CD45 is a protein found on the surface of nearly all white blood cells, also known as leukocytes. This protein acts as a distinguishing marker, helping to identify these immune cells from other cell types in the body. It is often referred to as the Leukocyte Common Antigen (LCA) because of its widespread presence on these cells.
The Role of CD45 in the Immune System
CD45 functions as a protein tyrosine phosphatase, an enzyme that plays a part in regulating cellular processes such as cell growth and differentiation. It works by removing phosphate groups from specific proteins within the cell, which either activates or deactivates these proteins, influencing the cell’s response.
This signaling mechanism is important for the proper activation of various immune cells, including T-cells and B-cells. CD45 is involved in the initial stages of signal transmission from the receptors on immune cells, allowing them to effectively respond to infections and foreign invaders. The appropriate balance of this phosphatase activity is needed for the immune system to function correctly.
Identifying Immune Cells with CD45
The presence of CD45 on the surface of white blood cells makes it a valuable tool for distinguishing them from other cell types in a sample. Scientists and medical professionals use CD45 as a “pan-leukocyte marker.” This allows for the identification and isolation of immune cells from various biological samples, such as blood or tissue.
A common technique for this identification is flow cytometry. In this method, cells from a sample are labeled with fluorescent tags that attach specifically to CD45. As the labeled cells flow one by one past a laser, the fluorescent tags light up, allowing instruments to count and sort the CD45-positive cells, helping in analyzing the composition of immune cell populations within a sample.
Different Forms of CD45
CD45 exists in various forms, known as isoforms, which are different versions of the same protein produced from a single gene. These isoforms arise from alternative splicing of specific gene segments, leading to variations in the protein’s extracellular part. Different types of immune cells express distinct CD45 isoforms, providing clues about their specific identity and developmental stage.
Two commonly studied isoforms are CD45RA and CD45RO. CD45RA is typically found on “naive” immune cells, such as T-cells. In contrast, CD45RO is expressed on “memory” immune cells, which have previously responded to an infection or antigen and are prepared to mount a faster, more effective response if they encounter it again.
Clinical Significance of CD45 Levels
Measuring CD45-positive cells holds considerable importance in medical diagnostics and monitoring. In the diagnosis of blood cancers like lymphomas and leukemias, identifying and counting CD45-positive cells is a key diagnostic step. The absence or very low expression of CD45 on certain malignant cells can also be a significant diagnostic indicator in some acute leukemias.
Monitoring CD45-positive cell levels can also indicate the overall health of the immune system. Low levels of these cells might suggest a weakened immune system. Following a bone marrow or hematopoietic stem cell transplant, tracking the recovery of CD45-positive cells is used to assess if the new immune system is successfully establishing itself, a process known as engraftment.