The immune system's defense against pathogens relies on two key mechanisms: cellular and humoral immunity. These processes involve specialized cells and molecules that work together to identify, target, and eliminate threats to the body's health.
T and B lymphocytes play crucial roles in these immune responses. T cells drive cellular immunity by directly attacking infected cells, while B cells produce antibodies for humoral immunity. Together, they form a powerful defense system against disease.
Lymphocytes and Immunity
T Lymphocytes and Cell-Mediated Immunity
- T lymphocytes mature in the thymus gland and play a central role in cell-mediated immunity
- Cytotoxic T cells (CD8+) directly attack and destroy infected or cancerous cells by releasing perforin and granzymes (cytotoxic granules)
- Helper T cells (CD4+) activate and coordinate the immune response by secreting cytokines to stimulate other immune cells (B cells, macrophages)
- Regulatory T cells (Tregs) suppress the immune response to prevent autoimmunity and maintain tolerance to self-antigens
- Memory T cells provide long-lasting immunity by quickly responding to previously encountered antigens
B Lymphocytes and Humoral Immunity
- B lymphocytes mature in the bone marrow and are responsible for humoral immunity (antibody-mediated immunity)
- B cells recognize specific antigens using their B cell receptors (membrane-bound antibodies)
- Upon activation by helper T cells, B cells differentiate into plasma cells that secrete large amounts of antibodies
- Memory B cells provide long-term immunity by rapidly producing antibodies upon re-exposure to the same antigen
- Antibodies neutralize pathogens, opsonize them for phagocytosis, and activate the complement system to enhance the immune response
Antibodies and Plasma Cells
Antibody Structure and Function
- Antibodies (immunoglobulins) are Y-shaped proteins composed of two heavy chains and two light chains connected by disulfide bonds
- The variable regions of antibodies determine their antigen specificity, while the constant regions determine their effector functions
- Five classes of antibodies exist (IgM, IgD, IgG, IgA, and IgE), each with distinct roles in the immune response
- IgG is the most abundant antibody in serum and provides the majority of antibody-based immunity
- IgA is found in mucosal secretions (saliva, tears, breast milk) and protects mucosal surfaces from pathogens
Plasma Cells and Antibody Production
- Plasma cells are terminally differentiated B cells that secrete large quantities of antibodies (up to thousands per second)
- Plasma cells are found in the bone marrow, lymph nodes, and spleen and have a lifespan of several days to months
- The process of antibody production involves somatic hypermutation and affinity maturation to generate high-affinity antibodies
- Class switching allows plasma cells to produce antibodies with different constant regions (isotypes) while maintaining antigen specificity
- Plasma cells and memory B cells are generated in germinal centers of lymphoid tissues following antigen exposure
Antigen Presentation and Signaling
Antigen-Presenting Cells and MHC
- Antigen-presenting cells (APCs) include dendritic cells, macrophages, and B cells that process and present antigens to T cells
- Major histocompatibility complex (MHC) molecules are cell surface proteins that display peptide fragments of antigens to T cells
- MHC class I molecules present intracellular antigens (viral proteins, tumor antigens) to CD8+ T cells
- MHC class II molecules present extracellular antigens (bacterial proteins) to CD4+ T cells
- The diversity of MHC genes ensures a wide range of antigen presentation and contributes to the uniqueness of an individual's immune response
Cytokines and Immune Cell Communication
- Cytokines are small proteins secreted by immune cells that regulate the immune response through cell signaling
- Interleukins (IL) are a group of cytokines that mediate communication between leukocytes (IL-2, IL-4, IL-10)
- Interferons (IFN) are cytokines that inhibit viral replication and modulate the immune response (IFN-α, IFN-β, IFN-γ)
- Tumor necrosis factors (TNF) are cytokines that promote inflammation and cell death (TNF-α)
- Chemokines are cytokines that attract immune cells to sites of infection or inflammation (CXCL8, CCL2)