Innate Immune Response: Mechanisms, Cells, and Pathogen Interactions

Innate Immune Response

Overview of Immunity

The immune system protects the body from pathogens such as viruses, bacteria, and fungi through a combination of physical barriers, cellular responses, and molecular mechanisms. Innate immunity provides the first line of defense and operates rapidly and non-specifically.

• Physical barriers: Skin, mucous membranes, and antimicrobial substances block pathogen entry.

• Inflammation, fever, and phagocytes: These mechanisms respond to infection and help eliminate pathogens.

• Humoral and cellular immunity: Adaptive responses that are more specific and develop after innate responses.

Physical Barriers to Infection

Physical barriers are the body's initial defense against infection, preventing pathogens from entering and establishing disease.

• Skin: Acts as a physical barrier and produces antimicrobial peptides.

• Mucous membranes: Line the gastrointestinal, respiratory, and genitourinary tracts; secrete mucus to trap microbes.

• Lacrimal apparatus: Drains tears and washes eyes, removing potential pathogens.

• Stomach acid: Low pH destroys ingested microbes.

• Flushing of urinary tract: Prevents colonization by pathogens.

• Ciliary elevator: Ciliated cells in the respiratory tract move mucus and trapped microbes out of the lungs.

Blood and Lymphatic Systems

The blood and lymphatic systems transport immune cells throughout the body and facilitate communication between different components of the immune system.

• Blood capillaries: Allow immune cells to pass in and out of the bloodstream.

• Lymph capillaries: Collect fluid and immune cells from tissues and return them to circulation.

Innate Immune Cells and Their Functions

Innate immunity relies on various white blood cells (WBCs) to detect, ingest, and destroy pathogens.

• Phagocytes (macrophages, neutrophils): Ingest and degrade pathogens; present antigens to trigger adaptive immunity.

• Granulocytes (basophils, eosinophils, neutrophils) BENG: Contain granules with enzymes or toxins that kill pathogens and stimulate inflammation.

• Natural Killer (NK) cells: Detect and destroy unhealthy or infected cells using activating and inhibitory receptors.

Table: Major Innate Immune Cell Types and Functions

Cell Type	Recognition	Function
Neutrophil	Non-specific	Phagocytosis and killing of bacteria
Macrophage	Non-specific	Phagocytosis, antigen presentation, cytokine production
Basophil	Non-specific	Release of histamine, role in inflammation
Eosinophil	Non-specific	Defense against parasites, allergic responses
NK cell	Non-specific	Destruction of virus-infected and tumor cells

Mechanism of Phagocytosis

Phagocytosis is a multi-step process by which phagocytes ingest and destroy pathogens.

• Chemotaxis: Chemical signals attract phagocytes to the site of infection.

• Adherence: Phagocyte attaches to the surface of the microorganism.

• Ingestion: Opsonization coats the microorganism with serum proteins, facilitating ingestion.

• Digestion: Microorganism is digested inside a phagolysosome.

Phagocyte Enzyme Activity

Phagocytes generate toxic oxygen compounds to kill ingested microbes.

• Reactive oxygen species (ROS): Include superoxide ($O_2^-$), hydrogen peroxide ($H_2O_2$), and nitric oxide ($NO$).

• Enzymatic reactions: Myeloperoxidase and NADPH oxidase are key enzymes in ROS production.

Microbial Evasion of Phagocytosis

Some pathogens have evolved mechanisms to evade destruction by phagocytes.

Mechanism	Example Organisms
Inhibit adherence (M protein, capsules)	Streptococcus pyogenes, S. pneumoniae
Kill phagocytes (leukocidins)	Staphylococcus aureus
Lyse phagocytes (membrane attack complex)	Listeria monocytogenes
Escape phagosome	Shigella, Rickettsia
Prevent phagosome-lysosome fusion	HIV, Mycobacterium tuberculosis
Survive in phagolysosome	Coxiella burnetii

Granulocytes: Basophils, Eosinophils, Neutrophils

Granulocytes are WBCs containing granules with enzymes or toxins that help eliminate pathogens and mediate inflammation.

• Basophils: Release histamine and other mediators of inflammation.

• Eosinophils: Combat parasites and participate in allergic responses.

• Neutrophils: Most abundant phagocyte; rapidly respond to infection.

Natural Killer (NK) Cells

NK cells are lymphocytes that recognize and kill virus-infected or abnormal cells by detecting changes in cell surface molecules.

• Activating receptors: Trigger killing of unhealthy cells.

• Inhibitory receptors: Prevent killing of healthy cells.

Pathogen Recognition and Signal Transduction

Innate immune cells recognize pathogens using pattern recognition receptors (PRRs) that bind pathogen-associated molecular patterns (PAMPs).

• PAMPs: Conserved microbial structures (e.g., peptidoglycan, flagellin, dsRNA, LPS).

• PRRs: Include Toll-like receptors (TLRs) that detect PAMPs and initiate immune responses.

Table: Toll-Like Receptors and Their Ligands

TLR	Ligand
TLR1	Lipoproteins
TLR2	Lipoproteins, lipoteichoic acid
TLR3	Double-stranded RNA
TLR4	Lipopolysaccharide (LPS)
TLR5	Flagellin
TLR7/8	Viral RNA
TLR9	Unmethylated CpG DNA

Inflammation and Fever

Inflammation is a localized response to infection or injury, characterized by redness, heat, swelling, and pain. Fever is a systemic response that raises body temperature to inhibit pathogen growth.

• Acute-phase proteins: Produced by the liver, promote vasodilation and increased permeability of blood vessels.

• Key mediators: Histamine, kinins, prostaglandins, leukotrienes, cytokines.

• Fever: Triggered by cytokines acting on the hypothalamus, increasing body temperature.

Complement System

The complement system is a group of plasma proteins that enhance immune responses and help eliminate pathogens.

• Activation pathways: Classical, alternative, and lectin pathways.

• Outcomes: Cytolysis (formation of membrane attack complex), opsonization (enhanced phagocytosis), inflammation (recruitment of immune cells).

Antiviral Action of Interferons (IFNs)

Alpha and beta interferons are cytokines produced in response to viral infection, inducing antiviral states in neighboring cells.

• IFN-α and IFN-β: Inhibit viral replication and activate immune cells.

• Mechanism: Induce production of antiviral proteins that degrade viral RNA and inhibit protein synthesis.

Iron-Binding Proteins

Iron-binding proteins limit microbial growth by sequestering iron, an essential nutrient for many pathogens.

• Transferrin: Found in blood and tissue fluids.

• Lactoferrin: Found in milk, saliva, and mucus.

• Ferritin: Stored in liver, spleen, and bone marrow.

• Hemoglobin: Located in red blood cells.

• Bacterial siderophores: Compete with host proteins for iron.

Antimicrobial Peptides

Antimicrobial peptides are short proteins produced by host cells in response to infection, with broad-spectrum activity against bacteria, fungi, and viruses.

• Mechanisms: Inhibit cell wall synthesis, form pores in microbial membranes.

• Examples: Defensins, cathelicidins.

Human Immunodeficiency Virus (HIV) and AIDS

HIV is a retrovirus that infects CD4+ T cells, leading to acquired immunodeficiency syndrome (AIDS).

• Pathogenesis: HIV infects cells with CD4 surface protein, primarily macrophages and T-helper cells.

• Genome integration: HIV integrates its genome into host DNA, favoring active transcription units.

• Replication cycle: Involves entry, reverse transcription, integration, assembly, and release of new virions.

• HAART: Highly active antiretroviral therapy reduces viral load and increases survival.

Table: HIV Evasion of Immune Response

Mechanism	Effect
Prevents phagosome-lysosome fusion	Allows survival in macrophages
Genome integration	Evades immune detection

Summary

The innate immune system provides rapid, non-specific defense against pathogens through physical barriers, cellular responses, and molecular mechanisms. Pathogens have evolved strategies to evade these defenses, and understanding these interactions is crucial for developing effective treatments and vaccines.

Additional info: The notes also briefly cover the role of the complement system, interferons, iron-binding proteins, and antimicrobial peptides, as well as the pathogenesis and treatment of HIV/AIDS, which are relevant to both innate and adaptive immunity.