CD45+ Cells: Function and Role in the Immune System

The body’s defense system relies on a specialized group of cells known as CD45+ cells. These cells are identified by a unique marker found on their surface. Their presence signifies an immune cell, making them easily distinguishable from other cell types within the body. They are fundamental components of the immune system, constantly working to protect the body from a variety of threats.

Understanding CD45: The Cellular ID Tag

CD45, or Cluster of Differentiation 45, is a protein located on the surface of most white blood cells, also known as leukocytes. It functions as a cellular identification tag, allowing identification of immune cells. This protein is classified as a transmembrane glycoprotein, meaning it spans the cell membrane and has sugar chains attached to its extracellular portion.

CD45 plays a role in cellular signaling pathways. Specifically, it acts as a receptor-linked protein tyrosine phosphatase (PTP). This means it removes phosphate groups from tyrosine amino acids on other proteins, a process known as dephosphorylation. By doing so, CD45 can regulate the activity of various signaling molecules inside immune cells, influencing their activation and overall function. The cytoplasmic domain of CD45 contains two phosphatase domains, with the first domain possessing enzymatic activity that influences T-cell activation through the antigen receptor.

The Diverse Family of CD45+ Cells

This broad group includes lymphocytes like T cells, B cells, and natural killer (NK) cells, as well as myeloid cells such as macrophages, neutrophils, eosinophils, basophils, and dendritic cells. Each type of CD45+ cell fulfills a distinct role in the immune system.

T cells, identifiable by markers like CD3, are involved in cell-mediated immunity, recognizing and eliminating infected cells or producing chemical signals that activate other immune responses.
B cells, marked by CD19, are primarily responsible for humoral immunity, producing antibodies to neutralize pathogens.
Natural killer (NK) cells, often identified by CD56, contribute to innate immunity by recognizing and destroying virally infected cells or tumor cells without prior antigen exposure.
Macrophages are large phagocytic cells that engulf and digest cellular debris, foreign substances, and pathogens.
Neutrophils are abundant white blood cells that are among the first responders to infection.
Eosinophils and basophils are involved in allergic reactions and defense against parasites.
Dendritic cells are specialized antigen-presenting cells that initiate adaptive immune responses.

CD45+ Cells in Action: Immune System Roles

CD45+ cells work together to defend the body against a wide range of threats. Their collective roles include recognizing and eliminating pathogens such as bacteria and viruses. They also clear damaged cells from the body, helping to maintain tissue health and prevent inflammation.

These cells are also involved in regulating immune responses, ensuring the body’s defense system is neither underactive nor overactive. This involves both activating immune responses when needed and suppressing them once a threat has been neutralized, preventing damage to healthy tissues. For example, CD45 facilitates the interaction between T cells and antigen-presenting cells, a key step in initiating an immune response. Without CD45, T cells might not respond effectively to antigens, leading to impaired immune function.

CD45+ cells also contribute to immune memory, allowing the immune system to remember previously encountered pathogens and mount a faster, more effective response upon re-exposure. CD45 modulates signals from various receptors, including integrin and cytokine receptors, influencing immune cell behavior.

CD45 and Health: Insights into Disease

The presence, absence, or altered expression of CD45+ cells can provide insights into various health conditions. Pathologists often test for CD45 to determine if cells in a tissue sample are from the immune system, which is useful in diagnosing conditions like leukemias and lymphomas. For instance, cancerous immune cells in leukemias and lymphomas often display distinct CD45 expression patterns that help in their identification. For example, in B-cell lymphoblastic leukemia, leukemic blasts typically show dim or even negative CD45 expression.

CD45 also plays a role in autoimmune diseases, where the immune system mistakenly attacks the body’s own tissues. Alterations in CD45 expression or function have been linked to conditions such as multiple sclerosis and rheumatoid arthritis.

The CD45 molecule is also being explored as a potential target in some therapies. Antibody-drug conjugates (ADCs) that specifically target CD45 are being developed to eliminate autoreactive immune cells in autoimmune diseases, aiming to “reset” the immune system. These targeted approaches may offer a way to remove disease-causing cells with fewer side effects.

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