Immune System: Body's Natural Defenses and Immunity Types

The Immune System: Body's Defense Mechanisms

The body's defense mechanisms protect us against various pathogens such as viruses, bacteria, fungi, protozoa, extraneous cellular tissue introduced from outside (e.g., in transplantation), and cells transformed into malignant or cancerous forms. The set of biological structures that enable specific defense against these agents constitutes the immune system, and all such defense mechanisms provide immunity. The science of studying the immune system is called immunology.

Types of Immunity: The Body's Defense Lines

The body has three lines of defense against microbial attacks:

• First Line: External Barriers

  These prevent microorganisms from entering the body.

• Second Line: Non-Specific Internal Defenses

  These fight invaders once they have entered.

• Third Line: Specific Immune Response

  The immune system directs its attack, an immune response, against specific microbes.

There are two basic types of immunity involved in defense against pathogens: innate or natural immunity. This includes mechanisms the body possesses to fight germs before infection occurs. The main components of innate immunity include:

Innate Immunity Components

Physical and Mechanical Barriers

• The thick skin is a physical barrier that hinders the entry of pathogens and also allows the elimination of microorganisms present on its surface. When the skin is broken by a cut or burn, it becomes a point of entry for infection.
• The mucous membranes line body cavities that communicate with the outside. The cells in the lining produce mucus, which lubricates and traps germs.
• Ciliated cells sweep the germs contained in the mucus towards the pharynx, where they are then swallowed and destroyed by stomach hydrochloric acid.

Chemical Barriers

These are discharges affecting the development of microbes or causing their death, directly or indirectly. Some examples include:

• Tears and saliva contain an enzyme called lysozyme, which destroys bacterial cell walls, leading to their death.
• Sebum (skin oil) determines the acidic pH of the skin, inhibiting bacterial growth.
• Sweat contains lysozyme and helps to flush pathogens out of the body.

Phagocytic Cells

These defenses are found after the first line. Some of these are non-specific, meaning they attack a variety of microbes. A broad range of these responses is represented by cells with phagocytic capacity. Among them are:

• Macrophages
• Neutrophils
• NK cells (Natural Killer cells)

Plasma Proteins

The last barrier participating in innate immunity processes consists of specific proteins, called:

• Cytokines
• Interleukins
• Lymphokines

These are involved in inducing the production and response of white blood cells.