Adaptive Immunity

Overview of the Adaptive Immune System

The adaptive immune system is a specialized defense mechanism that provides specific, long-lasting protection against pathogens. Unlike innate immunity, adaptive immunity develops after exposure to antigens and has memory, allowing for a faster and stronger response upon subsequent exposures.

• Specificity: Targets specific antigens using highly specialized cells and molecules.

• Memory: Remembers previous encounters with pathogens for enhanced future responses.

Types of Adaptive Immunity

• Humoral Immunity: Mediated by B lymphocytes and antibodies; defends against extracellular pathogens.

• Cellular Immunity: Mediated by T lymphocytes; targets infected or abnormal cells.

Comparison: Adaptive vs. Innate Immunity

• Innate Immunity: Non-specific, immediate response, no memory.

• Adaptive Immunity: Specific, delayed response, develops memory.

Humoral Immunity

Role of B Lymphocytes

B lymphocytes (B cells) are responsible for producing antibodies that neutralize pathogens in body fluids.

• B cells recognize antigens via their B cell receptors (BCRs).

• Upon activation, B cells differentiate into plasma cells (antibody producers) and memory B cells.

Antibodies (Immunoglobulins)

• Immunoglobulins (Ig): Y-shaped proteins that specifically bind antigens.

• Five main classes: IgG, IgM, IgA, IgD, IgE.

Cellular Immunity

Role of T Lymphocytes

T lymphocytes (T cells) recognize and destroy infected or abnormal cells.

• CD4+ T cells (Helper T cells): Coordinate immune responses by secreting cytokines.

• CD8+ T cells (Cytotoxic T cells): Kill infected or cancerous cells.

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

Interaction of Humoral and Cellular Immunity

• Helper T cells activate B cells and cytotoxic T cells, linking both arms of adaptive immunity.

Key Concepts and Definitions

Cytokines

• Cytokines: Small proteins released by immune cells that regulate immunity, inflammation, and hematopoiesis.

Antigens and Haptens

• Antigen: Any substance that elicits an immune response (e.g., proteins on bacteria or viruses).

• Example: Surface protein of Streptococcus pyogenes.

• Hapten: Small molecule that is not immunogenic by itself but can elicit an immune response when attached to a larger carrier protein.

Major Histocompatibility Complex (MHC)

• MHC: Cell surface proteins essential for antigen presentation to T cells.

• Class I MHC: Present on all nucleated cells; present endogenous antigens to CD8+ T cells.

• Class II MHC: Present on antigen-presenting cells; present exogenous antigens to CD4+ T cells.

B Cell Activation and Types

B Cell Activation

• B cells are activated by antigen binding and, often, help from helper T cells (T-dependent activation).

Types of B Cells

• Plasma Cells: Produce and secrete antibodies.

• Memory B Cells: Provide long-term immunity by responding rapidly to future exposures.

T-Dependent vs. T-Independent Antigens

• T-Dependent Antigens: Require T cell help for B cell activation; usually proteins.

• T-Independent Antigens: Can activate B cells without T cell help; often polysaccharides.

Antigen-Antibody Interactions

• Binding of antibodies to antigens can neutralize pathogens, opsonize for phagocytosis, or activate complement.

Antigen Presenting Cells (APCs)

Types and Functions

• Dendritic Cells and Macrophages are primary APCs.

• APCs process and present antigens via MHC molecules to T cells, initiating adaptive responses.

Specialized Immune Cells

CD4+, CD8+, and T Regulatory Cells

• CD4+ T cells: Activate B cells, cytotoxic T cells, and macrophages.

• CD8+ T cells: Destroy infected or abnormal cells.

• T Regulatory cells: Suppress immune responses to maintain tolerance.

Natural Killer (NK) Cells

• NK cells destroy infected or cancerous cells without prior sensitization.

Immunological Processes

Apoptosis

• Apoptosis: Programmed cell death; eliminates harmful or unnecessary cells without inflammation.

Primary vs. Secondary Immune Response

• Primary Response: First exposure; slower, lower antibody production.

• Secondary Response: Subsequent exposure; faster, higher antibody production due to memory cells.

Acquisition of Adaptive Immunity

Summary Table: Key Components of Adaptive Immunity

Additional info: Academic context and examples have been added to expand on brief points and ensure completeness.