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| | | Absolute CD4 | | The absolute CD4 count is a measurement of how many functional CD4 T-cells are circulating in the blood. The lower the absolute CD4 count, the weaker the immune system. The absolute CD4 count is measured by a simple blood test and is reported as the number of CD4 cells per cubic millimetre of blood. HIV-negative people typically have absolute CD4 counts between 600 and 1200 CD4 cells per cubic millimetre. HIV-infected people have counts that are typically less than 500, and people with AIDS can have 200 CD4 cells per cubic millimetre or fewer. | | | | | | Active site (N-site) | | The enzyme surface to which a substrate molecule binds to undergo a reaction. | | | | | | Allergens | | These are antigens that bring about hypersensitivity or allergic reactions. | | | | | | Allergic reaction | | This is a response to harmless environmental antigens due to a pre-existing antibody. The most common mechanism is the binding of the antigen or allergen to IgE antibody on mast cells causing asthma, hay fever, eczema and other common reactions. It is acquired and is classified as a type I (or immediate) hypersensitivity reaction. | | | | | | Allergy | | This is a symptomatic reaction to substances that are otherwise harmless environmental antigens. It occurs from the interaction between the antigen and antibody produced from prior exposure to the antigen. | | | | | | Amino acid | | Any organic acid containing one or more amino groups (NH2) and a carboxyl group (CO2H) forming the essential components of proteins. | | | | | | Anaphylaxis | | This is a type l IgE mediated hypersensitivity reaction. It is an immediate response to antigenic challenge with the release of histamines and other agents causing capillary leak, wheeze, cyanosis, oedema of the larynx, tongue, lips and eyelids and urticaria. Anaphylaxis can result in sharp drops in blood pressure, difficulty in breathing, and death if left untreated. | | | | | | Anti-HBe | | This is an antibody produced in response to the Hepatitis B e antigen. In those who have recovered from acute hepatitis B infection, anti-HBe will be present along with anti-HBc and anti-HBs. In those with chronic hepatitis B, usually anti-HBe becomes positive when the virus goes into hiding or is eliminated from the body. In strains that do not make HBe antigen, anti-HBe is also positive. | | | | | | Anti-hepatitis B core antigen (anti-HBc) | | This is an antibody to the hepatitis B core antigen. The core antigen is found on virus particles but disappears early in the course of infection. This antibody is produced during and after an acute HBV infection and is usually found in chronic HBV carriers as well as those who have cleared the virus, and usually persists for life. Anti-HBc testing is either specific for the IgM antibody, which indicates acute infection, or measures total antibody, anti-HBC, which indicates past infection which could be either acute or chronic. | | | | | | Antigen presenting cells (APC) | | Specialised cells that process antigens and display their peptide fragments on the cell surface. A subset of these cells, dendritic cells, can initiate naive T cell responses to antigen (Ag) by carrying out Ag processing, Ag presentation and stimulation of naïve cells into effector cells. Other APCs include macrophages and B cells, although these cannot initiate a response from naïve T cells. | | | | | | Association and dissociation constant | This is a useful measure to describe the strength of binding or affinity between receptors and their ligands. The concentration of ligand required is related to whether binding is strong or weak.
Association constant is a measure of the extent of a reversible association between two molecular species at equilibrium.
Dissociation constant is the reciprocal. | | | | | | ATP (Adenosine triphosphate) | | Adenosine triphosphate is a nucleotide present in all living cells which serves as an energy source for many metabolic processes and is required for ribonucleic acid synthesis. | | | |
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