The Immune System Overview

The Immune System - Overview

• The immune system protects the body from pathogens (bacteria, viruses, fungi, parasites) and abnormal cells (tumors).
• It's a complex system with multiple layers and interconnected components.
• It distinguishes between healthy and unhealthy cells based on markers on their membranes.
• Cells can become non-self due to infection, damage, or other factors.
• Infectious microbes release signals recognized by the immune system.
• The immune system consists of various cell types that circulate or reside in specific tissues.
• Immune cells communicate and work together to respond to threats.

Immune Cell Locations and Development

• Immune cells originate from precursors in the bone marrow.
• The bone marrow contains stem cells that develop into different cell types.
• Common myeloid progenitors produce innate immune cells like neutrophils, eosinophils, basophils, mast cells, monocytes, dendritic cells, and macrophages.
• These cells are important first-line responders to infection.
• Common lymphoid progenitors give rise to adaptive immune cells (B cells, T cells) and natural killer (NK) cells.
• B, T, and NK cells are also called lymphocytes.

Lymphatic System and Components

• The lymphatic system is a network of vessels and tissues, including lymph nodes.
• It carries immune cells and communicates with tissues and the bloodstream.
• Lymph nodes are hubs where immune cells sample antigens from the body, and if adaptive immune cells recognize pathogen fragments, they are activated and proliferate to address the pathogen.
• The spleen processes information from the bloodstream and responds to pathogens there.
• Mucosal surfaces are prime entry points for pathogens; immune hubs (Peyer's patches and tonsils) in mucosa are important.

Non-Specific (Innate) Immune Mechanisms

• Physical barriers (skin, mucous membranes) are the first line of defense.
• Inflammation helps to isolate the injured/infected area, clean up debris, and prepare for repair.
• Fever is a response to pyrogens (chemicals) that can interfere with the pathogen's metabolism.
• Cells like neutrophils, macrophages, and natural killer (NK) cells are part of the innate immune system.

Specific (Adaptive) Immune Mechanisms

• T cells (T-helper cells and cytotoxic T cells):
  • T helper cells direct the immune response by signaling other cells.
  • Cytotoxic T cells directly kill infected cells and tumor cells.
• B cells: Produce antibodies that specifically target and neutralize pathogens (bacteria, viruses).
• Antibody production and binding is critical to remove pathogens from the body.
• The activation and proliferation of B cells, after recognizing an antigen with the aid of an APC, are critical to immune defense.
• Memory cells provide long-lasting immunity to repeated exposures to pathogens.

Types of Immunity

• Naturally acquired passive immunity: temporary immunity through the transfer of antibodies (e.g., from mother to fetus).
• Naturally acquired active immunity: immunity resulting from an infection, which usually leads to long-term immunity.
• Artificially acquired passive immunity: temporary immunity from injected antibodies (e.g., serum).
• Artificially acquired active immunity: immunity from a vaccine that stimulates an immune response without causing the disease.