1. Antibodies consist of a huge variety of genetically coded protein molecules that bind to a correspondingly huge variety of antigens.

2. Antigens can be anything: proteins, carbohydrates, cells, viruses, etc.

3. Many antigens are normal cell surface structures that vary genetically from one person to another. If a person is exposed to the "foreign" molecule, antibodies may be produced.

1. All white cells arise from stem cells in the bone marrow. When lymphocytes are first formed, they are immature, meaning that they are not yet capable of functioning specifically.

2. Those that are destined to become B-cells mature in the bone marrow, becoming cells that have the ability to produce a single form of antibody.

1. The antibody tetramer is shaped like a Y. The two upper tips of the Y are the sites of interaction with antigen. These are the variable regions. The remainder of the molecule exists in a very limited number of forms and is the constant region.

2. The vast repertory of antibody structures is generated from a relatively small number of DNA sequences by a process of DNA rearrangement and mutation.

1. According to the clonal selection theory, there exists a large population of B cells, each of which is programmed to make a single type of antibody. An antigen stimulates growth and division of all cells whose antibodies can react with the antigen, but only those cells.

2. Some B-cells become memory cells and can respond rapidly to subsequent exposure to antigen. The memory cells are responsible for long-term immunity.

3. The immune system can distinguish between "self" and "nonself" and does not make antibodies to self. This is called immune tolerance and is developed during the fetal period

4. Production of antibodies by an organism is an example of active immunity. Antibodies that are received by injection (gamma globulin) or through the placenta is passive immunity.

5. Vaccination is the deliberate introduction of antigens into a person in order to stimulate antibody production and provide protection against infection. The antigens can be killed bacteria or viruses, attenuated strains (weak strains that do not cause disease), or proteins that occur on the surface membrane of the viruses or bacteria.

B. There are several inherited defects in immune response.

1. Agammaglobulinemia, an X-linked recessive trait, was the first discovered, in 1952. Mature B-cells are not made, but T-cells are normal. Therefore, humoral immunity is absent but cellular immunity is normal. As a result, there is little resistance against bacterial infections, but resistance against viral infections is retained.

2. Severe combined immunodeficiency (SCID) is due to defects at any of several loci. Most forms are autosomal recessive. Both humoral and cellular immunity are missing, Therefore, affected persons are highly susceptible to bacteria, viruses, and fungi. They cannot survive without marrow transplants from a normal person. Gene therapy is being actively pursued as a means of treating this disease.

1. HIV is a retrovirus that specifically targets T-helper cells (T4 cells). When the viral RNA enters the cell, DNA copies are made. These can integrate into the host genome, thereby largely escaping the immune system. However, transcription does occur, producing viral RNA and more infective viral particles. Eventually, destruction of virally-infected cells reduces the number of T4 cells to levels so low that the immune system is compromised. Various infections occur, eventually resulting in death of the host.

2. In order to invade a cell, the virus must bind to the specific plasma membrane receptors CD4 and CCR-5 simultaneously. There is a mutant form of CCR-5 that cannot bind HIV-1 (the common form of the virus). The mutant is polymorphic in Caucasians, with an allele frequency of 0.092. Homozygotes occur at a frequency of 0.008 and appear not to be susceptible to HIV infection. Heterozygotes occur at a frequency of 0.167 and are relatively resistant to infection. This allele has not been detected in African or Asian (Japanese) populations.