Immunity

The Immune System

Overview of the Immune System

The immune system's primary job is to prevent illness. It is a widespread collection of cells and chemicals that defend the body against infections, cancer, and foreign substances.

Components of the Immune System

The immune system has several components that work together:

• White Blood Cells:

  • Phagocytes: Consume pathogens.

  • Lymphocytes: Key players in adaptive immunity.

  • Basophils: Involved in inflammatory responses.

• Lymphatic System:

  • Lymph: Colorless fluid containing white blood cells.

  • Lymph Vessels: Transport lymph throughout the body.

  • Lymph Nodes: Filter foreign substances from lymph.

• Defensive Proteins and Chemicals: Examples include antibodies and complement proteins.

White Blood Cells (Leukocytes)

White blood cells, generally referred to as leukocytes, are crucial defenders of the body with diverse functions in the immune system.

The Lymphatic System

Functions of the Lymphatic System

1. Production and Storage of Defense Cells:

   • Spleen: Produces and stores white blood cells.

   • Thymus: Site of T lymphocyte maturation.

   • Bone Marrow: Produces a variety of blood cells.

2. Transport of Defense Cells:

   • Lymph absorbs interstitial fluid and delivers it to larger lymph vessels, serving as a transport system for immune cells.

3. Filtering of Foreign Substances:

   • Lymph passes through lymph nodes, where foreign substances are removed and millions of lymphocytes are present.

Subdivisions of the Immune System

The immune system can be divided into two main subdivisions:

1. Innate Defenses: Provide broad and immediate responses against any pathogen.

2. Adaptive Immunity: Immune cells that specifically recognize and remember specific pathogens.

Innate Defenses

Characteristics of Innate Defenses

• Immediate and Nonspecific:

  • The first line of defense consists of physical and chemical barriers that block pathogens and foreign substances from entering the body.

    • Examples:

    • Skin

    • Mucus

    • Earwax

    • Tears

    • Stomach acid

Cellular Participants in Innate Defense

• Various leukocytes are involved:

  • Macrophages: Consume pathogens and promote fever.

  • Natural Killer Cells: Destroy cancerous or virus-infected body cells.

  • Basophils: Trigger inflammation by releasing histamine.

Antimicrobial Proteins

• Produced by white blood cells, complement proteins puncture bacterial cell membranes, while cytokines can cause a fever, which counters microbial growth indirectly.

Adaptive Theory of Fever

Fever has two proposed interpretations:

1. Manipulation Hypothesis: Fever may represent the manipulation of the host by the pathogen, suggesting that reducing fever might help combat infection.

2. Adaptive Hypothesis: Fever may serve as an adaptive defense against pathogens, indicating that alleviating fever could be counterproductive.

Evidence for Adaptive Hypothesis:
Studies on iguanas have shown they develop a behavioral fever in response to infections, reinforcing the adaptive defense theory.

Implications of Fever Behavior

• Human Considerations: Whether individuals should take fever-reducing medications for illnesses such as colds remains a relevant question.

Inflammation as an Innate Defense

• Inflammation is an immediate response to injury or infection that recruits white blood cells, clears debris, and creates a hostile environment for microbes.

Adaptive Immunity

Adaptive immune responses target specific invaders. The two main types of adaptive immunity involve:

• Cell-Mediated Immunity: Primarily mediated by T lymphocytes which kill defective or infected body cells.

• Humoral Immunity: Mediated by B cells that produce antibodies targeting specific antigens.

Role of Antigens

• Antigens are molecules found on the surfaces of pathogens that stimulate immune reactions from B and T cells, leading to antibody production.

Function of Antibodies

• Antibodies: Y-shaped proteins produced by B cells, designed to bind to specific antigens and facilitate pathogen neutralization.

Primary vs Secondary Immune Responses

• Primary Immune Response: The initial, slower response to a foreign antigen that can take weeks to build up enough antibodies to clear infection.

• Secondary Immune Response: A faster and stronger reaction due to the presence of memory cells which have been primed to respond to previously encountered antigens.

Vaccines and Immunity

• Vaccines: Stimulate active immunity against pathogens without causing illness. They contain antigens that prompt a primary immune response and promote the generation of memory cells.

• Mechanism of Action: Memory cells enable a rapid and robust secondary immune response upon subsequent exposure to the real pathogen.

Types of Vaccines

Limitations of Vaccines

• Not all infectious diseases can be prevented by vaccines:

  • HIV: Mutates rapidly, complicating vaccine development.

  • Flu Vaccines: Have limited effectiveness due to the evolution of new antigens each season.

  • Cancer Vaccines: Few vaccines available for cancer prevention (e.g., Gardasil).

Autoimmune Disorders

• Autoimmune Reactions: Occur when the immune system mistakenly targets the body’s own molecules due to failures in clonal deletion, leading to diseases such as type I diabetes (antibodies attack insulin-producing cells) and arthritis (immune attack on joint cells).

Allergies

• Allergic Reactions: Exaggerated immune responses to benign substances. For example, B cells recognizing pollen antigens activate and proliferate, leading to the secretion of antibodies.

Mechanism of Allergic Responses

• Upon re-exposure to an allergen, antibodies bound to mast cells release histamine and other chemicals, causing an exaggerated immune response characterized by allergy symptoms.