Immunity: Refers to the body’s ability to prevent the invasion of pathogens.

Pathogens: These are foreign disease-causing substances, such as bacteria and viruses, that people are exposed to every day.

Antigens: These are attached to the surface of pathogens and stimulate an immune response in the body.

Immune Response: This is the body’s defense system to fight against antigens and protect the body.

Types of Immunity

The body’s defense mechanisms are broadly classified into several types of immunity:

1. Innate Immunity

2. Acquired (Adaptive) Immunity

3. Passive Immunity

1. Innate Immunity

This type of immunity represents the inherent protection a person is born with. It comprises general defenses, such as physical barriers, defense mechanisms, and generalized immune responses like inflammation. These are non-specific defense mechanisms, meaning they react rapidly to any pathogen without precise identification of the antigen.

Aspects of Innate Immunity

This type of immunity is functionally divided into:

1. First Line of Defenses

2. Second Line of Defenses

First Line of Defenses: External/Physical/Barrier Defenses

These are the initial obstacles that impede pathogen entry, functioning as the first type of protection:

1. The stratum corneum of the epidermis, being a non-living layer, acts as an excellent barrier when unbroken, protecting against various pathogens.

2. Fatty acids in sebum help to inhibit bacterial growth on the skin; living epidermal cells also produce antimicrobial defensins.

3. Mucous membranes lining the respiratory, digestive, urinary, and reproductive tracts are living barriers, preventing entry.

4. The ciliated epithelium of the upper respiratory tract is especially effective; dust and pathogens are trapped in mucus and then swept to the pharynx for disposal.

5. Hydrochloric acid in gastric juice destroys most pathogens entering the stomach via mucus or through ingestion.

6. Lysozyme, an enzyme present in saliva, perspiration, nasal secretions, and tissue fluids, limits bacterial growth in the oral cavity and on the eye’s surface.

7. The unidirectional flow of urine through the urethra prevents the ascending of microorganisms.

8. The lacrimal apparatus of the eyes produces and drains away tears in response to irritants.

9. Blinking helps to spread tears across the eye surface, diluting microbes and preventing settling. Tears also contain lysozyme, which can degrade bacterial cell walls.

10. Saliva washes microbes away from teeth and the mouth’s mucous membrane, reducing colonization similar to how tears act in the eyes.

11. Defecation and vomiting expel microbes. For instance, some toxins cause vigorous smooth muscle contractions in the lower GI tract, resulting in diarrhea that rapidly eliminates microbes.

12. The skin’s acidity (pH 3–5) is due in part to fatty and lactic acid secretion.

13. Perspiration helps flush microbes from the surface of the skin.

14. Gastric juice, which includes hydrochloric acid, enzymes, and mucus, has a high acidity (pH 1.2–3.0), destroying many bacteria and bacterial toxins.

15. Acidic vaginal secretions prevent bacterial growth and, similarly, move microbes out of the body in females.

Second Line of Defenses: Internal Defenses

Once pathogens breach the initial barriers, they encounter internal defenses, representing another type of response, including:

1. Internal Antimicrobial Substances:

   • Interferons: (alpha, beta, and gamma) Proteins produced by virus-infected cells or T cells; they inhibit viral reproduction.

   • Complement: A group of >20 plasma proteins that circulate until activated; they are involved in lysing cellular antigens, tagging non-cellular antigens, and stimulating histamine release.

   • Iron-binding proteins: Inhibit the growth of certain bacteria by limiting available iron. Examples include transferrin, lactoferrin, ferritin, and hemoglobin.

   • Antimicrobial proteins (AMPs): Short peptides with broad antimicrobial activity. Examples include dermicidin, defensins, cathelicidins, and thrombocidin.

2. Defensive Cells:

   • Phagocytes: Such as macrophages (fixed and wandering), and dendritic cells (Langerhans cells) engulf foreign material through phagocytosis.

   • Neutrophils: Rapidly produced and deployed to injury or infection sites; they are phagocytes that attack foreign substances.

   • Eosinophils: Myelocytic leukocytes involved in allergic reactions and removing immune response components from the site of an allergic reaction

   • Basophils: Myelocytic leukocytes that release mediators (e.g., histamine, heparin) for initiating and sustaining immune or inflammatory responses.

   • Mast cells: Fixed basophils that release chemical mediators when stimulated by local irritation, found in the respiratory and GI tracts, as well as skin.

3. Inflammation:

   A physiological response to tissue damage characterized by local changes, designed to isolate, inactivate, and remove causative agents and damaged tissue to facilitate healing.

   • Redness: Resulting from increased blood flow due to local vasodilation, initiated by chemical mediators.

   • Heat: Caused by an endogenous pyrogen (interleukin 1) that resets the hypothalamus thermostat at a higher level, leading to pyrexia. The increased temperature helps to inhibit microbial growth while promoting phagocyte activity.

   • Swelling: Occurs due to fluid leaving blood vessels and entering interstitial spaces. This increases tissue fluid osmotic pressure, drawing more fluid from the blood.

   • Pain: Caused by swelling that compresses sensory nerve endings, exacerbated by mediators such as bradykinin and prostaglandins. Although unpleasant, it encourages protection of the damaged area.

    4. Immunological surveillance; see IMMUNOLOGY for details