Background & Introduction:

An OVERVIEW (Adapted in part from Nester et. al. Page 371) : 

Immunology is the study of the way in which animals, especially humans, protect themselves from foreign invaders such as bacteria, viruses, non-self proteins etc. Human defense systems can be categorized into one of two main groups: Innate responses and adaptive (specific) responses. Innate mechanisms of defense develop quickly in response to molecular patterns associated with invading microbes or tissue damage. These mechanisms include physical barriers such as skin, mucous membranes, inflammation and fever. In contrast, adaptive defense mechanisms develop more slowly; they are triggered by and directed at a particular foreign invader (called an antigen (Ag)). These adaptive defense responses are primarily mediated by B and T lymphocytes and are classified as either humoral or cell-mediated immune responses. T lymphocytes destroy self cells that have been invaded by parasites or that have become cancerous (cell-mediated immune response). In contrast, B lymphocytes produce antibodies (Ab), also called immunoglobulins, in response to antigens (humoral immune response). Antibodies are glycoproteins that effectively “tag” antigens for removal or destruction. Antibodies are bivalent, Y shaped molecules

 Immunoglobulin (antibody) structure

antibody diagram

Unlike antibodies, antigens are multivalent. That is, they have many antigenic determining sites (epitopes) that stimulate the production of and combine with antibodies. 

antigen epitope diagram

Antigens and antibodies interact with one another in detectable ways. Serology is the science of detecting and measuring these interactions. Because antibodies have two antigen binding sites (bivalence), they can react with two antigen molecules or cells of the same type. This allows them to cross-link multivalent antigens into a large lattice network of alternating Ag-Ab-Ag-Ab molecules that eventually becomes so large that it settles out of solution as a visible precipitate. If the antigen is a cell or particle (ex. red blood cells or bacteria), the reaction is called an agglutination. If the antigen is soluble (ex. proteins, carbohydrates, toxins, etc), the reaction is called a precipitation.

agglutination diagram