Blood Physiology: Immunity and Inflammation

Blood Physiology: Immunity, Infection, and Inflammation

• Immunity is the body's ability to resist or eliminate potentially harmful foreign materials or abnormal cells. These materials include viruses, toxins, carcinogens, fungi, pollution, bacteria, germs, and parasites.

• Immunity involves various activities, including defense against invading pathogens (like viruses and bacteria), removal of worn-out cells and tissue debris, identifying and destroying abnormal or mutated cells, and rejection of foreign cells (like in organ transplants). Inappropriate responses like allergies and autoimmune diseases are also part of the immune system.

• The immune system is a complex, functional system, not an organ system. Its components include the skin (a physical barrier), lining of mucus membranes, secretions (like tears and mucus – antimicrobial), blood cells and vasculature (WBCs), bone marrow, the liver (complement proteins), resident immune cells in most tissues, the lymphatic system, and lymphoid organs.

The Lymphatic System

• The lymphatic system is part of the circulatory system and crucial for immunity. It's a network of lymphatic vessels, lymphoid organs, lymphoid tissues, and circulating lymph.
• Lymphatic vessels carry clear fluid (lymph) directionally towards the heart.
• Unlike the cardiovascular system, the lymphatic system isn't a closed system.

Lymphoid Organs

• Primary lymphoid organs generate immature lymphocytes from progenitor cells and include the thymus and bone marrow.
• Secondary lymphoid organs (like lymph nodes and the spleen) maintain mature lymphocytes and initiate adaptive immune responses. These organs are crucial for the production and maturation of some types of lymphocytes. Several lymph nodes, such as cervical, axillary, thoracic, etc. are part of the lymphatic system for bodily functions.

Relationship Between Cardiovascular and Lymphatic Systems

• The heart pumps blood, roughly 5 liters per minute, with 7200 liters passing per day.
• About 20 liters of plasma from blood pass into the interstitial space daily through capillary filtration.
• Roughly 17 liters of filtered plasma are reabsorbed into the blood vessels; the remaining 3 liters stay in interstitial fluid.
• The lymphatic system is a major drain of excess interstitial fluid that does not directly re-enter the blood vessels. This excess fluid is then filtered and drained via the lymphatic system.

Cellular Elements of Blood

• Blood consists of plasma (55%), red blood cells (4%), white blood cells and platelets (<1%).
• Key cell types include red blood cells, white blood cells, and platelets.

Phagocytosis

• Phagocytosis is a cellular process.

Immune Effector Cells: Granulocytes and Agranulocytes

• Granulocytes (polymorphonuclear leukocytes) include neutrophils, eosinophils, and basophils. These cells have cytoplasmic granules containing substances involved in inflammatory and allergic reactions.
• Neutrophils are the most abundant white blood cells, with a lifespan of 6 hours and are crucial for inflammatory responses, particularly infections. They are phagocytes.
• Eosinophils are involved with parasitic worms and allergic reactions; they are a less abundant type of white blood cell.
• Basophils release histamine and heparin and are crucial in allergic reactions.
• Agranulocytes include lymphocytes and monocytes.
• Monocytes (macrophages) are involved with tissue-based immunity and releasing toxins including reactive oxygen species. They are part of the initial innate immune response.
• Lymphocytes are key for adaptive immune responses, including B cells producing antibodies, and T cells directly destroying virus-invaded cells and cancer cells.

Examples of Cytokines

• Interferons (IFNs) are secreted proteins that interfere with viral replication.
• Interleukins (ILs) are signalling molecules affecting various aspects of the immune system and promote T and B lymphocyte development and differentiation.
• Tumor necrosis factor (TNF) is a cytokine produced in response to tissue injury.

Complement Proteins

• Complement proteins are plasma proteins that react with each other, essential for innate immunity, and enhancing adaptive immune responses.

Innate vs. Acquired Immunity

• Innate immunity is the body's first line of defense, fast and non-specific, with no memory.
• Acquired immunity is the body's second line of defense, slower, and specific, with memory.

Anatomical Barriers to Infection

• Mechanical factors like skin, mucous membranes, and secretions (saliva, tears) help protect the body from foreign organisms.
• Chemical factors like the acidic environment of the stomach, or antimicrobial peptides in sweat also help prevent infection.
• Biological factors, like normal flora, compete with pathogens for nutrients and space.

Inflammation

• Inflammation is a complex localized response to foreign agents (bacteria) or internally produced substances, initiating reactions including cytokines, neutrophils, adhesion molecules, complement proteins, and immunoglobulin G (IgG). It is often a key component of the body's innate immune response. Monocytes and lymphocytes are also involved later.
• Increased capillary permeability, vasodilation, and swelling are associated with inflammation.
• Inflammation has a key role in pathogen removal, limiting infection spread, stimulating an adaptive immune response, and initiating tissue repair.
• Inflammation can be acute (short-term) or chronic (long-term). Chronic inflammation is often associated with tissue damage.

Wound Healing

• When tissue is damaged, platelets bind to collagen and laminin.
• Blood coagulation produces thrombin; it encourages platelet aggregation and the release of granules, initiating an inflammatory response.
• Cytokines are released from white blood cells. Fibroblasts produce collagen fibers for scar-wound tissue formation.

Autoimmunity

• In some situations, immune responses target self-antigens, leading to autoimmune disorders.
• Examples include type 1 diabetes (pancreatic islet B cells), myasthenia gravis, and multiple sclerosis. Treatments focus on replacing damaged functions and reducing inflammation.

Vaccination (Active Immunity)

• Active immunity develops when encountering a foreign organism; antibodies and T cells are activated, and the body develops immunological memory. Vaccines stimulate this process using dead or weakened microorganisms or toxins
• Memory cells are crucial in forming this acquired immunity via vaccination

Passive Immunity

• Temporary immunity occurs when antibodies or T-cells from another source (person, animal) are introduced to an individual.
• This is often seen as antibody therapy