As we’ve seen over these past few months, a SARS-CoV-2 infection can result in widely different manifestations and severities in the subsequent course of the disease it causes, COVID-19. Many of those infected by SARS-CoV-2 experience a mild to severe illness, with symptoms that include fever, shortness of breath, cough, and fatigue that appear roughly 2-14 days after exposure to the virus. On the other hand, some individuals infected with the virus will remain asymptomatic. 

A healthy immune system requires a series of checkpoints to ensure self tolerance and prevent damage to other tissues during immune response. Binding of costimulatory signal transduction molecules (such as CD28, ICOS, GITR) on T cells to their receptors (such as CD80/CD86, ICOSL, GITRL) on antigen presenting cells (APCs) may contribute to T cell activation. However, in some states, inhibitory signals of T cell activation and response occur during the involvement of T cell receptors. These signals are generated by proteins involved in immune checkpoints (eg, PD-1, CTLA-4, TIM-3, and LAG3). Usually PD-1 and CTLA-4 immunological checkpoint proteins are upregulated in T cells infiltrating tumors and bind to their respective ligands, PD-L1 (ligand B7-H1)/PD-L2 (ligand B7- DC) and CD80/86, and down-regulate T cell responses. Immunological checkpoint ligands are often upregulated in cancer cells as a means of evading immune detection. Therefore, immunotherapy by blocking immunological checkpoint protein activation of anti-tumor immunity has become a popular research subject for cancer therapy.

Cluster of differentiation, or CD molecules, are cell surface markers that are used for identification of cell types in pathology and other bioscience disciplines. The expression levels of CD markers may increase or decrease (or disappear altogether, at least to undetectable levels) when cells (for example, leukocytes, red blood cells, platelets, and vascular endothelial cells, etc.) differentiate into new and different lineages. Depending on the CD marker, the expression level may identify a phenotype for different segments of cells, such as when they become active or diseased. Most CD molecules are transmembrane proteins or glycoproteins, including extracellular regions that bind a ligand or opposing receptor, transmembrane regions to anchor the CD marker into the cell, and cytoplasmic regions that may confer some adaptor or catalytic function. Some CD molecules can also be "anchored" on the cell membrane by means of inositol phospholipids. A few CD molecules are carbohydrate haptens. The study of CD molecules can be used in many basic immunology research fields, such as the relationship between CD antigen structure and function, cell activation pathway, signal transduction and cell differentiation, etc. It can be used clinically for disease mechanism research, clinical diagnosis, disease prognosis, efficacy tracking and treatment, and more. CD molecules such as CD4, CD8, CD25, etc. can be used to identify populations of cells when studying samples by flow cytometry or immunofluorescence.