MODULE SUPPLEMENT: IMMUNE SYSTEM
The Immune Response

Before a T or B cell can recognize an antigen it usually has to be processed by an antigen presenting cell (APC). You ask:

"Why is this?"

As noted earlier, the primary function of the immune system is to distinguish self from non-self. This is accomplished through the molecules of the major histocompatibility complex, the entirety of which is called the Human Leukocyte Antigen (HLA) Complex.

HLA molecules and genes that encode them fall into three categories, two of which are important to the current discussion:

Class I and II molecules are cell surface glycoproteins and are members of the immunoglobulin supergene family that include a wide variety of cell surface molecules such as immunoglobulins and T cell receptors like CD4 and CD8.

Class I molecules are present on all nucleated cells; it is the foreign class I molecules on transplanted tissues that are recognized by the host's T lymphocytes causing rejection.

Class II molecules are found chiefly on immunocompetent cells including the APCs.

An antigen has to be associated with one of these receptors in order for it to be recognized as foreign and thus initiate an immune response.

So, what really happens? Watch this streamed java-based presentation narrated by M. Wallhagen.

Immune Changes With Age
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