6.1 Introduction to Immunity

The immune system is the body's defense network against pathogens like bacteria, viruses, fungi, and parasites. For nursing pharmacology, understanding how immunity works is essential because so many drugs either boost, suppress, or modify immune function. This section covers the key components of the immune system, how innate and adaptive responses differ, and how antibodies and T cells work together to fight infection.

Overview of the Immune System

Functions of the Immune System

The immune system does three core jobs:

• Defends against invading pathogens (bacteria, viruses, fungi, parasites)
• Distinguishes self from non-self, recognizing foreign substances while tolerating the body's own cells (this is called immune tolerance)
• Maintains homeostasis by clearing out damaged or abnormal cells through processes like apoptosis (programmed cell death) and phagocytosis (engulfing and digesting debris)

When any of these functions break down, the result can be infection, autoimmune disease, or cancer. Many pharmacologic interventions target one or more of these functions directly.

Components of the Immune System

Lymphoid Organs

Primary lymphoid organs are where immune cells develop and mature:

• Bone marrow produces all blood cells through hematopoiesis and is where B cells mature
• Thymus is where T cells mature and learn to distinguish self from non-self

Secondary lymphoid organs are where immune responses are initiated:

• Lymph nodes filter lymph fluid and trap pathogens, bringing them into contact with immune cells
• Spleen filters blood and acts as a reservoir for immune cells
• Mucosa-associated lymphoid tissue (MALT) includes the tonsils, Peyer's patches (in the intestines), and appendix. These tissues provide immune surveillance at mucosal surfaces, which are common entry points for pathogens.

Cells of the Immune System

Lymphocytes drive the adaptive immune response:

• B cells produce antibodies and mediate humoral (antibody-mediated) immunity
• T cells mediate cell-mediated immunity and help activate B cells

Innate immune cells provide rapid, non-specific defense:

• Phagocytes (neutrophils, monocytes, macrophages) engulf and destroy pathogens
• Dendritic cells capture antigens and present them to T cells, acting as a bridge between innate and adaptive immunity
• Natural killer (NK) cells destroy virus-infected cells and tumor cells without needing prior antigen exposure

Granulocytes are white blood cells with granules containing enzymes and mediators:

• Neutrophils are the most abundant WBCs and the first responders to infection
• Eosinophils target parasitic infections and play a role in allergic reactions
• Basophils release histamine and other inflammatory mediators (relevant to allergic and hypersensitivity responses)

Soluble Factors

• Cytokines are signaling molecules that regulate communication between immune cells. Key types include interleukins, interferons, and tumor necrosis factors. Many biologic drugs used in nursing practice target specific cytokines.
• Complement proteins are plasma proteins that circulate in inactive form. When activated by pathogens or antibodies, they enhance phagocytosis, promote inflammation, and can directly lyse pathogens through the membrane attack complex.

Innate Immune Recognition

The innate immune system recognizes broad patterns rather than specific antigens:

• Pathogen-associated molecular patterns (PAMPs) are conserved molecular structures found on many pathogens but not on human cells (e.g., bacterial cell wall components like lipopolysaccharide)
• Pattern recognition receptors (PRRs) are receptors on innate immune cells that detect PAMPs and trigger an immediate defensive response
• Major histocompatibility complex (MHC) molecules are cell surface proteins that present antigens to T cells. MHC is critical for launching adaptive immune responses and plays a major role in transplant rejection and autoimmune disease.

Types of Immune Responses

Antibody-Mediated (Humoral) vs. Cell-Mediated Immunity

These are the two arms of adaptive immunity. They work together, but each handles different types of threats.

Antibody-Mediated (Humoral) Immunity

This arm is mediated by B cells and the antibodies they produce. It primarily targets extracellular pathogens and toxins circulating in blood and body fluids.

How it works:

1. B cells recognize a specific antigen using surface immunoglobulin receptors and differentiate into plasma cells
2. Plasma cells secrete large quantities of antigen-specific antibodies into the bloodstream and lymph
3. Antibodies bind to pathogens or toxins, marking them for destruction by phagocytes or complement activation

What antibodies do once they bind:

• Neutralization: Directly bind to and block pathogens (like viruses) or toxins, preventing them from entering or damaging cells
• Opsonization: Coat pathogens so phagocytes (macrophages, neutrophils) can recognize and engulf them more efficiently
• Complement activation: Trigger the complement cascade, which amplifies the immune response through enhanced phagocytosis, recruitment of immune cells (chemotaxis), and direct pathogen lysis via the membrane attack complex

Cell-Mediated Immunity

This arm is mediated by T cells and targets intracellular pathogens (viruses replicating inside cells, intracellular bacteria like Mycobacterium tuberculosis) and abnormal cells (cancer cells).

How it works:

1. Antigen-presenting cells (APCs) such as dendritic cells and macrophages capture foreign antigens and display them on their surface using MHC molecules
2. T cells recognize the antigen-MHC complex via their T cell receptors and become activated
3. CD4+ helper T cells secrete cytokines that coordinate the broader immune response by activating other immune cells
4. CD8+ cytotoxic T cells directly kill infected or abnormal cells by releasing cytotoxic granules

T cell subtypes and their roles:

• CD4+ helper T cells come in different subsets:
  • Th1 cells activate macrophages and promote cell-mediated immunity against intracellular pathogens
  • Th2 cells stimulate B cell antibody production and promote humoral immunity against extracellular pathogens
• CD8+ cytotoxic T cells kill target cells by releasing perforin (punches holes in the cell membrane) and granzymes (enter through the holes and trigger apoptosis). They also secrete cytokines like IFN-γ that inhibit viral replication.

Antigen-Antibody Interactions

Antigens

An antigen is any substance that can trigger a specific immune response. Antigens are typically proteins, polysaccharides, or lipids found on the surface of pathogens (like bacterial capsules) or foreign substances (like pollen).

The specific region on an antigen that an antibody or T cell receptor recognizes is called an epitope. A single antigen can have multiple epitopes, meaning different antibodies can bind to different parts of the same antigen.

Antibodies (Immunoglobulins)

Antibodies are Y-shaped glycoprotein molecules produced by B cells in response to specific antigens.

Structure:

• Composed of four polypeptide chains: two heavy chains and two light chains
• The Fab region (fragment antigen-binding) contains the variable region that binds to a specific epitope
• The Fc region (fragment crystallizable) is the constant region that interacts with immune cells and complement proteins. The Fc region determines what the antibody does after it binds.

Five classes (isotypes) of antibodies:

How Antigen-Antibody Binding Works

• Specificity: Each antibody binds to a specific epitope, like a lock and key
• Affinity: The strength of binding between one antibody binding site and one epitope. Higher affinity means stronger, more stable binding.
• Avidity: The overall strength of the interaction when multiple antibodies bind multiple epitopes simultaneously. Avidity depends on both affinity and the number of binding sites (this is why IgM, with 10 binding sites, has high avidity despite lower individual affinity).
• Cross-reactivity: Sometimes an antibody recognizes similar epitopes on different antigens. This can be useful (providing some protection against related pathogens) but can also contribute to autoimmune reactions when antibodies mistakenly target self-tissues.

Antibody Effector Functions

Once antibodies bind to their target, they trigger several downstream effects:

• Neutralization: Block pathogens or toxins from attaching to and entering cells
• Opsonization: Coat pathogens so phagocytes can more easily recognize and engulf them
• Complement activation: Initiate the complement cascade, leading to enhanced phagocytosis, inflammation, and direct cell lysis
• Antibody-dependent cell-mediated cytotoxicity (ADCC): Antibodies bound to infected or abnormal cells recruit NK cells or macrophages, which then release cytotoxic granules to destroy the tagged cell. Several monoclonal antibody drugs used in cancer treatment work through this mechanism.

Clonal Selection

Clonal selection is the process by which the immune system amplifies its response to a specific threat. When an antigen binds to a B cell or T cell with the matching receptor, that cell is activated and rapidly divides to produce a large population (clone) of identical cells, all targeting the same antigen. Some of these clones become memory cells, which is why your second exposure to a pathogen produces a faster, stronger response. This principle is the basis of vaccination.