Key Themes and Concepts:
The Immune System is Essential for Life:
Immune systems are necessary for the survival of all animals, protecting against parasites and pathogens. Without a functional immune system, animals are unable to defend themselves against microbial threats and rapidly succumb to infection. "Without means for self-defense, no individual plants or animals would exist on Earth."
The text highlights the evolutionary pressure that led to the development of these systems stating, "That constant evolutionary pressure, those threats to the survival of individual organisms, produced immune systems."
Immune deficiency, whether primary or acquired, leads to rapid disappearance of the individual.
Innate vs. Adaptive Immunity:
The immune system is divided into two interconnected branches: the innate and adaptive immune systems.
The innate immune system provides a rapid, non-specific, and immediate defense. It utilizes pattern-recognition receptors (PRRs) to identify foreign molecules and responds in a predictable manner to threats. It "represents the first line of defense against invading pathogens". However, the innate immune system's efficiency does not improve with repeated exposure to a given microbe and has no immunological memory.
The adaptive immune system provides a slower, but more specific and customized response. It develops over time and "is also referred to as the acquired immune response because the animal does not directly inherit these immune responses but acquires them within its lifetime." It is characterized by the development of immunological memory which allows for more effective responses upon subsequent exposure to the same pathogen.
Cells of Defense:
Leukocytes (white blood cells) are the primary cells of the immune system. This includes:
T lymphocytes (T cells): Primarily involved in adaptive immunity, further differentiating into T-helper (Th) cells and cytotoxic T (Tc) cells, which perform specialized functions.
B lymphocytes (B cells): Also involved in adaptive immunity, ultimately differentiating into plasma cells that produce antibodies.
Neutrophils, basophils, eosinophils, and NK cells: Primarily involved in innate immunity
Monocytes, Macrophages and Dendritic cells (DCs): Involved in both innate and adaptive immunity.
The text notes that neutrophils comprise the highest percentage of leukocytes, followed by lymphocytes.
Communication Within the Immune System:
The immune system relies on a complex communication network involving:
Cytokines: Soluble proteins or glycoproteins that act as messengers between immune cells. Different categories of cytokines include interleukins (ILs), interferons (IFNs), chemokines, and colony-stimulating factors (CSFs).
"Essentially, all cells use cytokines (soluble proteins or glycoproteins) to communicate with one another."
Other signaling molecules: such as histamine, serotonin, leukotrienes and prostaglandins.
The text notes that histamine and serotonin are released from mast cells and basophils.
Cell-to-cell contact: Direct interaction between proteins and receptors on cell surfaces are crucial for communication, especially for T-cell activation.
"Much of the communication between immune cells in the adaptive immune system relies on direct molecular interactions between proteins, complexes, and receptors on cell surfaces."
Mechanisms of Innate Immunity:
Pattern Recognition Receptors (PRRs): These receptors on immune cells recognize molecular patterns specific to pathogens, allowing for an immediate response.
Complement system: A series of plasma proteins that, when activated, can directly lyse microbes.
Phagocytosis: Phagocytes engulf and destroy microbes within phagolysosomes. The text lists macrophages, DCs, and neutrophils as phagocytes.
Natural Killer (NK) cells: These cells can induce apoptosis (programmed cell death) in infected or abnormal cells.
Mechanisms of Adaptive Immunity:
Antigens: Molecules that induce adaptive immune responses.
"Molecules that induce adaptive immune responses are called antigens. In the natural world, antigens are molecular bits of organisms, including pathogens."
Lymphocytes (T and B cells): The key players in adaptive immunity, possessing unique antigen receptors (TCRs and BCRs). These receptors recognize and respond to specific antigens.
Clonal expansion: Upon activation, lymphocytes specific to a particular antigen proliferate rapidly, creating a large army of cells with the same specificity.
"Once these cells are activated by APCs, they begin to divide rapidly and further differentiate."
Antibodies: Proteins produced by B cells that bind to specific antigens and neutralize them.
"Plasma cells secrete antibodies (essentially a soluble form of the BCR) into lymph and blood."
Immunological Memory: The ability of the adaptive immune system to remember prior encounters with antigens, leading to faster and stronger responses upon subsequent exposure.
"Those differences between primary and subsequent immune responses constitute immunological memory—the adaptive immune system’s ability to remember it has seen an antigen before and respond differently on a second encounter with that antigen."
Clinical Correlation: Simian Immunodeficiency Virus (SIV):
SIV infection in macaques serves as a model for understanding acquired immunodeficiency. The text highlights the story of Daniel et al who found a retrovirus as the cause of immunodeficiency in Rhesus Macaque monkeys.
SIV specifically targets and destroys Th cells, critical for both innate and adaptive immune responses.
"SIV specifically infected T cells, particularly Th cells, a type of T cell necessary for almost all types of adaptive immune responses..."
The resulting T-cell depletion leads to immunodeficiency, making the animals susceptible to opportunistic infections.
The text notes that in Macaques, SIV infection results in a chronic wasting disease and opportunistic infections like oroesophageal candidiasis.
The clinical follow-up question asks the reader to explain the opportunistic infections in SIV-infected monkeys.
The explanation is that SIV destroys T cells, weakening the immune system and allowing normally harmless microbes like Candida albicans to flourish.