Chapter 16: Adaptive Immunity – Structure and Function

Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity is the specialized branch of the immune system that provides a targeted, learned defense against specific pathogens. It is characterized by its ability to recognize and remember particular antigens, leading to a more effective response upon subsequent exposures.

• Specificity: The immune system recognizes unique antigens and mounts a precise response.

• Inducibility: Adaptive immunity is activated only after exposure to a specific antigen.

• Clonality: Activated B or T cells proliferate to form clones targeting the same antigen.

• Unresponsiveness to self: The immune system avoids attacking the body’s own cells, preventing autoimmunity.

• Memory: Memory cells persist after initial exposure, enabling a rapid response to future encounters with the same pathogen.

Lymphatic System and Organs

Structure and Function

The lymphatic system is a network of vessels, organs, and tissues essential for fluid balance and immune defense. It returns interstitial fluid to the bloodstream, filters lymph for pathogens, and houses immune cells.

• Primary Lymphoid Organs: Sites of lymphocyte formation and maturation.

  • Red Bone Marrow: All blood cells form here; B cells mature here.

  • Thymus: T cells mature and learn self-tolerance; largest during childhood.

• Secondary Lymphoid Organs: Sites where lymphocytes encounter antigens.

  • Lymph Nodes

  • Spleen

  • Tonsils

Lymphocytes and Their Roles

B Lymphocytes (B cells)

• Mature in red bone marrow.

• Produce antibodies (humoral immunity).

• Differentiate into plasma cells that secrete antibodies.

T Lymphocytes (T cells)

• Mature in the thymus.

• Responsible for cell-mediated immunity.

• Handle a variety of immune functions, including direct killing of infected cells and regulation of immune responses.

Antibodies (Immunoglobulins)

Functions of Antibodies

• Neutralization: Bind to pathogens or toxins, blocking their entry into cells.

• Opsonization: Coat pathogens, enhancing phagocytosis by immune cells.

• Agglutination: Bind multiple pathogens, causing clumping for easier removal.

• Antibody-Dependent Cellular Cytotoxicity (ADCC): Mark infected or abnormal cells for destruction by natural killer (NK) cells.

Classes of Antibodies

• IgM: First antibody produced; excellent at activating complement and agglutination.

• IgG: Most common in blood; crosses placenta; long-term immunity; effective in neutralization, opsonization, and complement activation.

• IgA: Found in mucosal secretions; protects mucosal surfaces.

• IgE: Involved in allergic responses and defense against parasitic worms; triggers histamine release from mast cells.

• IgD: Found on B cell surfaces; helps activate B cells; function not fully understood.

Major Histocompatibility Complex (MHC)

Structure and Function

The MHC is a group of glycoproteins on cell surfaces that present antigenic peptides to T cells, enabling the immune system to distinguish self from non-self.

• MHC I: Present on all nucleated cells; displays endogenous antigens; recognized by cytotoxic T cells (CD8+).

• MHC II: Present only on professional antigen-presenting cells (APCs); displays exogenous antigens; recognized by helper T cells (CD4+).

Importance: MHC molecules are critical for tissue compatibility in transplantation and for initiating adaptive immune responses.

Antigen-Presenting Cells (APCs)

Role in Immunity

• APCs ingest pathogens, process them, and present antigen fragments on MHC II molecules.

• Major APCs: Dendritic cells (most important), macrophages, and B cells.

• APCs activate helper T cells (CD4+), initiating adaptive immunity.

Antigens

Types and Recognition

• Antigens: Substances recognized as foreign, containing specific regions (epitopes) targeted by immune cells.

• Types of Antigens:

  • Exogenous antigens: Enter the body from the environment.

  • Endogenous antigens: Generated within cells (e.g., viral proteins).

  • Autoantigens: Derived from normal cellular processes; usually ignored by the immune system.

Types of T Lymphocytes

Classification and Functions

The following table summarizes the main types of T lymphocytes, their maturation site, surface markers, and notable secretions:

Lymphocyte	Site of Maturation	Representative Cell-Surface Glycoproteins	Notable Secretions
Helper T cell type 1 (Th1)	Thymus	CD4 and distinctive TCR	Interleukin 2, IFN-γ
Helper T cell type 2 (Th2)	Thymus	CD4 and distinctive TCR	Interleukin 4 and 5
Cytotoxic T cell (Tc)	Thymus	CD8, CD95L, and distinctive TCR	Perforin, granzyme
Regulatory T cell (Tr)	Thymus	CD4, CD25, and distinctive TCR	Interleukin 10

Clonal Deletion of T Cells

Mechanism and Importance

Clonal deletion is a quality-control process in the thymus that ensures T cells do not attack the body’s own tissues. Developing T cells are tested for their ability to recognize MHC and avoid strong recognition of self-antigens. Cells failing these tests undergo apoptosis.

• T cells that do not recognize MHC: Apoptosis (eliminated).

• T cells that strongly recognize self-antigens: Apoptosis (prevent autoimmunity).

• Some self-reactive T cells become Regulatory T cells (Tregs): Suppress overactive immune responses and prevent autoimmunity.

• T cells that recognize MHC and foreign epitopes: Survive and become functional T cells.

Types of Immunity

Active and Passive Immunity

• Naturally acquired active immunity: Long-lasting; results from infection and antibody production by the individual (e.g., recovering from illness).

• Naturally acquired passive immunity: Immediate, temporary; transfer of antibodies (e.g., via breast milk).

• Artificially acquired active immunity: Long-lasting; induced by vaccination, stimulating antibody production.

• Artificially acquired passive immunity: Immediate, short-term; injection of pre-made antibodies (e.g., gamma globulins, antivenoms).