21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types

Adaptive Immune Response

The adaptive immune response is your body's specialized defense system. Unlike innate immunity, it targets specific threats with precision and creates a memory of past invaders so future responses are faster and stronger. T and B lymphocytes drive this system, each playing distinct roles in recognizing and eliminating pathogens.

Adaptive vs. Innate Immune Responses

These two branches of immunity work together, but they differ in speed, specificity, and memory.

Adaptive immunity:

• Highly specific: targets particular antigens (proteins or polysaccharides) on pathogens or foreign substances
• Develops immunological memory, enabling faster, stronger responses to recurring pathogens (e.g., influenza on second exposure)
• Slower initial response: takes days to weeks to develop after first antigen exposure
• Relies on T lymphocytes and B lymphocytes as key cellular components

Innate immunity:

• Non-specific: provides broad-spectrum defense against a wide range of pathogens
• No immunological memory; each exposure triggers a similar-strength response
• Rapid response: activates within hours of pathogen exposure
• Relies on physical barriers (skin, mucous membranes), phagocytes (neutrophils, macrophages), and the complement system (plasma proteins)

The big advantage of adaptive immunity is its combination of antigen specificity and memory. It can also recognize a vast array of antigens because T and B cell receptors are generated through somatic recombination, a process that shuffles gene segments to create enormous receptor diversity.

Antigens in Immune Recognition

An antigen is any substance that can trigger an immune response. Most antigens are proteins or polysaccharides found on the surface of pathogens (bacteria, viruses) or foreign substances (like a transplanted organ).

Two properties define how antigens interact with the immune system:

• Immunogenicity: the ability to induce an immune response by stimulating lymphocyte activation and differentiation
• Antigenicity: the ability to be specifically recognized and bound by antibodies or T cell receptors

The specific region on an antigen that an immune receptor binds to is called an epitope. A single antigen can have multiple epitopes, meaning different antibodies or TCRs can recognize different parts of the same molecule.

For T cells to respond, antigens must first be processed and displayed by antigen-presenting cells (APCs). T cells cannot recognize free-floating antigens on their own; they need antigens "served up" on MHC molecules.

T Cell Antigen Receptor Structure

T cell receptors (TCRs) are membrane-bound proteins on the surface of T lymphocytes responsible for recognizing antigens.

• Each TCR is a heterodimer made of two polypeptide chains (most commonly α and β) linked by disulfide bonds
• The variable regions at the N-terminal end of each chain form the antigen-binding site, which determines what antigen the TCR can recognize

A critical detail: TCRs do not bind free antigens. They only recognize antigens that are presented on major histocompatibility complex (MHC) molecules on the surface of other cells.

• MHC class I presents intracellular antigens (e.g., viral proteins made inside an infected cell) to CD8+ T cells
• MHC class II presents extracellular antigens (e.g., bacterial peptides that were engulfed and digested by an APC) to CD4+ T cells

TCR diversity is generated through somatic recombination of gene segments during T cell development in the thymus. This process creates millions of unique TCRs, giving the immune system the ability to recognize an enormous range of potential threats.

T Cell Development and Types

T Cell Development Process

T cell maturation happens in the thymus, and the selection process is designed to produce T cells that are functional but not self-reactive.

1. Migration: T cell precursors originate in the bone marrow and migrate to the thymus
2. Positive selection: T cells whose TCRs can bind to MHC molecules (at least weakly) receive survival signals and continue developing. T cells that fail to bind MHC at all undergo apoptosis, since they'd be useless in recognizing presented antigens.
3. Negative selection: T cells whose TCRs bind too strongly to self-antigens are eliminated. This prevents autoimmunity by removing cells that would attack the body's own tissues.
4. Lineage commitment: Surviving T cells differentiate into either CD4+ cells (if they recognize MHC class II) or CD8+ cells (if they recognize MHC class I)
5. Release: Mature, naïve T cells exit the thymus and circulate through the blood and lymph, ready to encounter their specific antigen

Only about 2-5% of developing T cells survive both positive and negative selection. This strict screening ensures the T cells that enter circulation are both functional and self-tolerant.

Types of T Cells

CD4+ T Helper (Th) Cells

Helper T cells recognize antigens presented by MHC class II on APCs such as dendritic cells and macrophages. Rather than killing targets directly, they coordinate the immune response by secreting cytokines that activate and regulate other immune cells. Different subsets handle different types of threats:

• Th1 cells promote cell-mediated immunity against intracellular pathogens (viruses, intracellular bacteria) by activating macrophages and CD8+ T cells
• Th2 cells stimulate B cells and antibody production, targeting extracellular pathogens like helminths (parasitic worms) and playing a role in allergic responses
• Th17 cells recruit neutrophils and promote inflammation, particularly in response to fungal and extracellular bacterial infections
• Regulatory T cells (Tregs) suppress immune responses and maintain self-tolerance, preventing the immune system from attacking the body's own tissues

CD8+ Cytotoxic T (Tc) Cells

Cytotoxic T cells recognize antigens presented by MHC class I on infected or abnormal cells. They are the direct killers of the adaptive immune system. When a Tc cell binds its target, it releases cytotoxic granules containing:

• Perforin: forms pores in the target cell's membrane
• Granzymes: enter through those pores and trigger apoptosis (programmed cell death)

This mechanism is critical for defense against viruses (influenza, HIV) and for eliminating tumor cells.

Memory T Cells

After an infection is cleared, most effector T cells die off, but a subset persists as memory T cells. These are long-lived cells that can survive for years or even decades. Upon re-exposure to the same antigen, memory T cells rapidly differentiate into effector cells (Th or Tc), producing a faster and stronger response than the initial encounter. This is the cellular basis of immunological memory.

Lymphoid Organs and Immune Cell Interactions

T cells develop and become activated in specific locations throughout the body.

• Primary lymphoid organs (bone marrow and thymus) are where lymphocytes develop and mature
• Secondary lymphoid organs (lymph nodes, spleen, and mucosa-associated lymphoid tissues like tonsils and Peyer's patches) are where lymphocytes encounter antigens and become activated

Naïve T cells constantly circulate through blood and lymph, passing through secondary lymphoid organs where APCs display antigens. When a T cell's TCR matches a presented antigen, that T cell becomes activated and undergoes clonal expansion, rapidly dividing to produce a large population of antigen-specific effector cells.

B cells also participate in adaptive immunity by producing antibodies (immunoglobulins) that recognize and neutralize specific antigens. Helper T cells play a key role in activating B cells, linking the cellular and humoral branches of adaptive immunity.