Innate and Adaptive Immunity: Key Concepts, Processes, and Clinical Applications

Innate and Adaptive Immunity

Overview of Immune Responses

The immune system is divided into two main branches: innate immunity and adaptive (acquired) immunity. Each branch has distinct features, mechanisms, and roles in defending the body against pathogens.

• Innate Immunity: The first line of defense, present from birth, non-specific, and responds rapidly to pathogens.

• Adaptive Immunity: Develops after exposure to specific antigens, is highly specific, and provides immunological memory for faster responses upon re-exposure.

Comparison of Innate and Adaptive Immunity

The following table summarizes the key differences and features of innate and adaptive immunity:

Feature	Innate Immunity	Adaptive Immunity
Bacterial Microbiota	Yes	No
Present at birth	Yes	No
Non-specific immune response	Yes	No
Faster immune response	Yes (minutes to hours)	No (days to weeks)
Physical barriers	Yes (skin, mucous membranes)	No
Inflammation, fever, & complement activation	Yes	Limited (complement can be activated by antibodies)
Phagocytosis	Yes (neutrophils, macrophages)	No (but can be enhanced by antibodies)
Antibody production	No	Yes (B cells)
Activated T-helper cells	No	Yes
Immunological memory	No	Yes
Specific immune response	No	Yes
Slower immune response	No	Yes (primary response is slower)
Cytotoxic cells production	No	Yes (Cytotoxic T cells)

Key Concepts in Immunology

Antigens and Epitopes

• Antigen: Any substance that can induce an immune response, typically a protein or polysaccharide on the surface of pathogens.

• Epitope: The specific part of an antigen recognized by antibodies or T cell receptors.

Physical and Chemical Barriers

• Skin: Acts as a physical barrier to prevent pathogen entry.

• Mucous membranes: Trap and help expel microbes.

• Chemical barriers: Include lysozyme in saliva and tears, acidic pH in the stomach.

Cells of the Immune System

• Phagocytes: Cells such as neutrophils and macrophages that engulf and destroy pathogens.

• Lymphocytes: Include B cells (produce antibodies) and T cells (helper and cytotoxic functions).

• Natural Killer (NK) cells: Part of innate immunity, target virus-infected and tumor cells.

Inflammation

Inflammation is a complex response to infection or injury, characterized by redness, heat, swelling, pain, and loss of function. It serves to contain and eliminate pathogens and initiate tissue repair.

• Key events: Vasodilation, increased vascular permeability, migration of phagocytes, and tissue repair.

• Benefits: Limits spread of infection, recruits immune cells, and promotes healing.

Complement System

• A group of plasma proteins that enhance (complement) the ability of antibodies and phagocytic cells to clear microbes.

• Activated via classical, alternative, or lectin pathways.

• Functions include opsonization, chemotaxis, and cell lysis.

Adaptive Immunity: Humoral and Cell-Mediated Responses

• Humoral immunity: Mediated by B cells and antibodies, effective against extracellular pathogens.

• Cell-mediated immunity: Mediated by T cells (helper and cytotoxic), effective against intracellular pathogens.

Immunological Memory

• After initial exposure, memory B and T cells are generated, leading to a faster and stronger response upon re-exposure to the same antigen.

• Primary response: Slower, lower antibody titer.

• Secondary response: Faster, higher antibody titer, mainly IgG.

Types of Immunity

• Active immunity: Results from direct exposure to antigen (infection or vaccination); long-lasting.

• Passive immunity: Transfer of antibodies from another source (e.g., maternal antibodies, antiserum); temporary.

• Natural vs. Artificial: Natural (infection, maternal transfer), Artificial (vaccination, antibody therapy).

Clinical Applications and Laboratory Tests

Serological Tests

• ELISA (Enzyme-Linked Immunosorbent Assay): Detects and quantifies antibodies or antigens in serum.

• Agglutination tests: Detect antibodies by their ability to cause clumping of antigen-coated particles.

• Titer: The concentration of antibodies in serum, determined by serial dilution.

Vaccines

• Live attenuated vaccines: Contain weakened pathogens.

• Inactivated vaccines: Contain killed pathogens.

• Subunit, conjugate, and toxoid vaccines: Contain specific components of pathogens or inactivated toxins.

Hypersensitivity Reactions

• Type I (Immediate): IgE-mediated, includes allergies and anaphylaxis.

• Type II (Cytotoxic): Antibody-mediated destruction of cells (e.g., blood transfusion reactions).

• Type III (Immune complex): Deposition of antigen-antibody complexes (e.g., serum sickness).

• Type IV (Delayed-type): T cell-mediated (e.g., contact dermatitis, transplant rejection).

Key Equations and Concepts

• Antibody titer calculation:

$\text{Titer} = \frac{1}{\text{highest dilution showing a positive reaction}}$

• Opsonization: The process by which antibodies or complement proteins coat a pathogen to enhance phagocytosis.

• Margination: The movement of leukocytes to the periphery of blood vessels at sites of inflammation.

Summary Table: Types of Immunity

Type	Natural	Artificial
Active	Infection	Vaccination
Passive	Maternal antibodies	Antibody therapy (antiserum)

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the detailed comparison tables and definitions.

• Specific answers to the numbered questions would require further elaboration, but the above notes provide a comprehensive framework for understanding the key concepts in innate and adaptive immunity, as well as their clinical relevance.