Immunology Terms Starting With A

Adaptive Immunity is the antigen-specific arm of the immune system that develops over days after first exposure, generates immunological memory, and produces highly targeted responses through T cells and B cells.

Unlike innate immunity, adaptive immunity recognizes specific molecular features of pathogens through diverse T cell receptors and antibodies generated by somatic recombination of receptor genes. Clonal selection amplifies the lymphocytes whose receptors bind a particular antigen, and a subset of these cells differentiate into long-lived memory cells. Secondary exposure to the same antigen triggers a faster, stronger response through these memory cells, forming the basis of vaccination.

In humans, this secondary response can be mounted within hours rather than the days required for a primary response, and antibody titers can reach ten to a hundred times higher than those seen after first exposure.

Adjuvant is a substance added to a vaccine or immunotherapy formulation that enhances and prolongs the immune response to an antigen without itself being the target of the immune response.

Adjuvants activate innate immune pathways that trigger local danger signals, recruit antigen-presenting cells, promote antigen uptake and processing, and stimulate the cytokine environment needed for strong T and B cell activation. Aluminum salts have been used as adjuvants in vaccines since the 1920s and remain the most widely used globally. Modern adjuvants use different mechanisms: AS04 combines alum with the TLR4 agonist monophosphoryl lipid A, while MF59 is an oil-in-water emulsion that recruits and activates innate immune cells at the injection site rather than acting as a classic TLR agonist.

By amplifying the immune response, adjuvants can reduce the amount of antigen needed per dose, which is particularly valuable when antigen supply is limited during a pandemic.

Allergen is any substance, usually a protein, that triggers an inappropriate IgE-mediated immune response in a sensitized individual, causing allergic symptoms ranging from mild itching to life-threatening anaphylaxis.

Common allergens include pollen, dust mite proteins, animal dander, mold spores, and food proteins from peanuts, tree nuts, shellfish, milk, and eggs. On first exposure, susceptible individuals produce IgE antibodies against the allergen, which bind to mast cells and basophils throughout the body. Subsequent exposure causes these IgE-sensitized cells to degranulate and release histamine, leukotrienes, and other mediators that produce allergic symptoms.

Dust mite allergens such as Der p 1, a cysteine protease from the house dust mite (Dermatophagoides pteronyssinus), can disrupt epithelial tight junctions and directly promote IgE sensitization.

Allergy is an exaggerated immune response to a normally harmless environmental substance, mediated by IgE antibodies and mast cell degranulation, producing symptoms such as sneezing, itching, urticaria, and potentially anaphylaxis.

Allergic diseases include hay fever, allergic asthma, atopic dermatitis, and food allergies, and they share a common mechanism in which the immune system mounts IgE responses against substances that pose no genuine threat. Genetic predisposition, particularly variants in genes controlling epithelial barrier function and cytokine signaling such as IL-4 and IL-13, raises susceptibility. Individuals with atopy, a hereditary tendency toward allergic sensitization, often react to several unrelated allergens simultaneously.

Prevalence of allergic disease has risen sharply in industrialized countries over the past fifty years, a trend linked by the hygiene hypothesis to reduced early-life exposure to diverse microbial environments.

Anaphylaxis is a severe, rapid-onset, systemic allergic reaction triggered by IgE-mediated mast cell degranulation, producing potentially fatal circulatory collapse, airway obstruction, and multi-organ involvement.

Anaphylaxis can begin within seconds to minutes of exposure to the causative allergen, most commonly foods, insect venom, latex, or medications. The massive release of histamine, tryptase, prostaglandins, and leukotrienes from mast cells and basophils throughout the body causes vasodilation, increased vascular permeability, bronchoconstriction, and mucus hypersecretion. Intramuscular epinephrine injected into the lateral thigh is the first-line treatment, reversing the vasodilation and bronchoconstriction that otherwise cause circulatory and respiratory failure.

Biphasic anaphylaxis, in which symptoms return hours after initial resolution, occurs in roughly five to twenty percent of cases, making observation for at least four to six hours after treatment standard practice.

Anti-LAG3 is a class of immunotherapy drugs that block the LAG-3 inhibitory checkpoint receptor on T cells, restoring their anti-tumor or anti-viral activity by preventing LAG-3 from dampening T cell responses.

LAG-3 (Lymphocyte Activation Gene 3) is expressed on exhausted T cells within tumors and binds MHC class II molecules with roughly one hundred times greater affinity than CD4, delivering an inhibitory signal that suppresses T cell proliferation and cytokine production. In tumors, LAG-3 expression on tumor-infiltrating lymphocytes contributes to immune evasion alongside PD-1 and CTLA-4, and co-expression of multiple checkpoint receptors correlates with deeper T cell exhaustion. Relatlimab, the first approved anti-LAG3 therapy, combined with nivolumab (an anti-PD-1 antibody) produced a median progression-free survival of 10.1 months in advanced melanoma compared to 4.6 months with nivolumab alone in the RELATIVITY-047 trial published in 2022.

This combination received FDA approval in March 2022, making it the first approved dual checkpoint blockade regimen targeting LAG-3 and PD-1 simultaneously.

Antibody is a Y-shaped glycoprotein produced by plasma cells that binds specifically to an antigen through its variable region, neutralizing pathogens, marking them for destruction, or activating complement.

Each antibody consists of two heavy chains and two light chains joined by disulfide bonds, with the variable regions at the tips of the Y forming the antigen-binding sites and the constant region determining the antibody’s class and effector functions. Five antibody classes exist in humans: IgG, IgA, IgM, IgD, and IgE, each with distinct structures, tissue distributions, and functions. IgM is the first antibody produced during a primary immune response and circulates as a pentamer of five Y-shaped units, giving it ten antigen-binding sites and making it highly effective at activating complement.

IgG, the most abundant antibody in serum, crosses the placenta and provides passive immunity to the fetus during the final trimester of pregnancy.

Antigen is any molecule recognized by the adaptive immune system that binds to B cell receptors, T cell receptors, or antibodies, triggering a specific immune response when present in a host.

Antigens are usually proteins, polysaccharides, lipopolysaccharides, or nucleic acids that are foreign to the host, though self-antigens can be targeted in autoimmune disease. The specific region of an antigen recognized by an antibody or T cell receptor is called an epitope, and a single antigen molecule may carry multiple distinct epitopes recognized by different lymphocytes. T cells do not recognize intact free antigens; instead, they recognize short peptide fragments, typically eight to twenty-four amino acids long, displayed on MHC molecules by antigen-presenting cells.

Haptens are small molecules that can bind antibodies but must be conjugated to a larger carrier protein to stimulate a primary immune response, a distinction first clarified by Karl Landsteiner in the early twentieth century.

Antigen Presenting Cell is an immune cell that processes protein antigens into peptide fragments, displays them on MHC molecules on its surface, and provides the costimulatory signals required for T cell activation, linking innate detection of pathogens to adaptive immune responses.

Professional antigen-presenting cells include dendritic cells, macrophages, and B cells, each differing in location, efficiency, and the type of T cell response they preferentially stimulate. MHC class I molecules present peptides derived from intracellular proteins to CD8 cytotoxic T cells, while MHC class II molecules present peptides from extracellular or phagocytosed proteins to CD4 helper T cells. Dendritic cells are considered the most potent professional antigen-presenting cells because they express high levels of both MHC class II and costimulatory molecules such as CD80 and CD86, and they migrate from peripheral tissues to lymph nodes after capturing antigen.

A process called cross-presentation, carried out mainly by a subset of dendritic cells expressing the marker XCR1, loads exogenous antigen onto MHC class I, allowing cytotoxic T cell responses to be generated against pathogens that do not directly infect antigen-presenting cells.

Autoimmune Disease is a condition in which the immune system fails to maintain self-tolerance and launches pathogenic responses against the body's own tissues, causing chronic inflammation and organ damage.

Autoimmune diseases affect approximately five percent of the population and are broadly categorized as organ-specific, such as type 1 diabetes targeting pancreatic beta cells, or systemic, such as systemic lupus erythematosus targeting DNA, proteins, and multiple organ systems. Loss of self-tolerance can result from molecular mimicry between self and foreign antigens, defects in regulatory T cells, genetic susceptibility concentrated in HLA genes, or disruption of the gut microbiome. Women are about three times more likely than men to develop autoimmune diseases, a disparity linked to sex hormone effects on immune regulation and to X-chromosome dosage effects on immune gene expression.

More than eighty distinct autoimmune diseases have been identified, and their combined prevalence makes them a leading cause of chronic illness in people under sixty-five in the United States.