Lymphatic Organs Primary lymph organs are the bone marrow and thymus. – Sites where stem cells divide and become immunocompetent (capable of mounting an immune response) Secondary lymphatic organs are sites where most immune responses occur, including the spleen and lymph nodes, and other lymphoid tissues such as the tonsils.

Lymphatic Organs Thymus The outer cortex is composed of a large number of immature T cells which migrate from their birth- place in red bone marrow. They proliferate and begin to mature with the help of Dendritic cells (derived from monocytes) and specialized epithelial cells (help educate T cells through positive selection) – only about 25% survive. The inner medulla is composed of more mature T cells.

Lymphatic Organs The thymus slightly protrudes from the mediastinum into the lower neck. It is a palpable 70g in infants, atrophies by puberty, and is scarcely distinguishable from surrounding fatty tissue by old age.

Lymphatic Organs There are about 600 lymph nodes scattered along lymphatic vessels (in superficial and deep groups) that serve as filters to trap and destroy foreign objects in lymph fluid. Important group of regional lymph nodes include: – Submandibular – Cervical – Axillary Mediastinal Inguinal

Lymph fluid enters the node through afferent vessels and is directed towards the central medullary sinuses. Efferent vessels convey lymph, antibodies and activated T cells out of the node at an indentation called the hilum. Lymphatic Organs

The spleen is the body’s largest mass of lymphatic tissue. The parenchyma of the organ consists of: White pulp - lymphatic tissue where lymphocytes and macrophages carry out immune function Red pulp – blood-filled venous sinuses where platelets are stored and old red cells are destroyed Lymphatic Organs

The Immune Response Our immune response includes innate and adaptive responses:

Innate Immunity The innate immune response is present at birth. It is non-specific and non-adaptive. It includes our first line of external, physical, and chemical barriers provided by the skin and mucous membranes.

Innate Immunity Our nonspecific innate immune response also includes various internal defenses such as antimicrobial substances, natural killer cells, phagocytes, inflammation, and fever.

Innate Immunity Internal defenses: – Phagocytes Wandering and fixed macrophages – Natural killer (NK) cells – Endogenous antimicrobials – Complement system – Iron-binding proteins – Interferon

Innate Immunity Phagocytosis is a non-specific process wherein neutrophils and macrophages (from monocytes) migrate to an infected area. There are 5 steps: Chemotaxis Adherence Ingestion Digestion Killing

Innate Immunity