Defensive Strategies - Immune System (Part 1)

Defensive Strategies - Immune System (Part 1)

• The immune system's evolution is linked to endosymbiosis.
• Eukaryotes, like animals, have a multi-celled immune system.
• Immune system is crucial for the survival of these organisms.
• The complexities of the system highlight its importance in survival and have real-world benefits, e.g., vaccines.
• The notes are based on Campbell 12th Edition, Chapter 47.
• It discusses animal defenses against infection.
• Specific concepts within the chapter are 47.1, 47.2, and 47.3.

General Introduction 1

• The immune system includes external defenses, innate immunity, adaptive immunity (acquired immunity), and the inflammatory response.
• The system comprises of cells and soluble factors (secreted by cells).

General Introduction 2

• The primary function of the immune system is to identify and destroy invading pathogens (e.g., viruses, bacteria, protists, worms).
• It aims to minimize the damage these invaders inflict.

Immune System Response

• An immune response involves recognizing invaders (A) and mounting a reaction to eliminate them (B).
• This involves recognizing the invader and a subsequent reaction against it, to eliminate it.
• These concepts were noted by Roitt et al., 1998.

Infectious Agents-1

• Infectious agents are mainly microbes like viruses and bacteria.
• Worms, fleas, lice, and ticks are other infectious agents that can cause harm to host organisms.
• Viruses, such as influenza, HIV, COVID-19, measles, and mumps, are examples of infectious agents capable of causing harm or even death to their host.

Infectious Agents-2

• Examples of bacterial pathogens include typhoid, cholera, MRSA, E. coli, bubonic plague, and Lyme disease.
• Protozoa, such as malaria, sleeping sickness, and red water fever, are also categorized as infectious agents.
• Worms like liver flukes, tapeworms, and roundworms are included as infectious agents as well.

Infectious Agents-3

• All viruses and many bacteria and protists/protozoa only replicate within host cells.
• Cells provide a safe haven for pathogens.
• The cell’s plasma membrane presents a barrier against soluble factors like antibodies.
• The immune system's crucial role is to identify and destroy infected cells to eradicate pathogens and minimize damage.

Infectious Agents-4

• Other types of bacteria, protozoa, and parasites reside outside of host cells in body fluids and tissues, hence, requiring a different immune response compared to intracellular pathogens.
• A different immune response is required to handle these agents.

Types of Pathogens

• The immune system defends against four categories of pathogens:
• Extracellular bacteria, parasites, and fungi.
• Intracellular bacteria, parasites.
• Intracellular viruses.
• Parasitic worms (extracellular).

Inflammatory Response-1

• Pathogens trigger inflammation.
• Inflammation is characterized by increased blood supply to the affected area.
• Increased permeability of capillaries allows larger molecules and cells to escape the blood vessels.
• White blood cells migrate from the capillaries to the infected tissues.

Innate Immunity-1

• Innate immunity is a natural or native immunity present before infection.
• The immune response is "poised" and ready to fight infection.
• It uses physical barriers, cells, and soluble factors to respond to infectious agents.
• This response is non-specific and does not have memory. This response remains non-specific regardless of whether the agent has been encountered before.

Innate Immunity-2 Cells

• The myeloid cell line includes phagocytes.
• Phagocytes are categorized as granulocytes.
• They include neutrophils, eosinophils, mast cells, and basophils.
• Monocytes (macrophages and dendritic cells): All three categories of these cells function as antigen-presenting cells (APCs).

Hematopoiesis

• Pluripotent hematopoietic stem cells differentiate into myeloid or lymphoid stem cells.
• Myeloid and lymphoid stem cells are crucial in the production of immune system cells that differentiate to combat infection.

Origin of Immune System Cells

• Cells develop in the thymus, bone marrow, and other secondary lymphoid tissues.
• Mature T-cells develop in the thymus gland.
• Mature B-cells develop in the bone marrow.
• Cells recirculate through blood.

Innate Immunity-Soluble Factors

• The innate immune system has soluble factors comprising proteins and peptides.
• Examples of these factors include complement proteins and interferons.
• Complement proteins play roles like lysis, chemotaxis, and opsonization.
• Interferons protect cells from viral invasion.
• Cytokines and chemokines are other proteins that participate in the response.

Opsonization

• Opsonins (e.g. antibodies, complement component C3b) bind to the surface of pathogens.
• This enhances phagocytosis by increasing effectiveness.
• Phagocytes have surface receptors to recognize opsonins and enhance the pathogen ingestion process.

Phagocytosis

• Phagocytosis is the process of engulfing and digesting microorganisms by certain immune cells.
• Phagocytes use pseudopodia to surround and engulf particles.
• The engulfed material (phagosome) merges with a lysosome for digestion.

Antigen-Presenting Cells (APCs)

• Dendritic cells (DCs) are a heterogeneous family of immune cells.
• They are crucial for bridging innate and adaptive immunity.
• They phagocytose pathogens and present antigens to T-cells.
• Antigens are presented in the form of MHC molecules.

T-cell Antigen Recognition

• T cells recognize antigens presented by MHC molecules on the surface of infected cells or APCs.
• This recognition triggers T-cell responses.

Adaptive Immune Response

• The adaptive immune response develops in reaction to infection; it is pathogen-specific and has memory.
• Adaptive immunity has increased potency with repeated exposure to a pathogen.
• The initial response is slower than the innate immune response.