O antigen and H antigen- Definition and 21 Key Differences


Bacteria are microscopic organisms that have diverse shapes, sizes, and metabolic abilities. They are surrounded by a cell membrane that regulates the transport of molecules in and out of the cell. Most bacteria also have a cell wall that provides structural support and protection from environmental stresses. However, bacteria can also have additional layers or structures outside the cell wall that contribute to their survival and interaction with other organisms. These surface structures include capsules, slime layers, S-layers, fimbriae, pili, and flagella. They are often composed of proteins, polysaccharides, or glycoproteins that are secreted by the cell and assembled on the outer part of the cell wall or membrane.

One of the important functions of these surface structures is to act as antigens, which are molecules that can elicit an immune response from a host organism. Antigens are recognized by specific antibodies or immune cells that bind to them and trigger a series of reactions to eliminate or neutralize the foreign invaders. However, bacteria have evolved various mechanisms to evade or modulate the host immune response by altering their surface antigens. This phenomenon is known as antigenic variation or antigenic alteration.

Antigenic variation can occur by changing the expression, structure, or composition of the surface antigens. For example, bacteria can switch on or off the production of certain antigens, modify them by adding or removing chemical groups, or recombine different segments of DNA to generate new variants of antigens. Antigenic variation can result from gene conversion, site-specific DNA inversions, hypermutation, or recombination of sequence cassettes. The advantage of antigenic variation is that it allows bacteria to escape from the host immune recognition and memory, and to adapt to different environmental conditions or niches.

Antigenic variation is especially common among bacterial pathogens that cause chronic or recurrent infections in humans and animals. For instance, Neisseria meningitidis and Neisseria gonorrhoeae can vary their pili and outer membrane proteins; Streptococcus pyogenes can vary its M protein; Borrelia burgdorferi can vary its surface lipoprotein VlsE; and Salmonella enterica can vary its O and H antigens . These antigens play a crucial role in bacterial virulence, adhesion, invasion, and colonization.

In this article, we will focus on the O and H antigens of Salmonella enterica, which are used as part of a serologic classification system for this genus. We will explain what these antigens are, how they are expressed and varied by the bacteria, and how they affect the host immune response. We will also compare and contrast the O and H antigens in terms of their structure, function, distribution, and properties.