Immunity - Biomedical Science 1

First line of defense: Characterized by a rapid response, occurring within minutes to hours.
Comprises physical, chemical, molecular, and cellular defenses.
Major components include:
- Skin
- Mucous membranes
- Phagocytic cells (e.g., neutrophils and macrophages)
- Lymphoid cells
- Plasma proteins
Key capabilities:
- Can distinguish self from non-self.
- Recognizes and reacts against microbial agents.
- Cells recognize common components of disease rather than specific invaders, unlike the adaptive immune system.

Serve as physical and chemical barriers against infection.
Skin:
- Creates a salty, acidic environment.
- Possesses antibacterial proteins that inhibit colonization and aid in microbial destruction.
Gastrointestinal (GI), respiratory, and Genitourinary (GU) tracts:
- Lined with mucus membranes composed of tightly packed epithelial cells.
- These membranes block and destroy microbes.
- They also secrete antimicrobial enzymes, proteins, and other substances.

Phagocytosis: Derived from "Phago" meaning "eat," literally "cell eating." This process involves engulfing invading cells.
Neutrophils:
- Kill invading microbes and then undergo programmed cell death.
- Pus is primarily composed of dead neutrophils.
Macrophages:
- Stationed in various organs, waiting to encounter pathogens.
- Highly efficient, capable of eating up to $100$ microbes.

Both belong to the family of leukocytes.
Innate Lymphoid Cells (ILCs):
- Recognize broad patterns associated with classes of microbes.
- Upon pathogen detection, ILCs release cytokines (proteins that mediate the actions of immune cells).
- Cytokines trigger the rapid mobilization of specific cells and mediators appropriate for the offending agent.
Natural Killer (NK) Cells:
- A type of cytotoxic lymphocyte.
- Capable of destroying other human cells.
- Seeks out invaded or damaged cells, surrounds them, and releases enzymes that destroy the cell membrane.

Characterized by a slower response, typically taking days to weeks.
Highly effective, employing focused recognition for each type of foreign agent.
Consists of lymphocytes and their products, including antibodies.
Cells in this system recognize a vast array of substances to develop specific immune responses for each.
Substances that elicit an adaptive immune response are called antigens.

Humoral Immunity:
- Generated by B lymphocytes.
- Mediated by antibodies.
- The principal defense against extracellular microbes and toxins.
Cell-Mediated Immunity (Cellular Immunity):
- Mediated by specific T lymphocytes (e.g., T-Helper, T-Cytotoxic lymphocytes).
- Defends against intracellular microbes, such as viruses.

Feature	Innate Immunity	Adaptive Immunity
Time of response	Rapid (minutes/hours)	Slower (days/weeks)
Diversity	Limited to classes or groups of microbes	Very large; specific for each unique antigen
Microbe recognition	General patterns on microbes; nonspecific	Specific to microbes and antigens
Nonself recognition	Yes	Yes
Response to repeated infection	Similar with each exposure	Immunologic memory; more rapid and efficient with subsequent exposure
Defense	Barriers (skin, mucous membranes), phagocytes, inflammation, fever	Cell killing; tagging of antigen by antibody for removal
Cells	Phagocytes (macrophages, neutrophils), innate lymphoid cells (natural killer cells), dendritic cells	T and B lymphocytes, dendritic cells
Molecules	Cytokines, complement proteins, acute-phase proteins, soluble mediators	Antibodies, cytokines, complement proteins

Final line of defense, activated once the innate immune system has initiated the inflammatory process.
Capabilities:
- Able to recognize and react to microbes and non-microbial substances, unlike the innate immune system.
- Distinguishes among different microbes and molecules, even closely related ones.
- Remembers the pathogen, leading to immunologic memory.
Components: Lymphocytes and their products.

Definition: Substances that elicit an immune response from the host.
Foreign molecules are recognized by receptors on immune cells and secreted proteins called antibodies (immunoglobulins).
Antibodies are specifically made in response to a particular antigen.
Examples:
- Microbial: Bacteria, fungi, viruses, protozoa, parasites.
- Non-microbial: Plant pollens, poison ivy resin, insect venom, transplanted organs.

Lymphocytes: The primary cells that recognize and respond to foreign antigens.
- T-lymphocytes:
  - Generated from stem cells in the bone marrow.
  - Mature in the thymus.
  - Function in peripheral tissues to produce cell-mediated immunity.
  - Also assist with antibody (immunoglobulin) production.
- B-lymphocytes:
  - Generated and mature in the bone marrow.
  - Essential for humoral (antibody-mediated) immunity.
Key properties of T & B lymphocytes for adaptive immune response:
- Diversity
- Memory
- Self-non-self recognition

Considered the "call the shots" directors for the immune system.
Cannot kill pathogens directly.
Their primary role is to activate and direct other cells that will kill pathogens.

Part of the humoral immune response.
Patrol the bloodstream until they are signaled by Helper T cells.
Covered in antibodies that will bind to specific antigens.
- For example, a B-cell may have up to $100,000$ antibodies for the virus causing the common cold, while another may have antibodies for chickenpox.
When a B-cell encounters and recognizes a pathogen, it attaches and then clones itself numerous times.
The new clones become plasma/effector cells that rapidly produce large quantities of specific antibodies (+++).

IgE:
- Primarily involved in inflammation and allergic responses.
- Also plays a role in combating parasitic infections.
- Binds to mast cells (and basophils).
- Binding triggers the release of histamine and other mediators of inflammation.
- Overstimulation of IgE-mediated responses can lead to complications such as low blood pressure and leaky capillaries, resulting in swelling/edema.