Adaptive Immunity: Mechanisms and Cellular Players

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Adaptive Immunity: The Third Line of Defense

Overview of Immune Defense Lines

The immune system is organized into three main lines of defense: innate barrier defenses, innate cellular and molecular defenses, and adaptive defenses. The adaptive immune response is the body's third and final line of defense, providing a highly specific and long-lasting response to pathogens.

Innate Immunity: Immediate, non-specific defense mechanisms (e.g., skin, mucous membranes, phagocytes).
Adaptive Immunity: Delayed, highly specific response involving lymphocytes (T cells and B cells) and immunological memory.

Three lines of immune defense diagram

Key Features of Adaptive Immunity

Specificity: Targets specific antigens unique to each pathogen.
Memory: Remembers previous encounters, resulting in a faster and stronger response upon re-exposure.
Delayed Response: Takes days to weeks to mount a primary response, but secondary responses are rapid and effective.
Interaction with Innate Immunity: Adaptive responses are activated when innate defenses are insufficient.

Primary and secondary antibody response graph

Example: Upon first exposure to an antigen, IgM antibodies are produced first, followed by IgG. Upon secondary exposure, IgG levels rise rapidly and to a higher level, demonstrating immunological memory.

Branches and Stages of Adaptive Immunity

Cellular and Humoral Responses

The adaptive immune system is divided into two main branches:

Cellular Response (T cell–mediated immunity): Involves T cells that target infected or abnormal cells.
Humoral Response (B cell and antibody–mediated immunity): Involves B cells and the production of antibodies that neutralize pathogens in body fluids.

Both branches progress through four main stages: antigen presentation, lymphocyte activation, proliferation and differentiation, and antigen elimination with memory formation.

Stages of adaptive immunity diagram

Stage 1: Antigen Presentation

Antigen-presenting cells (APCs), such as dendritic cells and macrophages, capture antigens and present them to T cells using major histocompatibility complex (MHC) molecules. B cells can recognize antigens directly without APCs.

APCs: Display processed antigens on their surface to T cells.
B Cells: Can bind directly to antigens via their B cell receptors (BCRs).

Antigen presenting cell and helper T cell cartoon

Stage 2: Lymphocyte Activation

Upon antigen recognition, lymphocytes (T and B cells) are activated by cytokines. Activated T cells can further stimulate B cell activation, leading to a coordinated immune response.

Stage 3: Lymphocyte Proliferation and Differentiation

Activated lymphocytes undergo clonal expansion, producing many identical cells. These differentiate into:

Effector Cells: Actively engage in eliminating the antigen.
Memory Cells: Persist in lymphatic tissues for rapid response upon re-exposure.

Lymphocyte proliferation and differentiation diagram

Stage 4: Antigen Elimination and Memory

Effector cells eliminate the antigen, after which most die off. Memory cells remain, providing long-term immunity.

Antigen Recognition and Immunogenicity

Antigen Structure and Immunogenicity

An antigen is any substance that can trigger an immune response. Most antigens are proteins or polysaccharides from pathogens, but they can also be lipids or small molecules (haptens).

Immunogenicity: The ability of an antigen to provoke an immune response. Proteins are the most immunogenic, followed by polysaccharides, then lipids.
Haptens: Small molecules that are only immunogenic when attached to a larger carrier molecule.

Immunogenicity of different antigen types

Epitopes and Antigen Receptors

Epitopes are specific regions on antigens recognized by B and T cells. Each lymphocyte expresses receptors (TCRs or BCRs) that recognize a single epitope.

Antigen with multiple epitopes T cell and B cell receptors binding epitopes

Clonal Selection and Expansion

When a lymphocyte binds its specific epitope, it is activated and undergoes clonal expansion, producing effector and memory cells. B cells differentiate into plasma cells that secrete antibodies.

B cell activation and antibody secretion

T Cells and B Cells: Types and Functions

T Cell Subtypes

T Helper Cells (TH, CD4+): Coordinate immune responses by releasing cytokines and activating other immune cells. Subclasses include TH1 (cellular response), TH2 (humoral response), and T regulatory cells (immune regulation).
T Cytotoxic Cells (TC, CD8+): Directly destroy infected, cancerous, or foreign cells.

T helper and T cytotoxic cell roles T helper cell subclasses

B Cells

B Cells: Produce antibodies and present antigens. Upon activation, they differentiate into plasma cells (antibody-secreting) and memory B cells.

Summary Table: Comparing T Cells and B Cells

	T cells	B cells
Adaptive immunity mediated	Cellular branch	Humoral branch
Include	TH (CD4+), TC (CD8+)	Plasma cells
Site of maturation	Thymus	Bone marrow
Found in	Mainly lymphatic tissues	Mainly lymphatic tissues
Antigen recognition receptors	TCRs	BCRs (antibodies)
Require APC for activation?	Yes	No
Memory cells made?	Yes	Yes
MHC proteins present	MHC I	MHC I and II
Considered APCs?	No	Yes

Major Histocompatibility Complex (MHC) and Antigen Presentation

MHC I and MHC II Pathways

MHC I: Present on all nucleated cells; presents intracellular antigens to CD8+ T cytotoxic cells.
MHC II: Present only on APCs; presents extracellular antigens to CD4+ T helper cells.

MHC I antigen presentation pathway

Example: Viral proteins produced inside an infected cell are processed and presented on MHC I molecules to cytotoxic T cells.

APC presenting antigens to T cells in lymph node

Activation of T Cells

T cells require two signals for full activation:

Primary Signal: TCR binds to MHC–antigen complex.
Secondary Signal: Co-stimulatory proteins (e.g., B7 on APC binds CD28 on T cell).

T cell activation signals

Clonal Expansion and Differentiation of T Cells

Activated T cells proliferate and differentiate into effector and memory cells. Cytokines released during this process determine the specific subclass of T helper cells (e.g., TH1, TH2).

T cell proliferation and differentiation Cytokine influence on T helper cell differentiation

Effector Functions and Immunological Memory

Antigen Elimination

T Cytotoxic Cells: Release perforins and granzymes to induce apoptosis in infected or abnormal cells.
T Helper Cells: Release cytokines to activate B cells, cytotoxic T cells, and innate immune cells.

T cytotoxic cell killing mechanism

Immunological Memory

Memory T and B cells persist in lymphatic tissues, enabling a rapid and robust response upon subsequent exposures to the same antigen. This underlies the effectiveness of vaccines and long-term immunity.

Summary

Adaptive immunity is highly specific, involves memory, and is mediated by T and B lymphocytes.
It is divided into cellular (T cell-mediated) and humoral (B cell/antibody-mediated) branches.
Antigen presentation, lymphocyte activation, proliferation/differentiation, and antigen elimination/memory are the four main stages.
MHC molecules are essential for antigen presentation and self/non-self recognition.
Immunological memory ensures faster and stronger responses upon re-exposure to pathogens.