Histocompatibility molecules are genetically determined proteins present in membranes of all nucleated cells. They form a unique system for which the term “Major Histocompatibility Complex (MHC)” was coined. Each vertebrate species is characterised by its own MHC; that of humans has an abbreviation HLA. This acronym is derived from the words: “human leucocyte antigens” as its fist molecules were discovered in membranes of leucocytes.
Protein antigens are no able to induce an immune response without being previously processed by antigen presenting cells (APCs). Following their processing that comprises their splitting to smaller fragments – peptides, APs subsequently present them to T cells; moreover, they activate them and polarise to a specific biological functions. Depending of antigen origin, there are two presentation pathways, exogenous and endogenous. Antigens originated from outside of APC, e.g. bacterial toxins, enzymes, etc., are presented by exogenous pathway and presented molecules are class II HLA molecules. T cell, that recognise presented peptides belong to helper subset of T cells. Antigens originated in the cytosol, such as antigens that appear in the cytoplasm of virus infected cells, are presented by endogenous pathway and presented molecules belong to class I HL-A molecules. T cells, that recognise presented peptides, represent cytotoxic T cells.
Phagocytosis is a process of bacteria engulfment and their killing. The phagocytic cells of mammals belong to two complementary systems, the myeloid and mononuclear-phagocytic systems. The myeloid system consists of cells that phagocyte rapidly, however they are not able of sustained effort. Tthey are represented by neutrophils and eosinophils. In contrast, cells of the mononuclear-phagocytic system act more slowly but are capable of repeated phagocytosis. Mmonocytes and macrophages are principal cell responsible for. Process of phagocytosis can be divided into several steps: chemotactic movement of phagocytes towards invading bacteria, opsonisation and recognition of bacteria by specific receptors, their engulfment and phagolysosome formation, bacteria killing and resolution. Bacteria can be killed by oxygen or nitrogen dependent mechanisms and by various antibacterial substances.
Antibodies manage to protect us against germs principally by two ways: by enveloping their surfaces (opsonisation), what makes their engulfment and killing by phagocytes easier and more effective or by cell lysis. However, in this case, they need a help from the complement system.
Antibodies are molecules of glycoprotein nature that have ability to bind antigens either soluble or bound in membranes of cells in a specific way. They are present in plasma and in tissue fluids (lymph, saliva, tears etc.). As most of antibodies belong to the gamma globulins of plasma proteins, they were previously known under the term gammaglobulins; their contemporary nomenclature is immunoglobulins (Ig). There are five classes of immunoglobulins: IgG, IgM, IgA, IgE, and IgE, respectively. They have specific biochemical structure and function. The principal biological role is to bind antigens; except it, they can activate the complement, opsonise bacteria, bind to receptors of various cell of the immune system etc.
Common people perceive Immunology as a scientific branch that studies defence of an organism against germs, cancer cells, toxins etc. Nowadays, Immunology is defined a scientific branch that studies immunity, its cellular and molecular processes that proceed after a foreign substance, an antigen, enters our body. There are many notable personalities who contributed to its development. Let us mention only some of them – Louis Pasteur, Ilja Mecnikov, Jean Dausset and many, many others. They contributed to the fact that Immunology belongs to those scientific disciplines that develop the most intensively in recent years.