Immunology Terms Starting With H

Helper T Cell is a CD4-positive T lymphocyte that coordinates adaptive immune responses by secreting cytokines that activate cytotoxic T cells, stimulate B cell antibody production, and enhance macrophage killing of intracellular pathogens.

Helper T cells differentiate into distinct functional subsets determined by the cytokine environment present during initial activation. Th1 cells produce interferon-gamma and support cellular immunity against intracellular bacteria and viruses, while Th2 cells produce IL-4 and IL-13 to drive humoral immunity and anti-parasite responses. Th17 cells secrete IL-17 and recruit neutrophils to sites of fungal and extracellular bacterial infection, and T follicular helper cells migrate into germinal centers to sustain high-affinity antibody production.

B cells require helper T cell co-stimulation through CD40-CD40L interaction and cytokine signals to undergo class switching and affinity maturation. HIV selectively destroys CD4-positive helper T cells, and when their count falls below 200 cells per microliter of blood, the coordinated adaptive response collapses and opportunistic infections define AIDS.

Humoral Immunity is the branch of adaptive immunity mediated by antibodies secreted by plasma cells into blood and body secretions, providing protection against extracellular pathogens, toxins, and viruses before they enter host cells.

The term humoral derives from the ancient medical concept of body humors, reflecting early observations that immunity could be transferred by cell-free serum. Humoral immune responses require collaboration between B cells, helper T cells, and follicular dendritic cells in germinal centers, where affinity maturation and class switching refine antibody quality over one to three weeks. Different antibody classes provide specialized functions: IgA protects mucosal surfaces, IgG provides long-term systemic protection and crosses the placenta, IgM is the first antibody produced in a primary response, and IgE drives allergic responses and anti-parasite defense.

Emil von Behring and Shibasaburo Kitasato demonstrated in 1890 that serum from diphtheria-immune animals could protect naive animals, establishing the concept of humoral protection and earning von Behring the first Nobel Prize in Physiology or Medicine in 1901. Maternal IgG transferred across the human placenta gives newborns passive humoral protection for roughly the first three to six months of life.

Hypersensitivity is an immune response that damages the body's own tissues rather than protecting against pathogens, classified into four types based on the effector mechanism involved.

Immunologists Peter Gell and Robin Coombs proposed the four-type classification in 1963, and it remains the standard framework for understanding immune-mediated tissue damage. The first category is IgE-mediated and includes allergic rhinitis and anaphylaxis, driven by mast cell and basophil degranulation within minutes of allergen exposure. Cytotoxic antibodies targeting cell-surface antigens define the second category, as seen in autoimmune hemolytic anemia, while immune complex deposition in vessel walls and tissues produces the third, causing conditions such as serum sickness.

T cell-mediated reactions make up the fourth, peaking 48 to 72 hours after antigen contact as in contact dermatitis from poison ivy or tuberculin skin test reactions. Each category can cause significant tissue injury, and some diseases such as systemic lupus erythematosus involve more than one mechanism simultaneously.