Comprehensive Guide to Adaptive Immunity: T‑Cells, B‑Cells, and Humoral Response

Adaptive Immunity

3rd Line of Defense activated when innate (1st & 2nd line) fail; specific response to particular antigens; learns and remembers for faster response upon re-exposure.

Specificity

Targets a specific antigen.

Memory

Secondary exposure leads to rapid, strong response.

Delayed onset

Takes days-weeks on first exposure.

T-cell mediated

Branch of adaptive immunity involving T cells that destroys infected or cancerous cells.

Humoral Response

Branch of adaptive immunity involving B cells that produces antibodies to neutralize pathogens.

Antigen Presentation

APCs (e.g., dendritic cells) process antigen and present it to T cells; B cells can bind antigen directly.

Lymphocyte Activation

T or B cell activated by antigen and cytokines, starting clonal expansion.

Proliferation & Differentiation

Activated cells divide to form effector (active fighters) and memory cells.

Antigen Elimination & Memory

Effector cells remove antigen and die off; memory cells remain in lymphatic tissue for years.

T Cells

Mature in Thymus; involved in direct cellular defense and immune regulation.

B Cells

Mature in Bone marrow; involved in antibody production (humoral defense).

Antigen

Any molecule that triggers an immune response.

Immunogenicity

Ability of antigen to provoke a response, influenced by size, complexity, and composition.

Epitope

Specific part of antigen recognized by TCR/BCR; one antigen can have multiple epitopes.

T Cell Receptor (TCR)

Recognizes epitopes presented by APCs.

B Cell Receptor (BCR)

Directly binds free antigen and secretes antibodies upon activation.

Clonal Expansion

Activation leads to many identical clones; some become effector cells, others become memory cells.

T Helper Cells (CD4⁺)

Coordinate immune response via cytokines; stimulate B cells, macrophages, and T cytotoxic cells.

T Cytotoxic Cells (CD8⁺)

Directly kill infected, cancerous, or transplanted cells; recognize antigen + MHC I on target cells.

Self-Tolerance Screening

Purpose is to prevent autoimmune reactions.

MHC Proteins

Body's 'uniform' that identifies self; must match for organ transplants to prevent rejection.

Thymus

Site where T cells undergo self-tolerance screening.

Bone Marrow

Site where B cells undergo self-tolerance screening.

Maturation Site of T Cells

Thymus.

Maturation Site of B Cells

Bone marrow.

Found in

Both B and T cells are found in lymphatic tissues.

Receptor of T Cells

TCR (T Cell Receptor).

Receptor of B Cells

BCR (B Cell Receptor) which leads to antibody production.

Need APC?

T Cells require Antigen Presenting Cells (APC), while B Cells do not.

Memory Cells

Both B and T cells can form memory cells.

MHC Present

MHC I is present on all nucleated cells; MHC II is present on APCs.

APC

Antigen Presenting Cells, which include dendritic cells, macrophages, and B cells.

Cellular Immunity

T-cell driven branch of adaptive defense.

Stages of Immune Response

1️⃣ Antigen presentation → 2️⃣ T cell activation → 3️⃣ Proliferation & differentiation → 4️⃣ Antigen elimination + memory.

MHC (Major Histocompatibility Complex)

Presents antigen fragments to T cells and is responsible for self-recognition.

MHC I

Presents intracellular antigens to CD8 (T cytotoxic) cells.

MHC II

Presents extracellular antigens to CD4 (T helper) cells.

Viral Evasion Examples

Adenovirus blocks MHC I transport; Herpes virus blocks peptide transport into ER; Cytomegalovirus destroys MHC I-antigen complexes.

Self-tolerance failure

Can lead to autoimmunity.

MHC match

Crucial for organ transplants.

Allorecognition

The recognition of foreign MHCs by recipient immune cells leading to transplant rejection.

Graft-versus-host disease (GVHD)

A condition that occurs when transplanted bone marrow cells attack the recipient's tissues.

Cytokine cascade

A series of signaling events triggered by co-stimulation that defines T cell subclass.

Superantigens

Antigens that bypass normal activation by cross-linking MHC II and TCR, causing massive T cell activation.

Cytokine storm

A severe immune reaction characterized by the release of large amounts of cytokines, leading to shock and organ failure.

Th1

A T helper cell subclass involved in stimulating Tc cells and macrophages, favoring cellular immune responses.

Th2

A T helper cell subclass that stimulates B cells and antibody production, favoring humoral immune responses.

T-dependent activation

B cell activation that requires help from T helper cells, leading to strong memory formation.

T-independent activation

B cell activation that does not require T cell help, resulting in weak or short-lived memory.

Plasma cells

Short-lived cells that secrete large amounts of antibodies specific to an activating epitope.

Memory B cells

Long-lived cells that remain in lymphoid tissue and respond rapidly upon re-exposure to the same antigen.

Neutralization

The process by which antibodies block viral entry by binding to toxins or viruses.

Complement activation

The process triggered by antibody-antigen complexes that leads to lysis of bacterial cells.

Opsonization

The tagging of antigens by antibodies to enhance recognition and phagocytosis by immune cells.

Isotype switching

The process of changing the constant region of an antibody while maintaining the same variable region.

IgG

An antibody isotype that provides strong neutralization, opsonization, and crosses the placenta.

IgA

An antibody isotype found in mucosal secretions, important for neutralization at mucosal surfaces.

IgM

The first antibody produced during an infection, known for strong agglutination and complement activation.

IgE

An antibody isotype that binds to mast cells and basophils, mediating allergic reactions.

IgD

An antibody isotype with a poorly understood function, primarily found on B cell surfaces.

Antibody titer

A quantitative measure of the amount of specific antibody in the blood, expressed as a dilution ratio.

Serology

The study of antibodies in serum to determine exposure or vaccine response.

Convalescent plasma therapy

The use of plasma from recovered individuals with high antibody titers for temporary passive therapy.

Monoclonal antibodies (mAbs)

Lab-made antibodies targeting a single specific epitope, used for treatment of various conditions.

Naturally acquired active immunity

Immunity developed after recovery from an infection, involving the production of memory cells.

Artificially acquired active immunity

Immunity developed through vaccination, leading to long-term memory cell formation.

Naturally acquired passive immunity

Temporary immunity provided by maternal antibodies transferred through the placenta or breast milk.

Artificially acquired passive immunity

Temporary immunity provided by injection of antibodies from another individual or animal.