Host Defenses: Innate and Adaptive Immunity

Host Defenses

Innate versus Adaptive Immunity

The immune system protects the body from pathogens through two main types of immunity: innate and adaptive. These systems differ in their mechanisms, speed, and memory capabilities.

• Innate Immunity: Present from birth and provides immediate, non-specific defense against pathogens. The response does not change upon repeated exposure, and no immunological memory is produced.

• Adaptive Immunity: Acquired over time and provides a specific response to pathogens. Upon subsequent exposures, the response is faster and stronger due to immunological memory.

• Cell Types and Chemical Messengers: Both systems involve a variety of cell types and chemical messengers that coordinate immune responses.

Cells Involved in the Immune Response

Immune cells originate from hematopoietic stem cells in the bone marrow and differentiate into various lineages with specialized functions.

• Myeloid Lineage: Includes mast cells, basophils, neutrophils, eosinophils, monocytes, macrophages, and dendritic cells. These cells are primarily involved in innate immunity.

• Lymphoid Lineage: Includes natural killer (NK) cells (innate immunity), T lymphocytes, and B lymphocytes (adaptive immunity). B lymphocytes can further differentiate into plasma cells that produce antibodies.

Example: Neutrophils are the most abundant white blood cells and are the first responders to infection, performing phagocytosis of pathogens.

Chemical Messengers – Cytokines

Cytokines are small proteins released by cells that act as chemical messengers to regulate immunity, inflammation, and hematopoiesis.

Type	Actions
Interferons	Regulation of innate immunity, activation of antiviral properties, antiproliferative effects
Interleukins	Growth and differentiation of leukocytes; many are proinflammatory
Chemokines	Control of chemotaxis, leukocyte recruitment; many are proinflammatory
Colony-stimulating factors	Stimulation of hematopoietic progenitor cell proliferation and differentiation
Tumor necrosis factor	Proinflammatory, activates cytotoxic T lymphocytes

Example: Interferon-alpha is produced in response to viral infection and helps neighboring cells resist viral replication.

Innate Responses

Innate immunity provides the first line of defense against pathogens through physical, chemical, and cellular mechanisms.

Physical Barriers

• Skin and Mucous Membranes: Act as physical barriers to prevent pathogen entry. Secretions such as mucus, saliva, and tears contain antimicrobial substances.

• Hair and Epithelium: Trap and remove microbes from the body surface.

Phagocytes

• Neutrophils, Macrophages, Eosinophils, Monocytes: Engulf and destroy pathogens and debris through phagocytosis.

• Process: Recognition, ingestion, and digestion of microbes.

Extracellular Killing

• Natural Killer (NK) Cells: Destroy abnormal or infected host cells by releasing cytotoxic granules.

Inflammatory Response

• Purpose: Localizes infection, recruits immune cells, and promotes tissue repair.

• Signs: Redness, heat, swelling, pain, and loss of function.

• Process: Increased blood flow, capillary permeability, and migration of leukocytes to the site of infection.

Fever

• Definition: Elevated body temperature in response to infection.

• Function: Inhibits pathogen growth and enhances immune cell activity.

• Mechanism: Pyrogens (e.g., cytokines) stimulate the hypothalamus to raise the body’s set point temperature.

Interferons

• Type I Interferons (IFN-α, IFN-β): Released by virus-infected cells to warn neighboring cells and induce antiviral states.

Complement System

• Definition: A group of plasma proteins that enhance (complement) the ability of antibodies and phagocytic cells to clear microbes.

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

• Functions: Opsonization, chemotaxis, cell lysis via the membrane attack complex (MAC), and inflammation.

Example: The classical pathway is triggered by antigen-antibody complexes, leading to the formation of the MAC and lysis of the pathogen.

Summary Table: Innate Immune Responses

Component	Main Function
Physical Barriers	Prevent pathogen entry
Phagocytes	Engulf and destroy pathogens
NK Cells	Kill infected or abnormal cells
Inflammation	Recruit immune cells, promote healing
Fever	Inhibit pathogens, enhance immunity
Interferons	Induce antiviral states
Complement	Opsonization, lysis, inflammation

Additional info:

• Opsonization: The process by which pathogens are marked for ingestion and destruction by phagocytes. Complement proteins and antibodies can act as opsonins.

• Cytokine Storm: An excessive and uncontrolled release of proinflammatory cytokines, which can lead to tissue damage and severe disease.