42.2 Adaptive Immune Response

Overview of Adaptive Immunity

Adaptive immunity is the body's targeted defense system. While innate immunity responds quickly to broad categories of threats, adaptive immunity takes longer to activate but can recognize specific pathogens and remember them for future encounters. This combination of specificity and memory is what makes vaccines work and why you typically only get diseases like chickenpox once.

Two main cell types drive adaptive immunity: T cells and B cells, both of which are lymphocytes (a type of white blood cell). Each individual T or B cell carries a unique receptor on its surface that recognizes one specific antigen, giving the system extraordinary precision.

Features of Adaptive Immunity

• Specificity: Each lymphocyte recognizes a particular antigen through its unique receptor (T cell receptor or B cell receptor). This means the immune system can distinguish between closely related pathogens.
• Diversity: The body generates a huge repertoire of different antigen receptors through somatic recombination, a process that shuffles receptor gene segments during lymphocyte development. This produces millions of distinct receptor variants from a limited number of genes.
• Memory: After an initial infection, some activated lymphocytes become long-lived memory cells. On a second exposure to the same antigen, these memory cells mount a faster, stronger secondary response compared to the slower primary response.
• Adaptability: The system can respond to new and evolving pathogens. In B cells specifically, processes like somatic hypermutation and affinity maturation progressively improve antibody binding strength during an immune response.

These features work together to eliminate pathogens and infected cells, generate antibodies for neutralization and opsonization (tagging pathogens so phagocytes can engulf them more easily), and regulate immune responses through cytokines and direct cell-to-cell interactions.

Adaptive vs. Innate Immunity

A key point: innate and adaptive immunity don't work in isolation. Innate immune cells like dendritic cells act as a bridge between the two systems by capturing pathogens and presenting their antigens to T cells, which kicks off the adaptive response.

Mechanisms of Adaptive Immunity

Cell-Mediated and Humoral Responses

Adaptive immunity has two major arms. Cell-mediated immunity is handled by T cells and targets infected or abnormal cells directly. Humoral immunity is handled by B cells and the antibodies they produce, and it targets pathogens circulating in body fluids (blood, lymph, tissue fluid). Both arms typically work together during an infection.

Cell-Mediated Immune Response (T Cells)

T cell activation requires three signals:

1. Antigen presentation: An antigen-presenting cell (APC), such as a dendritic cell, digests a pathogen and displays antigenic peptide fragments on its surface using MHC molecules (major histocompatibility complex).
2. Antigen recognition: The T cell receptor (TCR) binds to the specific antigen-MHC complex on the APC. CD8+ T cells recognize antigens on MHC class I; CD4+ T cells recognize antigens on MHC class II.
3. Co-stimulation: A second signal, typically the interaction between CD28 on the T cell and B7 on the APC, is required for full activation. Without this co-stimulatory signal, the T cell becomes unresponsive (anergic) rather than activated. This acts as a safety check.

Once activated, T cells differentiate into effector types:

• Cytotoxic T cells (CD8+) directly kill infected or abnormal cells. They release cytotoxic granules containing perforin (which punches holes in the target cell membrane) and granzymes (which enter through those holes and trigger apoptosis).
• Helper T cells (CD4+) don't kill directly. Instead, they coordinate the immune response by secreting cytokines that activate and regulate other immune cells. Different subsets (Th1, Th2, Th17, and regulatory T cells) secrete different cytokine profiles and promote different types of immune responses.

Humoral Immune Response (B Cells and Antibodies)

B cell activation follows a similar pattern:

1. Antigen recognition: B cell receptors (BCRs) on the B cell surface bind directly to a specific antigen. Unlike T cells, B cells can often recognize antigens in their native (unprocessed) form.
2. T cell help: For most antigens (called T-dependent antigens), full B cell activation requires signals from helper T cells. This involves CD40-CD40L interaction between the two cells, plus cytokines from the helper T cell.

Activated B cells then differentiate into:

• Plasma cells, which are antibody-producing factories. They secrete large quantities of antibodies (immunoglobulins). The five main classes are IgM, IgD, IgG, IgA, and IgE, each with distinct functions and locations in the body. Antibodies work by neutralizing pathogens and toxins (blocking their ability to infect cells), opsonizing pathogens (coating them to enhance phagocytosis), and activating the complement system.
• Memory B cells, which are long-lived and persist after the infection clears. They respond rapidly upon re-exposure to the same antigen, producing the faster, stronger secondary response.

Lymphocyte Activation and Differentiation

Here's how the adaptive response unfolds from start to finish:

1. Antigen encounter: A pathogen enters the body and is captured by APCs.
2. Antigen presentation: APCs carry antigens to secondary lymphoid organs (lymph nodes, spleen) where T and B cells are concentrated.
3. Antigen recognition: The specific T or B cell whose receptor matches the antigen becomes activated.
4. Clonal expansion: That activated lymphocyte rapidly divides, producing many identical copies of itself. This is why the primary response takes days: the body needs time to build up enough antigen-specific cells.
5. Differentiation: The expanded clones differentiate into effector cells (which fight the current infection) and memory cells (which provide future protection).

The primary response (first encounter) is slower and produces lower antibody levels, with IgM appearing first. The secondary response (subsequent encounters) is faster, stronger, and produces predominantly IgG, thanks to memory cells that are already primed and ready.

Immune Tolerance and Autoimmunity

The adaptive immune system's ability to recognize millions of different antigens creates a dangerous possibility: some lymphocytes will inevitably have receptors that recognize the body's own molecules (self-antigens). Immune tolerance is the set of mechanisms that prevent these self-reactive cells from attacking your own tissues.

Tolerance operates at two levels:

• Central tolerance occurs during lymphocyte development in the primary lymphoid organs (thymus for T cells, bone marrow for B cells). Developing lymphocytes that strongly react to self-antigens are eliminated through negative selection, meaning they undergo apoptosis before they ever enter circulation.
• Peripheral tolerance catches self-reactive cells that escape central tolerance. It operates in secondary lymphoid organs and peripheral tissues through several mechanisms:
  • Anergy: Self-reactive lymphocytes that encounter their antigen without proper co-stimulation become functionally inactivated.
  • Deletion: Self-reactive lymphocytes are triggered to undergo apoptosis.
  • Suppression: Regulatory T cells actively suppress self-reactive lymphocytes, preventing them from causing damage.

When tolerance mechanisms fail, the immune system attacks the body's own tissues, resulting in autoimmune diseases. Examples include type 1 diabetes (immune destruction of insulin-producing beta cells in the pancreas), rheumatoid arthritis (immune attack on joint tissues), and multiple sclerosis (immune attack on the myelin sheath surrounding nerve fibers). Understanding tolerance is central to understanding why the immune system doesn't normally destroy healthy tissue and what goes wrong when it does.