Haemoglobin: Red blood dye which binds oxygen.

Haemolytic plaque test: A haemolytic plaque test uses so-called haemolytic plaques to detect B cells which form antibodies. The cells are applied to an erythrocyte layer. The antibodies released by the B cells bind to receptors on the erythrocytes in the proximity of the B cells. Finally, complements are added which are stimulated by the bound antibodies to lysate the erythrocytes around the B cells.

Hairless mice have a mutation (nude) which causes a lack of body hair and an abnormal formation of the thymus stroma. Homocygote hairless mice do not have any mature T cells.

Haptens are molecules which bind to antibodies, but which cannot themselves trigger an acquired immune response. To initiate the formation of antibodies or a T cell response, haptens must be bound to carrier proteins (transport proteins).

Hay fever (allergic rhinitis) is an allergic reaction in the nasal mucous membranes which causes a runny nose, sneezing and heavy tear flow.

Heavy chains: All immunoglobulin molecules are constructed from two types of peptide chains, the heavy chains with a molecular weight of 50-70 kd and the light chains with 25 kd. An immunoglobulin consists of two identical heavy and two identical light chains. There are several classes, or isotypes, of heavy chains, and each of these classes is the basis for a particular function of the antibody molecule.

Histamine (biogenic amine) is stored in the granulomas of mastocytes. It is released when antigens bind to IgE molecules on mastocytes and causes a local expansion of the blood vessels and contraction of the smooth muscles. It is consequently responsible for some of the symptoms of a hypersensitivity reaction of the immediate types.

Histocompatibility: The term is used in immunology to describe the genetic system which causes the rejection of tissue or organ transplants. The rejection comes about from the immunological recognition of foreign histocompatibility antigens or H antigens.

HIV is the common term used for the human immunodeficiency virus.

HIV positive: People infected with HIV whose blood has been shown to contain antibodies against the HI virus.

HLA stands for human leukocyte antigen and is the genetic designation for human main histocompatibility complexes (genetic systems which lead to the rejection of tissue or organ transplants). The individual gene loci are designated by capital letters, such as HLA-A, and alleles by number, e.g., HLA-A*0201.

Humoral immunity is one form of protective immunity (the other is the cellular). It is based on antibodies which are produced in the course of a humoral immune response. Humoral immunity can be passed on to a recipient by the transfusion of serum containing the specific antibody. A transmission of the cellular immunity, on the other hand, is possible only with the help of specific immune cells.

Hypersensitivity reactions are immune responses to harmless antigens. When there is renewed contact with the antigen, it results in symptomatic reactions. They are classified according to their mechanism: Type 1 is a result of the activation of mastocytes by IgE antibodies, IgG antibodies against cell surface or matrix antigens are involved in Type II, Type III comes about due to antigen-antibody complexes, and Type IV is mediated by T cells.

Hypersensitivity reactions of the immediate type occur within minutes after the contact with an antigen. They are mediated by antibodies. Hypersensitivity of the delayed type, by contrast, does not lead to an immune reaction until hours or even days after contact with the antigen. In this case, the reaction is caused by T cells.

Hypersensitivity of the delayed type is a result of a cellular immune response which is triggered by antigens in the skin. It is initiated by inflammatory CD4 T cells. The reaction is called delayed because it does not occur for hours or days after injection of the antigen. This is the difference between it and the hypersensitivity of the immediate type, which produces an observable skin reaction only minutes after the injection.