Immunology Terms Starting With P

Passive Immunity is temporary protection conferred when preformed antibodies produced by one individual are transferred to another, bypassing the need for the recipient to mount an active immune response.

Maternal IgG crosses the human placenta during the third trimester, reaching concentrations in the newborn that can exceed those in the mother by the time of delivery. Breast milk supplies secretory IgA that coats mucosal surfaces in the infant gut, blocking pathogen attachment during the first months of life. Clinicians also administer passive immunity therapeutically: rabies immune globulin injected at a bite wound provides immediate neutralizing antibodies before the recipient’s own response can develop, and antivenom for snakebite delivers antibodies raised in horses (Equus caballus) or sheep (Ovis aries) that neutralize venom toxins within hours.

Because the transferred antibodies are not produced by the recipient’s own lymphocytes, they are gradually catabolized and protection typically wanes within weeks to a few months.

Pathogen Recognition is the process by which immune cells detect conserved molecular features of microbes and initiate defensive responses before antigen-specific lymphocytes can be recruited.

Toll-like receptors expressed on the surface and in endosomes of macrophages and dendritic cells bind pathogen-associated molecular patterns such as bacterial lipopolysaccharide, peptidoglycan, flagellin, and viral RNA. TLR4 heterodimerizes with the co-receptor MD-2 and binds lipopolysaccharide from Gram-negative bacteria with nanomolar affinity, triggering NF-kB-dependent transcription of pro-inflammatory cytokines within minutes. NOD-like receptors in the cytoplasm detect intracellular bacterial fragments and viral nucleic acids, with NOD1 and NOD2 activating NF-kB and the NLRP3 inflammasome cleaving pro-IL-1-beta into its active form.

Complement protein C3 deposits covalent C3b fragments onto microbial surfaces, and complement receptors on macrophages and neutrophils recognize these tags to enhance phagocytosis through opsonization. This layered system of pattern recognition ensures that even novel pathogens sharing conserved microbial structures trigger an immediate innate response while adaptive immunity develops over the following one to two weeks.

Phagocyte is an immune cell that engulfs and destroys microbes, dead cells, and foreign particles by enclosing them in an internal membrane-bound compartment and digesting them with enzymes and antimicrobial chemicals.

Neutrophils make up 50 to 70 percent of circulating white blood cells and are the first cells recruited to sites of bacterial infection, where they engulf pathogens into phagosomes that fuse with granules containing myeloperoxidase, lactoferrin, and defensins. Macrophages reside permanently in tissues, including the liver Kupffer cells and brain microglia, and extend pseudopodia around targets to form phagolysosomes where an acidic pH below 5 and hydrolytic enzymes degrade ingested material. Dendritic cells capture antigens by phagocytosis or macropinocytosis and then migrate to lymph nodes, where they display processed peptides on MHC molecules to naive T cells, linking the innate and adaptive arms of immunity.

Opsonization with IgG antibodies or complement fragment C3b dramatically increases phagocytic efficiency by engaging Fc receptors and complement receptors on the phagocyte surface.

Plasma Cell is the terminally differentiated effector form of a B lymphocyte that has reorganized its biosynthetic machinery to secrete large quantities of antigen-specific antibody into the blood and tissues.

Plasma cells develop from B cells activated by antigen and T cell help, with short-lived plasmablasts appearing in the blood within three to five days of stimulation and long-lived plasma cells differentiating after germinal center reactions over two to three weeks. Long-lived plasma cells migrate to specialized survival niches in the bone marrow, where stromal cell-derived APRIL and BAFF signals, along with contact with eosinophils and basophils, sustain antibody secretion for decades without requiring re-exposure to antigen. Each plasma cell expands its endoplasmic reticulum to an extraordinary degree and can secrete up to 10,000 antibody molecules per second, a rate that demands tight coordination between the unfolded protein response and the cell’s metabolic output.

Unlike naive B cells, most plasma cells downregulate surface B cell receptor expression and the transcription factor PAX5, committing fully to secretion rather than further antigen recognition.

Plasmacytoid Dendritic Cells are a specialized subset of dendritic cells that detect viral nucleic acids through endosomal Toll-like receptors and respond by rapidly secreting massive quantities of type I interferons to suppress viral replication.

Unlike conventional dendritic cells that efficiently capture and present antigens to T cells, plasmacytoid dendritic cells are optimized for antiviral cytokine production through constitutive high-level expression of the transcription factor IRF7 and the endosomal receptors TLR7 and TLR9. A single activated plasmacytoid dendritic cell can produce thousands of times more IFN-alpha than any other cell type, and the secreted interferon induces an antiviral state in surrounding cells by upregulating hundreds of interferon-stimulated genes within hours. These cells circulate in blood at low frequency, comprising less than 0.5 percent of peripheral blood mononuclear cells in healthy humans, yet their collective output during acute viral infection can dominate the early cytokine environment.

Plasmacytoid dendritic cells also migrate to lymph nodes and cross-present viral antigens to CD8-positive cytotoxic T cells, linking the rapid innate interferon response to the slower development of antigen-specific adaptive immunity.

Primary Immune Response is the adaptive immune response generated upon first exposure to a specific antigen, characterized by a lag period of one to two weeks, modest antibody titers dominated initially by IgM, and the formation of immunological memory.

Naive B and T cells specific for the antigen must first be located among the enormous polyclonal repertoire, a process that can take several days as lymphocytes traffic through secondary lymphoid organs. Once activated, these cells undergo clonal expansion and differentiation: B cells in germinal centers accumulate somatic hypermutations that increase antibody affinity over two to three weeks, and class switching shifts production from IgM toward IgG, IgA, or IgE. Peak antibody titers during a primary response are typically 10- to 100-fold lower than those generated by the same antigen during a secondary response, reflecting the small starting frequency of antigen-specific cells.

A minority of activated B and T cells differentiate into long-lived memory cells that persist in bone marrow and lymphoid tissue, enabling the faster and stronger secondary response upon re-exposure.

Psychoneuroimmunology is the scientific field that investigates the bidirectional communication pathways linking the nervous system, the endocrine system, and the immune system, and how psychological states and behaviors influence immune function and vice versa.

Glucocorticoids such as cortisol, released from the adrenal cortex during stress, bind receptors on lymphocytes and macrophages to suppress pro-inflammatory cytokine production, shift T helper cell balance toward Th2 responses, and reduce natural killer cell activity. Sympathetic nerve fibers innervate the spleen, thymus, and lymph nodes, releasing norepinephrine that binds beta-adrenergic receptors on immune cells and modulates their proliferation and cytokine secretion. Cytokines produced during infection, particularly IL-1-beta, IL-6, and TNF-alpha, cross the blood-brain barrier or signal through vagal afferents to induce sickness behavior, fever, and altered sleep, demonstrating that immune activity feeds back to change brain function.

Janice Kiecolt-Glaser and Ronald Glaser showed in the 1980s and 1990s that medical students under examination stress had measurably reduced natural killer cell activity and slower wound healing compared with low-stress periods, providing early human evidence for clinically meaningful neuroimmune interactions.