The Immune Response

The Immune Response

The immune response is a defence mechanism by which the body fights infection. It is divided into two functional systems, the innate immune system and the adaptive immune system. The innate system is the first line of defence by which the host combats infectious agents and pathogenic microbes. This is a non-specific response, which proves effective against most infective agents. In case this primary response is not affective and the invading microbe is lead to proliferate the adaptive immune system comes into action. The adaptive immune system produces a specific response in the form of antibodies to the infective agent that normally proves effective in neutralising that agent. In the case of the innate system resistance (immunity) is not improved by repeated infection. The adaptive immune response involves memory and gives rise to resistance to repeated exposure to the same infective agent. Childhood diseases such as mumps, measles, and chickenpox produce life-long immunity following an infection.

Most infectious agents enter the body proper via the epithelial surfaces of either the upper respiratory, digestive or genito-urinary tract. Once infectious agents have penetrated the body a variety of physical and chemical defence mechanisms come into play to help protect these tissues from most infections. This involves a specialised set of cells called leukocytes (white blood cells) and their products that have evolved to help combat infection and disease. Leukocytes fall into two broad categories of cell types: phagocytes and lymphocytes. Phagocytes form part of the innate immune system and provide non-specific immunity. Included in this category are neutrophils, basophils, eosinophils, monocytes, and macrophages. Lymphocytes form the adaptive immune system and provide specific immunity. There are two types of lymphocytes B cells and T cells. The B cells are differentiated in the bone marrow and found mainly in the lymph nodes and spleen. They are the cells that make antibodies (immunoglobulins). The T cells are differentiated in the thymus and fulfill two major functions. They regulate the activity of the B cells and directly attack infectious agents.

Antibodies

Antibodies belong to the specialised group of bio-active proteins called immunoglobulins (Ig's).They are glycoproteins and are present in serum and other tissue fluids, including the milk, of all mammals. Antibodies are produced in response to the host being exposed to immunogenic foreign (antigens) substances such as infectious microbes. They are an important element in the adaptive immune response in that they are directed specifically to the antigen that induced their formation and that they impart memory. In this manner the body is effectively prepared to repel any later invasion by the same organism.

Antibodies are produced by activated B cell lymphocytes (plasma cells). Each plasma cell secretes one class of antibody and all the antibody produced by a single plasma cell is of the same specificity. There are five classes of immunoglobulin that are recognised in mammals IgG, IgA, IgM, IgE, and IgD. The most prevalent class of immunoglobulin in all species of animals is IgG. The function of these molecules is to bind to invading organisms and to activate specific actions that help rid the body of disease causing agents. They function in cell killing, inflammation, and prevention of bacterial and viral attachment.