HLA Overview, continued

HLAs are categorized into different classes. The two classes that play pivotal roles in immune cell activation include HLA Class I (MHC Class I Antigens) and HLA Class II (MHC Class II Antigens), as shown in Table 2.

1. Specifically HLA Class I molecules are further classified into two subclasses:

2. HLA Class II contains genes HLA-DR, HLA-DP, HLA-DQ, HLA-DM, and HLA-DO.

HLA genes are richly polymorphic and contain many alleles. Alleles refer to the variations of the same gene. The advantage of having many alleles is to allow the immune system to recognize a remarkably great variety of antigens for better protection. A quick recap on alleles: A gene can have different alleles all of which serve the same function such as coding a protein that determines eye color. Different alleles may be responsible for a different phenotypic feature of the genetic function of eye color, for example, brown eyes versus blue eyes versus hazel eyes. HLA genes bear a remarkable number of alleles as summarized in Table 2. Worth noting is the number of alleles in HLA class I (classical). This class contains the richest number of alleles and corresponding proteins. Note: See how HLA-B stands out, containing 9,146 alleles and 5,511 encoded proteins.