recap

41.4 recap

The humoral immune response is based on the synthesis by B cells of specific immunoglobulins. The specificity of an immunoglobulin derives from the amino acid sequence of its variable regions. B cells can make millions of immunoglobulins with different specificities by rearranging the genes that encode the variable regions of the heavy and light chains.

learning outcomes

You should be able to:

  • Compare the antigen-binding site of an antibody with other molecules such as enzymes that bind other molecules.

  • Describe the roles of molecules such as immunoglobulins in humoral immunity and of T cell receptors in cellular immunity.

  • Compare the diversity of specific antibodies in an individual with the diversity of other biological molecules.

Question 1

Describe the part of an antibody molecule that interacts with an antigen. How is it similar to the active site of an enzyme? How does it differ?

See Figure 41.8. The antigen-binding site of an antibody has heavy and light chains in a unique three-dimensional configuration that binds a particular antigenic determinant. This is similar to an enzyme active site that binds a substrate. In both cases, binding is noncovalent. A major difference is in the result of binding: an antigen does not change its covalent structure when it binds to an antibody, whereas a substrate does change covalently when it binds to an active site.

Question 2

Compare immunoglobulins and T cell receptors with respect to structure and function.

Both immunoglobulins and T cell receptors have constant and variable protein regions, bind antigens, and have great variability in primary structure. T cell receptors are membrane proteins of T cells. Immunoglobulins can be either membrane proteins of B cells or secreted proteins in the blood.

Question 3

Discuss the diversity of antibody specificities in an individual in relation to the diversity of enzymes. Does every cell in an animal contain genetic information for all of that organism’s enzymes? For all of the immunoglobulins? Explain your answer.

There are thousands of different enzymes in an individual but potentially millions of different specific antibodies. Every cell in an animal has the genetic information for all enzymes. Each immunoglobulin, however, is derived from a unique gene (produced by DNA rearrangements) in a B cell or a clone.

By making antibodies, B cells are the major players in the humoral immune response. We will now turn to the cellular immune response, and the T cells responsible for it.