Humoral and Cellular Immune Responses Explained

Posted on Mar 6, 2025 in Biology

Humoral and Cellular Immune Responses

Humoral Immune-Mediated Replies

Humoral immune response is triggered by B cells in response to a particular antigen, leading to the production of specific antibodies. B cells are formed and differentiate in the bone marrow, where they acquire the capacity to produce antibodies, becoming immunocompetent.

The bone marrow generates millions of genetically distinct B cells, each producing different antibodies that bind to a vast number of different antigens. Some antibodies remain anchored in the B cell membrane. The humoral response can be summarized as follows:

  1. Antigen binding: An antigen binds to an antibody present in the membrane of a specific B cell.
  2. B cell activation: After recognition, B cells are activated.
  3. Clonal expansion: Activated B cells divide to form cells producing the same type of antibody, increasing the number of lymphocytes producing antibodies against that antigen. This theory, developed by Burnet, explains the large quantities of specific antibodies following antigen exposure.
  4. Plasma cell differentiation: Activated B cells become plasma cells responsible for the large-scale production of specific antibodies against the antigen.
  5. Memory B cell formation: Some B cells remain as memory B cells, which have an extended lifespan and serve as an immune reserve for future exposures to the same antigen.

Plasma cells and memory B cells accumulate in the cortex of lymph nodes, then pass into the lymph and subsequently the blood. The activation of B cells is significantly enhanced by interleukins.

Cellular Immune Response

Two types of lymphocytes are involved in this response: T cells, which originate in the bone marrow and mature in the thymus. They are divided into two main groups:

T4 Lymphocytes: These cells contain CD4 receptor proteins in their cell membrane. There are two types:

  • TH Lymphocytes (Helper): Stimulate other T cells and B cells.
  • TD Lymphocytes: Increase the number and activity of macrophages.

T8 Lymphocytes: These cells have a membrane protein known as CD8. There are also two subgroups:

  • TC Lymphocytes (Cytotoxic): Cause the destruction of target cells.
  • TS Lymphocytes (Suppressor): Prevent excessive or disproportionate immune responses.

T cells act specifically against target cells bearing recognized antigens, distinguishing them from the body’s own molecules (autoantigens). A foreign antigen combines with an autoantigen in the membrane, resulting in an antigenic complex, transforming the macrophage into an antigen-presenting cell. This complex binds to the T cell receptor, leading to activation.

Autoantigens are glycoproteins specific to an individual, encoded by the Major Histocompatibility Complex (MHC). There are two kinds of self-antigens encoded by MHC:

  • Class I: Found in all nucleated cells of the body.
  • Class II: Localized only in antigen-presenting cells, such as macrophages and B lymphocytes.

T8 lymphocytes recognize MHC class I, while T4 lymphocytes (TH and TD) recognize only class II. TH cells are activated, and their activation is enhanced by interleukin 1 and interleukin 2, which also activates TC and TS lymphocytes. TC cells secrete perforin, causing the target cell to lyse and die. Memory cells are also generated in this response.

Non-B Non-T Lymphocytes

These lymphocytes are a minority, have a larger size, and contain cytoplasmic granules. They do not recognize antigens specifically, so their action is nonspecific. There are two different types:

  • K cells (Killer cells): Attack cells coated with antibodies and secrete perforin.
  • NK cells (Natural Killer cells): Act similarly to K cells, destroying virus-infected cells, cancer cells, or cells from transplanted organs.