Innate and Adaptive Immunity: Key Concepts and Mechanisms

Innate Immunity

Overview of Innate Immunity

Innate immunity is the body's first and immediate defense against pathogens. It is non-specific, meaning it does not target particular pathogens, and is present from birth. The innate immune system includes physical barriers, chemical factors, cellular components, and processes that work together to prevent infection.

• Innate immunity: The natural, non-specific defense mechanisms present at birth.

• Competitive inhibition: Normal microbiota compete with pathogens for nutrients and space, limiting pathogen growth.

First Line of Defense: Physical and Chemical Barriers

The first line of defense consists of barriers that prevent pathogens from entering the body. These include the skin, mucous membranes, and associated secretions.

• Skin: Acts as a physical barrier; contains sebum (an oily substance) that lowers skin pH and inhibits microbial growth.

• Mucous membranes: Line the respiratory, digestive, urinary, and reproductive tracts; trap microbes in mucus.

• Lysozyme: An enzyme found in tears, saliva, and mucus that breaks down bacterial cell walls.

• Physical factors: Include skin, mucous membranes, cilia, and tears.

• Chemical factors: Include sebum, lysozyme, gastric acid, and antimicrobial peptides.

Role of Normal Microbiota

Normal microbiota are beneficial microorganisms that reside on and within the body. They play a crucial role in innate immunity by outcompeting pathogens and producing substances that inhibit their growth.

• Competitive inhibition: Microbiota prevent colonization by pathogens.

• Example: Lactobacillus in the vagina produces acid, lowering pH and inhibiting pathogens.

Second Line of Defense: Cellular and Chemical Components

If pathogens breach the first line of defense, the second line of defense is activated. This includes various cells and proteins that attack invaders.

• Formed elements: Cellular components of blood, including leukocytes (white blood cells), platelets, and others.

• Plasma: The liquid portion of blood containing proteins, including complement proteins.

• Leukocytes: White blood cells; classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes, lymphocytes).

• Phagocytes: Cells that ingest and destroy pathogens; include neutrophils and monocytes (which become macrophages).

• Phagocytosis: The process by which phagocytes engulf and digest microbes. Steps include formation of a phagosome, fusion with a lysosome to form a phagolysosome, and destruction of the microbe.

• NK cells (Natural Killer cells): Destroy infected or abnormal cells by releasing cytotoxic granules.

• Interferons: Proteins released by virus-infected cells that inhibit viral replication in neighboring cells.

• Complement system: A group of proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

• Toll-like receptors: Proteins on immune cells that recognize pathogen-associated molecular patterns (PAMPs) and trigger immune responses.

Inflammation and Fever

Inflammation and fever are important responses to infection and injury.

• Inflammation: Localized response to infection or injury characterized by redness, heat, swelling, and pain. Can be acute (short-term) or chronic (long-term).

• Fever: Elevated body temperature induced by pyrogens (substances that trigger fever), which enhances immune activity and inhibits pathogen growth.

Classification of Leukocytes

Leukocytes are classified based on their appearance and function.

Leukocyte Type	Granulocyte/Agranulocyte	Main Function
Neutrophil	Granulocyte	Phagocytosis of bacteria and debris
Eosinophil	Granulocyte	Defense against parasites; involved in allergic responses
Basophil	Granulocyte	Release histamine; involved in inflammation
Monocyte	Agranulocyte	Differentiate into macrophages; phagocytosis
Lymphocyte	Agranulocyte	Includes NK cells (innate immunity) and B/T cells (adaptive immunity)

Stages of Phagocytosis

Phagocytosis is a multi-step process:

1. Chemotaxis: Phagocyte moves toward the site of infection.

2. Adherence: Phagocyte attaches to the microbe.

3. Ingestion: Microbe is engulfed, forming a phagosome.

4. Fusion: Phagosome fuses with lysosome to form a phagolysosome.

5. Digestion: Enzymes destroy the microbe.

6. Exocytosis: Debris is expelled from the cell.

Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity is a specific defense system that develops in response to exposure to pathogens. It involves recognition of specific antigens and provides long-lasting protection, including memory of past infections.

• Adaptive immunity: Specific, acquired defense mechanisms; includes humoral and cell-mediated responses.

• Antigen: Any substance that triggers an immune response; includes exogenous (external), endogenous (internal), and autoantigens (self).

• Major histocompatibility complex (MHC): Proteins on cell surfaces that present antigens to T cells.

• Antigen-presenting cell (APC): Cells (e.g., dendritic cells, macrophages) that process and present antigens to T cells.

Humoral and Cell-Mediated Immune Responses

Adaptive immunity is divided into humoral (antibody-mediated) and cell-mediated responses.

• Humoral immune response: Involves B lymphocytes and production of antibodies (immunoglobulins).

• Cell-mediated immune response: Involves T lymphocytes (cytotoxic, helper, regulatory) that target infected or abnormal cells.

• Lymph and lymph nodes: Lymphatic system transports immune cells and filters pathogens.

Antibodies and Immunoglobulins

Antibodies are proteins produced by B cells that bind to specific antigens. There are five main classes, each with distinct functions.

Class	Function	Location	Response	Prevalence	Crosses Placenta
IgM	First antibody produced; agglutination	Blood, lymph	Primary	Low	No
IgG	Main antibody; opsonization, neutralization	Blood, tissues	Secondary	High	Yes
IgA	Protects mucosal surfaces	Mucus, saliva, tears	Primary/Secondary	Moderate	No
IgE	Allergic responses, defense against parasites	Blood	Primary/Secondary	Low	No
IgD	Unknown, B cell receptor	Blood, B cell surface	Primary	Low	No

• Fab: Antigen-binding fragment of antibody.

• Fc: Constant region; interacts with immune cells.

Types of T Cells and B Cells

T cells are classified based on their function:

• Cytotoxic T cells (CD8+): Destroy infected cells.

• Helper T cells (CD4+): Activate B cells and other immune cells.

• Regulatory T cells: Suppress immune responses to prevent autoimmunity.

• B lymphocytes: Produce antibodies; differentiate into plasma cells and memory cells.

• Memory cells: Provide long-term immunity by responding rapidly to subsequent exposures.

Antigen-Antibody Reactions

Antigen-antibody interactions lead to several outcomes:

• Agglutination: Clumping of pathogens for easier removal.

• Opsonization: Coating pathogens to enhance phagocytosis.

• Neutralization: Blocking pathogen activity.

• Complement activation: Triggering the complement system for pathogen destruction.

• Antibody-dependent cellular cytotoxicity (ADCC): Immune cells destroy antibody-coated targets.

Primary vs. Secondary Immune Response

The immune system responds differently to first and subsequent exposures to an antigen.

• Primary response: First exposure; slower, mainly IgM produced.

• Secondary response: Subsequent exposure; faster, mainly IgG produced due to memory cells.

Key Equations and Concepts

• Antibody structure: Antibodies are Y-shaped molecules with two antigen-binding sites (Fab) and a constant region (Fc).

• Complement activation: Classical pathway is triggered by antigen-antibody complexes.

Equation for Antibody-Antigen Binding:

$\text{Ag} + \text{Ab} \rightarrow \text{Ag-Ab complex}$

Equation for Complement Activation (Classical Pathway):

$\text{Ag-Ab complex} + \text{C1} \rightarrow \text{C1 activation} \rightarrow \text{Cascade of complement proteins}$

Equation for Phagocytosis:

$\text{Phagocyte} + \text{Microbe} \rightarrow \text{Phagosome} \rightarrow \text{Phagolysosome} \rightarrow \text{Digestion}$

Additional info: Academic context was added to expand brief terminology and learning objectives into full explanations, including tables and equations for clarity.