Immunology Overview and Innate Immunity

Immunology Overview

• Immunology is the branch of biology studying the body's defense reactions.
• Immunity is derived from the Latin word "immunis," meaning "free of burden."
• Pathogenic microorganisms (bacteria, viruses, fungi, and parasites) are present in the environment and can cause disease.
• Without an immune system, humans would constantly become ill and possibly die due to infections.

Innate Immunity

• Innate immunity constitutes the body's first line of defense against infectious agents, serving as the body's early warning system.
• It provides a non-specific defense against any pathogen, regardless of its type.
• Innate immunity is a two-line defense system, including physical factors (skin and mucous membranes) and a second line of defense involving phagocytes, inflammation, fever, and antimicrobial substances.
• Innate immunity lacks a memory component, meaning it cannot recognize or respond to previously encountered pathogens.

Specific (Adaptive) Immunity

• Specific immunity is the body's third line of defense, which is acquired throughout life.
• It's a specific response to a particular microbe.
• Specific immunity has long-term memory for specific antigens.
• This immunity is based on specialized immune cells (B and T cells) and specific proteins like antibodies.

Innate vs. Adaptive Immunity

Nonspecific (Innate) Resistance

• First Line of Defense:

  • Physical factors: skin, mucous membranes, mucus, tears, saliva, hairs, cilia, urine, vomiting, and defecation.
  • Chemical factors: sebum, perspiration, lysozyme, gastric juice. The skin's epidermis (outer layer) is a strong physical barrier due to the keratin protein.

• Mucous Membranes: Consist of epithelial layers and connective tissue; line the gastrointestinal, respiratory, and genitourinary tracts. Mucus, a glycoprotein, is produced by goblet cells.

• Lachrymal Apparatus: Tears produced by the lacrimal glands protect the eye surface.

• Salivary Glands: Saliva produced by salivary glands dilutes microorganisms and removes them from the teeth and mouth;

• Hair: Nose hairs trap airborne microorganisms, dust, and pollutants.

• Cilia: Cilia in the respiratory tract move foreign particles upward toward the throat.

• Urine: Flow of urine prevents microbial colonization of the urinary tract.

Chemical Factors

• Sebum: Produced by sebaceous glands, contains unsaturated fatty acids to prevent bacterial and fungal growth.
• Perspiration: Produced by sweat glands helps regulate temperature and remove microorganisms.
• Lysozyme: Enzyme found in tears, saliva, nasal secretions, urine, and tissue fluids, that breaks down peptidoglycan in the cell walls of gram-positive bacteria.
• Gastric Juice: The high acidity of the stomach destroys most bacteria and bacterial toxins (pH 1.2-3), while some pathogens survive acidic conditions.

Normal Microbiota

• Normal microbiota prevents pathogen overgrowth by competing for nutrients, producing harmful substances that inhibit pathogens, and altering environmental conditions.

Phagocytosis

• This is the cellular process of engulfing and destroying solid particles like bacteria, debris, and pathogens.
• Phagocytosis has four steps: chemotaxis, adherence, ingestion, and digestion.
• Lysosomal bactericidal factors include hydrolytic enzymes (e.g., lysozyme, lipase, protease) and toxic oxygen products (e.g., hydrogen peroxide, singlet oxygen).

Inflammation

• Inflammation is the body's defensive response to tissue damage (from microbial infections, physical damage, or chemical agents).
• Inflammation is characterized by redness, pain, heat, and swelling.
• Inflammation has beneficial functions, including destroying and removing injurious agents, limiting microbial effects, and repairing damaged tissues.
• Inflammation is divided into Vasodilation and increased permeability of blood vessels (caused by chemical mediators), Phagocyte migration (Neutrophils, Monocytes), and Tissue Repair (skin regeneration).

Fever

• Fever is an abnormally high body temperature, controlled by the hypothalamus.
• Typically set at 37°C, infection can cause a rise in the body temperature related to the cytokines (e.g., IL-1) released from macrophages that triggers the hypothalamus to release prostaglandins that reset the hypothalamic thermostat, and increase body temperature.
• High temperatures can slow down the growth rate of bacteria and viruses and increase the body's immune response.

Antimicrobial Substances

• Interferons and complement proteins are a part of the body's antimicrobial arsenal.
• Interferons are proteins that help prevent viral infections.
• Complement proteins enhance various aspects of the immune system.

Formed Elements of Blood

• Blood consists of plasma and formed elements (erythrocytes, leukocytes, and platelets).
• Leukocytes (white blood cells) are crucial for the immune system, further categorized into granulocytes and agranulocytes.

Granulocytes

• Neutrophils are highly phagocytic and motile, active in initial infection phases.
• Basophils release substances like histamine, playing a role in inflammation and allergic responses.
• Eosinophils produce toxic proteins against parasites.

Agranulocytes

• Dendritic cells have extensions resembling nerve cells, abundant in skin, mucosal membranes, and lymph nodes. They are antigen-presenting cells.
• Monocytes are phagocytic after leaving the blood and becoming macrophages.
• Lymphocytes (B and T cells) are vital for specific immunity, responsible for antibody production (B cells) and cell-mediated immunity (T cells).

Natural Killer (NK) Cells

• Found in blood, spleen, lymph nodes, and bone marrow.
• NK cells kill various infected body cells and tumor cells by recognizing and destroying abnormal cells.