes).

• Phagocytosis Steps: Adherence, opsonization, engulfment, digestion, exocytosis.

• NK Cells: Kill virus-infected and cancer cells by inducing apoptosis.

• Inflammation: Redness, heat, swelling, pain; prevents spread, disposes debris, alerts adaptive system, sets stage for repair.

• Antimicrobial Proteins: Interferons (warn neighboring cells, activate NK cells), complement proteins (destroy bacteria, enhance inflammation).

• Fever: Systemic response; increases metabolic rate, sequesters iron/zinc.

Adaptive (Specific) Defenses

• Characteristics: Specificity, systemic action, memory.

• Branches: Humoral (antibody-mediated, B cells), Cellular (cell-mediated, T cells).

Humoral Immunity: Antibodies bind to extracellular targets, inactivate and mark for destruction.

Cellular Immunity: T cells act directly (kill infected cells) or indirectly (release chemicals, activate other cells).

Antigens and Immune Response

• Antigens: Substances that provoke immune response; can be complete (immunogenicity, reactivity) or incomplete (haptens).

• Antigenic Determinants: Specific regions recognized by antibodies or lymphocyte receptors.

• Self-Antigens: MHC proteins; not antigenic to self, but may be to others.

Lymphocyte Development and Activation

• Origin: Red bone marrow.

• Maturation: B cells mature in bone marrow, T cells in thymus; immunocompetence and self-tolerance are established.

• Seeding: Naive cells colonize secondary lymphoid organs.

• Activation: Encounter with antigen triggers clonal selection, proliferation, differentiation into effector and memory cells.

• Antigen-Presenting Cells (APCs): Dendritic cells (most effective), macrophages, B cells (present antigens to helper T cells).

Additional info: The diversity of antigen receptors is generated by gene rearrangement, allowing recognition of a vast array of pathogens.