Antibodies, Viruses, and Prions: An Immunology Primer

Posted on Apr 1, 2025 in Biology

Antibodies: Structure and Function

Antibodies are glycoproteins found in a soluble form in blood or other bodily fluids of vertebrates. They act as receptors on B lymphocytes and are employed by the immune system to identify and neutralize foreign materials. Each antibody consists of basic structural units with two large heavy chains and two light chains.

Antibodies are synthesized by a type of white blood cell called a B lymphocyte. Although the general structure of antibodies is very similar, a small region at the tip of the protein is variable; this part is referred to as the hypervariable region. The great diversity of antibodies allows the immune system to recognize many antigens, which are recognized by the epitope. There are five classes of antibodies: IgG, IgA, IgM, IgD, and IgE.

Antigen-Antibody Reactions

Neutralization Reaction

The antigen binds to the antibody and loses its negative effects on the body.

Precipitation Reaction

Soluble antigens are macromolecules possessing several antigenic determinants. Free antibodies bind to them and form insoluble complexes that precipitate.

Agglutination Reaction

This occurs when antibodies react with antigens located on the surface of bacteria or other cells. As a result, these cells form aggregates that sediment.

Opsonization Reaction

This is the process by which antibodies bind to antigenic determinants in the area of germs and lining. This favors the phagocytosis of the same because the antibodies facilitate adhesion to the surface of phagocytes.

Virus Composition and Classification

Membership is based on two types of molecules:

  • Nucleic Acid: Can be DNA or RNA, single-stranded or double-stranded.
  • Proteins: Called capsomeres. They form the protective cover or coat inside of which is the genetic material. According to the shape of the capsid, viruses are classified as icosahedral, helical, or complex.

Examples of Virus Types

  • Capsid DNA: Helical, icosahedral, complex (e.g., Mosaic snuff, Herpes simplex, Pox)
  • Capsid RNA: Helical, icosahedral, complex (e.g., Influenza, Rubella, Phage)

Retroviruses Explained

A retrovirus is a virus that has RNA in its genome, not DNA. It uses reverse transcriptase to convert RNA into DNA. The viral RNA is translated into DNA, which inserts itself into the DNA of the infected cell. Retroviruses are enveloped viruses with uncertain symmetry and various types of proteins on their cover.

Retroviruses (e.g., HIV) have an exclusive replication cycle by which genetic information is encoded in RNA rather than DNA. Retroviruses contain a DNA-dependent RNA polymerase, which directs the synthesis in the form of DNA, the genome of the virus after it has infected a host cell.

Retrovirus Cycle Phases

  1. The virus is fixed by the envelope protein to CD4 receptors on T4 lymphocytes.
  2. The viral envelope fuses with the cell membrane and releases the viral nucleocapsid within.
  3. The capsid breaks, and viral RNA is released along with reverse transcriptase.
  4. By the action of reverse transcriptase, a double-stranded DNA molecule of the viral genome is synthesized from each RNA viral strand.
  5. Later, components are formed and assembled, and finally released from the lymphocyte T4 membrane by exocytosis.

Specific Immune Response

The specific immune response is effective against those antigens against which it has been initiated and developed. This type of response is mediated by other cells such as lymphocytes and macrophages.

Lymphocytes are of two types: B and T, which in turn may be T helper (TH) and cytotoxic (TC).

The specific immune response may be humoral or cellular. Although the separation of both types of response is not very real, it is generally considered that when the elements involved are B lymphocytes, it is a humoral response, whereas when T lymphocytes are involved (TH or TC), it is cellular.

Prions and Viroids

Prions

Prions are infectious proteins with a smaller capacity. A prion is responsible for Jakob’s disease, related to mad cow disease.

Viroids

Viroids are infectious RNA fragments linked to illnesses in vegetables, such as fosiforme tuber disease of potato.