Table of Contents
Overview of Immunobiology
Immunobiology deals with how organisms defend themselves against harmful agents (pathogens, toxins, damaged cells) and how these defense mechanisms are organized, controlled, and sometimes misdirected. In humans and other vertebrates, this involves a complex immune system that combines fast, broadly acting defenses with slower, highly specific responses and long-lasting memory.
In this chapter, the focus is on principles that apply to the immune system as a whole. Details of individual topics such as antibodies, immunization, autoimmune diseases, and allergies are treated in their own chapters.
Tasks of the Immune System
The immune system must perform several fundamental tasks:
- Distinguish “self” from “non-self”
- Recognize the body’s own cells and molecules.
- Detect foreign structures such as those from bacteria, viruses, fungi, parasites, and transplanted tissues.
- Eliminate harmful agents
- Destroy or neutralize pathogens before they cause serious damage.
- Remove toxins and foreign particles.
- Handle altered self
- Eliminate body cells that have become dangerous, for example:
- Virus-infected cells.
- Tumor cells.
- Strongly damaged or dying cells.
- Maintain balance (homeostasis)
- Limit and eventually switch off immune reactions once the threat is removed.
- Avoid excessive tissue damage from inflammation.
To accomplish these tasks, the immune system uses a network of cells, tissues, soluble molecules (such as signal substances and antibodies), and physical barriers.
Components and Organization of the Immune System
The immune system is not a single organ, but a distributed system with specialized structures throughout the body.
Primary and Secondary Lymphatic Organs
Immune cells (lymphocytes) develop and mature in primary lymphatic organs and are activated in secondary lymphatic organs:
- Primary lymphatic organs
- Bone marrow: Origin of almost all blood cells, including immune cells; B lymphocytes mature here.
- Thymus: Organ above the heart where T lymphocytes mature and learn self–non-self discrimination.
- Secondary lymphatic organs
- Lymph nodes: Filter lymph (tissue fluid), trap pathogens, and provide meeting points for antigens and lymphocytes.
- Spleen: Filters blood, removes old blood cells, and initiates immune responses against blood-borne pathogens.
- Mucosa-associated lymphoid tissue (MALT): Immune tissue in mucous membranes (e.g. tonsils, Peyer’s patches in the intestine, bronchial-associated tissue) that protects entry sites of pathogens.
Immune Cells (Overview)
Important groups of immune cells include:
- Phagocytes (“eating cells”)
- E.g. macrophages, neutrophil granulocytes.
- Ingest and digest microbes and debris (phagocytosis).
- Important effectors of the nonspecific immune response and key for initiating specific responses.
- Lymphocytes
- B cells: Can develop into plasma cells that produce antibodies.
- T cells: Different subtypes, e.g.
- Helper T cells (coordinate other immune cells).
- Cytotoxic T cells (kill infected or altered body cells).
- Regulatory T cells (help prevent excessive or misdirected immune reactions).
- Natural killer (NK) cells: Kill virus-infected cells and certain tumor cells without the same specificity rules as T cells.
- Antigen-presenting cells (APCs)
- Include dendritic cells, macrophages, and certain B cells.
- Take up antigens, process them, and present fragments on their surface to T cells, linking nonspecific and specific immunity.
Molecular Mediators
Many molecules orchestrate immune responses:
- Cytokines
- Small signaling proteins (e.g. interleukins, interferons) that control growth, activation, and differentiation of immune cells.
- Coordinate inflammation and communication between cells of both nonspecific and specific immunity.
- Complement system
- Group of plasma proteins that, once activated, can:
- Damage pathogen membranes directly.
- Tag pathogens for phagocytosis (opsonization).
- Enhance inflammation by attracting immune cells.
- Pattern recognition molecules
- Receptors and soluble proteins that recognize common structures on microbes, helping to initiate nonspecific responses.
These components work together in time and space, forming an integrated defense network.
Recognition of Self and Non-Self
A central problem in immunobiology is how immune cells discriminate between the body’s own structures and foreign material.
Antigens
- An antigen is any structure that can be specifically recognized by components of the immune system and elicit an immune response.
- Typical antigens:
- Proteins or parts of proteins from pathogens.
- Polysaccharides on the surface of bacteria.
- Modified self-proteins (e.g. in some autoimmune diseases).
- Often, only small regions of a molecule, epitopes, are directly recognized.
Major Histocompatibility Complex (MHC)
- MHC molecules are cell-surface proteins that present peptides to T cells.
- They play a critical role in:
- Displaying fragments of proteins from inside the cell (self or non-self).
- Enabling T cells to scan for infected or altered cells.
- MHC molecules are also central to transplantation biology:
- Differences in MHC between donor and recipient can lead to rejection reactions.
Central and Peripheral Tolerance
To avoid immune attacks against the body’s own tissues, tolerance mechanisms are essential:
- Central tolerance
- Occurs during T and B cell development in thymus and bone marrow.
- Cells that strongly recognize self-structures are eliminated or inactivated.
- Peripheral tolerance
- Acts on mature lymphocytes that have left the primary organs.
- Mechanisms include:
- Regulatory T cells that suppress harmful responses.
- Requirement for additional signals to fully activate lymphocytes.
- Induction of anergy (functional silence) or cell death in self-reactive cells.
Disturbances of tolerance can lead to autoimmune diseases, which are discussed in a separate chapter.
Nonspecific and Specific Immune Responses
Immunobiology distinguishes two major, interacting defense systems:
Nonspecific (Innate) Immunity
Characteristics:
- Present from birth and acts immediately.
- Broad recognition: Detects general patterns typical of pathogens (not individual antigens).
- No lasting memory of previous encounters.
Main components:
- Physical and chemical barriers (e.g. skin, mucous membranes, acidic environment).
- Phagocytic cells and NK cells.
- Soluble factors such as complement and certain cytokines.
- Inflammatory reactions that increase blood flow and recruit immune cells to sites of infection or injury.
Nonspecific immunity provides the first line of defense and shapes the subsequent specific response.
Specific (Adaptive) Immunity
Characteristics:
- Highly specific recognition of individual antigens via unique receptors on B and T cells.
- Develops during life through exposure to antigens.
- Immunological memory: Repeat contact with the same antigen leads to a faster and stronger response.
Main features:
- Clonal selection: Each lymphocyte has a particular receptor. When an antigen fits, that cell proliferates and forms a clone of effector and memory cells.
- Diversity: The body can generate a vast repertoire of receptors to recognize many different antigens.
- Memory: Long-lived memory cells enable more efficient responses to previously encountered pathogens.
The interplay between innate and adaptive immunity is central: innate mechanisms detect danger and present antigens, while adaptive mechanisms refine and remember the response.
Inflammation as a Protective Reaction
Inflammation is a local reaction of tissues to damage or infection and a key concept in immunobiology.
Hallmarks of Inflammation
Typical signs (classically described for skin):
- Redness and warmth (increased blood flow).
- Swelling (fluid and cell influx).
- Pain (mediators affecting nerve endings; pressure from swelling).
- Impaired function (due to pain and structural changes).
Functions of Inflammation
- Containment of pathogens
- Blood vessels become more permeable, enabling plasma proteins and immune cells to leave the bloodstream and reach the affected tissue.
- Clotting mechanisms can physically limit the spread of pathogens.
- Recruitment of immune cells
- Chemical signals (chemokines) attract phagocytes and other immune cells to the site of infection or injury.
- Initiation of repair
- Clearance of dead cells and debris.
- Activation of processes that restore tissue structure (regeneration or scar formation).
Inflammation is thus a necessary part of effective defense. However, if it becomes excessive, chronic, or misdirected, it contributes to tissue damage and disease.
Immunological Memory
Immunological memory is a defining feature of the specific immune response and a key subject of immunobiology.
Primary and Secondary Responses
- Primary response
- First contact with a particular antigen.
- Slower onset (days) and relatively low intensity.
- Generates effector cells that fight the current infection and memory cells that persist.
- Secondary (and further) responses
- Subsequent encounters with the same antigen.
- Faster and stronger reaction with more rapid antibody production and more efficient cellular responses.
- Often prevents the development of symptoms or leads to milder disease.
Basis of Memory
Memory is based on:
- Long-lived memory B cells
- Quickly differentiate into antibody-producing plasma cells upon re-exposure to the antigen.
- Long-lived memory T cells
- Rapidly expand and exert helper or cytotoxic functions.
This biological principle underlies the concept of immunization, which is dealt with in detail in its own chapters.
Immune System and the Whole Organism
Immunobiology emphasizes that the immune system is tightly integrated with other body systems.
Interaction with the Nervous and Endocrine Systems
- Stress, hormones, and neural signals influence immune cell activity and distribution.
- Conversely, immune mediators can act on the brain and endocrine organs, affecting:
- Fever and sickness behavior (e.g. fatigue, loss of appetite).
- Hormone production and metabolism.
This bidirectional communication helps the organism coordinate defense, energy use, and behavior during infection or injury.
Development and Aging of the Immune System
- Early life
- The immune system is still maturing.
- Newborns rely in part on maternal antibodies (e.g. via placenta or breast milk).
- Adulthood
- Immune responses are usually well balanced and capable.
- Older age
- Many components of the immune system decline in function.
- Increased susceptibility to infections and reduced effectiveness of vaccinations.
Understanding these age-related changes is important for preventive and therapeutic strategies.
Summary
Immunobiology examines how the immune system is built, how it recognizes and responds to threats, and how it maintains the balance between effective defense and protection of the body’s own tissues. Central themes include:
- Organization of immune cells and organs.
- Mechanisms of self–non-self recognition and tolerance.
- Cooperation between nonspecific and specific immune responses.
- Inflammation and immunological memory as core processes.
- Interactions of the immune system with other bodily systems and across the lifespan.
The following chapters delve deeper into particular aspects of immune function, including nonspecific and specific responses, antibodies, immunization strategies, and disorders such as autoimmunity and allergies.