Innate Immunity: The Second Line of Defense

Innate Immunity: The Second Line of Defense

Overview of the Second Line of Defense

The second line of defense in innate immunity is activated when pathogens breach the skin or mucous membranes. This system is composed of various cells, antimicrobial chemicals, and physiological processes, many of which are present in or originate from the blood. These components work together to provide a rapid, nonspecific response to invading microorganisms.

Defense Components of Blood: Plasma

Plasma is the liquid portion of blood, primarily consisting of water, electrolytes, dissolved gases, nutrients, and proteins. When clotting factors are removed, the remaining fluid is called serum. Plasma contains important defense molecules such as iron-binding proteins, complement proteins, and antibodies. Iron is essential for microbial metabolism, and some microbes secrete proteins to capture iron from the host.

Defense Components of Blood: Formed Elements

The cellular components of blood, known as formed elements, include erythrocytes, platelets, and leukocytes. Each type plays a distinct role in immunity and homeostasis:

• Erythrocytes: Transport oxygen and carbon dioxide.

• Platelets: Involved in blood clotting and inflammation.

• Leukocytes: Defend the body against invaders and are subdivided into granulocytes and agranulocytes.

Leukocytes: Granulocytes

Granulocytes are leukocytes with visible granules in their cytoplasm, which stain differently with specific dyes. The three main types are:

• Basophils: Stain blue with basic dyes; release inflammatory chemicals but do not phagocytize pathogens.

• Eosinophils: Stain red/orange with acidic dyes; phagocytize pathogens, attack helminths, and can kill without phagocytosis.

• Neutrophils: Stain lilac with a mix of dyes; phagocytize pathogens, produce antimicrobial chemicals, and form neutrophil extracellular traps (NETs).

Leukocytes: Agranulocytes

Agranulocytes have a uniform cytoplasm under light microscopy and include:

• Lymphocytes: Primarily involved in adaptive immunity; natural killer (NK) lymphocytes are part of innate immunity.

• Monocytes: Differentiate into dendritic cells and macrophages, which are phagocytic and ingest foreign particles.

Laboratory Analysis of Leukocytes

Laboratory evaluation of leukocyte populations, such as a differential white blood cell count, can indicate disease states:

• Increased eosinophils suggest allergies or parasitic worm infection.

• Bacterial infections often cause elevated leukocytes and neutrophils.

• Viral infections typically increase lymphocyte numbers.

Phagocytosis: Mechanism and Stages

Phagocytosis is the process by which certain cells (phagocytes) ingest and destroy pathogens. The process involves six stages:

1. Chemotaxis: Movement of phagocytes toward chemical signals from pathogens or damaged cells.

2. Adhesion: Attachment of phagocyte to the pathogen.

3. Ingestion: Engulfment of the pathogen into a phagosome.

4. Maturation: Fusion of the phagosome with lysosomes to form a phagolysosome.

5. Killing: Destruction of the pathogen by enzymes and toxic substances.

6. Elimination: Expulsion of digested material by exocytosis.

Nonspecific Chemical Defenses Against Pathogens

The second line of defense also includes nonspecific chemical mechanisms:

• Toll-like receptors (TLRs): Integral membrane proteins on phagocytes that recognize pathogen-associated molecular patterns (PAMPs) and trigger defensive responses such as inflammation, apoptosis, and stimulation of adaptive immunity.

• NOD proteins: Cytosolic proteins that detect PAMPs and initiate inflammation and apoptosis.

• Interferons: Proteins released by host cells to slow the spread of viral infections. Type I interferons (alpha and beta) are produced early, while Type II (gamma) is produced later by T lymphocytes and NK cells.

TLR	PAMP (Microbial Molecule)
TLR1	Bacterial lipopeptides, certain parasite proteins
TLR2	Peptidoglycan, lipoteichoic acid, yeast cell wall
TLR4	Lipid A in LPS (Gram-negative bacteria)
TLR5	Flagellin (bacterial flagella)
TLR6	Bacterial lipopeptides, lipoteichoic acid, yeast cell wall
TLR10	Unknown component of influenzaviruses
TLR3	Double-stranded RNA (viruses)
TLR7/8	Single-stranded viral RNA
TLR9	Unmethylated CpG DNA (viral/bacterial)

Summary Table: Leukocyte Types and Functions

Cell Type	Main Function	Granules Present?
Erythrocyte	Gas transport	No
Platelet	Clotting, inflammation	No
Basophil	Inflammation	Yes
Neutrophil	Phagocytosis, innate immunity	Yes
Eosinophil	Phagocytosis, helminth defense	Yes
Monocyte	Phagocytosis (as macrophage/dendritic cell)	No
Lymphocyte	Adaptive immunity	No

Key Points and Examples

• Phagocytosis is essential for destroying pathogens; pathogens are killed in phagolysosomes by acid and oxidative radicals.

• Gamma interferon is produced late in infection by T lymphocytes and NK cells.

• Toll-like receptors do not target eukaryotic flagellar proteins, but do recognize bacterial flagellin, lipid A, and viral RNA.