Summary Principles of Immunology INTRODUCTION

Principles of Immunology INTRODUCTION • The immune system serves as a means to resist infection. • It is comprised of two major sub-divisions; The innate and adaptive/acquired immune systems. • Its main function is to discriminate between self and nonself, thus protecting against invading organisms and to eliminate modified or altered cells. • There are both cellular and Humoral aspects to both adaptive/acquired/specific and innate immune responses. • All the cells involved in the immune system are derived from bone marrow. • There are both myeloid cells i.e. neutrophils, basophils, eosinophils, macrophages and dendritic cells and lymphoid cells i.e. B and T lymphocytes and Natural Killer cells. • These cell types differentiate along distinct pathways from both myeloid and lymphoid progenitor (stem) cells. • Organisms such as bacteria, viruses and parasites are collectively referred to as pathogens. • Antigens are substances that elicit a specific immune response. • Most are large polysaccharides or proteins from a foreign organism. • They include pollen and foreign tissue as well as viral proteins and bacterial flagella, cell walls, capsids etc. • Haptens are small molecules that are not antigenic unless coupled with a carrier molecule. ORGANS OF THE IMMUNE SYSTEM ORGANS OF THE IMMUNE SYSTEM • Referred to as lymphoid organs, there are both primary (bone marrow and thymus) and secondary lymphoid organs (Lymph nodes and the spleen) • 1° lymphoid organs are the sites of development and maturation of lymphocytes (T and B cells, dendritic cells). • 2° lymphoid organs serve as a residence for a variety of lymphoid cells, traps for antigens and are the anatomical sites where immune responses are initiated. • The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood. • The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems. • The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Immune system organs: Lymphocytes develop in primary lymphoid tissue (bone marrow [BM], thymus) and circulate towards secondary lymphoid tissue (lymph nodes [LN], spleen, MALT). • Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. • Humans have about 500–600 lymph nodes throughout the body. • A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. • Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space, i.e., spaces between individual cells in the tissues. • In humans, 20 litres of plasma is released into the interstitial space of the tissues each day • due to capillary filtration, and once out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. • Of this, 17 litres is reabsorbed directly by the blood vessels. • The lymphatic system drains the remaining/excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. • Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. Lymphatic vessels in the arms and legs convey lymph to the larger lymphatic vessels in the torso. Lymphatic Capillaries: Lymphatic capillaries are interlaced with the arterioles and venules of the cardiovascular system. Collagen fibers anchor a lymphatic capillary in the tissue (inset). Interstitial fluid slips through spaces between the overlapping endothelial cells that compose the lymphatic capillary. • A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart. • It is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. • One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. • Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck. • Lymphatic capillaries (a.k.a. terminal lymphatics), are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. • Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body. • Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels. • The primary lymphoid organs are the bone marrow and thymus gland. • The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body. • The red bone marrow is a loose collection of cells where haematopoiesis occurs, and the yellow bone marrow is a site of energy storage, which consists largely of fat cells.