Organogenesis, Tissue Structure and Function of the Body Systems
Digital World Medical School
© 2025
Immune System
The human immune system is able to distinguish “nonself” organisms and molecules from “self,” that which is part of the body.
Threats may enter the body from the outside (e.g., infectious organisms, toxic agents, therapeutic drugs ) or may arise from potentially harmful changes occurring within the body (e.g., the malignant transformation of a previously normal cell into a cancer cell).
Fortunately, the immune system consists of three layers of defense.
The first line of defense is provided by a set of mechanical (e.g., skin), chemical (e.g., acidic environment of stomach), and biologic (e.g., commensal microbes) barriers that protect the body. If these barriers are breached, the second and third lines of protective systems are activated: first the innate immune system and then the adaptive immune system.
If the immune system malfunctions, it may attack the body’s own healthy cells, leading to autoimmune diseases.
Major Histocompatibility Complex
The immune system distinguishes self from nonself .
The immune system also has the capacity to recognize and destroy abnormal cells that derive from host tissues. It clears the body’s own cells that have become senescent or abnormal.
Any molecule capable of being recognized by the immune system is considered an antigen (Ag).
Occasionally normal host tissues become the subject of inappropriate immune attack.
The immune system is the major defense system of the bodoy. Non specific barreirs also contribute to the defense systes as physical brriers
The skin, corneas, and mucosa of the respiratory, gastrointestinal, and genitourinary tracts form a physical barrier that is the body's first line of defense.
Some of these barriers also have active immune function
Outer, keratinized epidermis: Keratinocytes in the skin secrete antimicrobial peptides (defensins), and sebaceous and sweat glands secrete microbe-inhibiting substances (eg, lactic acid, fatty acids). Also, many immune cells (eg, mast cells, intraepithelial lymphocytes, antigen-sampling Langerhans cells) reside in the skin.
Corneas: Tears contain defensins. Macrophages and dendritic cells reside in the corneas and other immune cells, including T cells and phagocytic neutrophils, are recruited through the limbal vasculature during infection.
Mucosa of the respiratory, gastrointestinal, and genitourinary tracts: The mucus contains antimicrobial substances, such as lysozyme, lactoferrin, and secretory immunoglobulin (Ig) A antibody (SIgA).