Complete or partial deficiency

- "Complete insufficiency" of T cell function can result from hereditary conditions (also called primary conditions) such as severe combined immunodeficiency (SCID), Omenn syndrome, and cartilage–hair hypoplasia.

- "Partial insufficiencies" of T cell function include acquired immune deficiency syndrome (AIDS), and hereditary conditions such as DiGeorge syndrome (DGS), chromosomal breakage syndromes (CBSs), and B-cell and T-cell combined disorders such as ataxia-telangiectasia (AT) and Wiskott–Aldrich syndrome (WAS).

- "Primary (or hereditary) immunodeficiencies" of T cells include some that cause complete insufficiency of T cells, such as severe combined immunodeficiency (SCID), Omenn syndrome, and Cartilage–hair hypoplasia.

- "Secondary causes" are more common than primary ones. Secondary (or acquired) causes are mainly:

Persons afflicted with X-SCID often have infections very early in life, before three months of age. This occurs due to the decreased amount of immunoglobulin G (IgG) levels in the infant during the three-month stage. This is followed by viral infections such as pneumonitis, an inflammation of the lung which produces common symptoms such as cough, fever, chills, and shortness of breath. A telltale sign of X-SCID is candidiasis, a type of fungal infection caused by "Candida albicans". Candidiasis involves moist areas of the body such as skin, the mouth, respiratory tract, and vagina; symptoms of oral candidiasis include difficulty in swallowing, pain on swallowing and oral lesions. Recurrent eczema-like rashes are also a common symptom. Other common infections experienced by individuals with X-SCID include diarrhea, sepsis, and otitis media. Some other common symptoms that are experienced by X-SCID patients include failure to thrive, gut problems, skin problems, and muscle hypotonia.

In some patients symptoms may not appear for the first six months after birth. This is likely due to passive immunity received from the mother in order to protect the baby from infections until the newborn is able to make its own antibodies. As a result, there can be a silent period where the baby displays no symptoms of X-SCID followed by the development of frequent infections.

Presentations differ among causes, but T cell insufficiency generally manifests as unusually severe common viral infections (respiratory syncytial virus, rotavirus), diarrhea, and eczematous or erythrodermatous rashes. Failure to thrive and cachexia are later signs of a T-cell deficiency.

Among the presentation consistent with hyper IgM syndrome are the following:

- Infection/"Pneumocystis" pneumonia (PCP), which is common in infants with hyper IgM syndrome, is a serious illness. PCP is one of the most frequent and severe opportunistic infections in people with weakened immune systems. Many CD40 Ligand Deficiency are first diagnosed after having PCP in their first year of life. The fungus is common and is present in over 70% of healthy people’s lungs, however, Hyper IgM patients are not able to fight it off without the administration of Bactrim)

- Hepatitis (Hepatitis C)

- Chronic diarrhea

- Hypothyroidism

- Neutropenia

- Arthritis

- Encephalopathy (degenerative)

X-linked severe combined immunodeficiency (X-SCID) is an immunodeficiency disorder in which the body produces very few T cells and NK cells. In the absence of T cell help, B cells become defective. It is an x-linked recessive trait, stemming from a mutated (abnormal) version of the IL2-RG gene located at xq13.1 on the X-chromosome, which is shared between receptors for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21.

Severe combined immunodeficiency, SCID, also known as alymphocytosis, Glanzmann–Riniker syndrome, severe mixed immunodeficiency syndrome, and thymic alymphoplasia, is a rare genetic disorder characterized by the disturbed development of functional T cells and B cells caused by numerous genetic mutations that result in heterogeneous clinical presentations. SCID involves defective antibody response due to either direct involvement with B lymphocytes or through improper B lymphocyte activation due to non-functional T-helper cells. Consequently, both "arms" (B cells and T cells) of the adaptive immune system are impaired due to a defect in one of several possible genes. SCID is the most severe form of primary immunodeficiencies, and there are now at least nine different known genes in which mutations lead to a form of SCID. It is also known as the bubble boy disease and bubble baby disease because its victims are extremely vulnerable to infectious diseases and some of them, such as David Vetter, have become famous for living in a sterile environment. SCID is the result of an immune system so highly compromised that it is considered almost absent.

SCID patients are usually affected by severe bacterial, viral, or fungal infections early in life and often present with interstitial lung disease, chronic diarrhoea, and failure to thrive. Ear infections, recurrent "Pneumocystis jirovecii" (previously carinii) pneumonia, and profuse oral candidiasis commonly occur. These babies, if untreated, usually die within one year due to severe, recurrent infections unless they have undergone successful hematopoietic stem cell transplantation.

Hyper IgM syndromes is a group of primary immune deficiency disorders characterized by defective CD40 signaling; "via" B cells affecting class switch recombination (CSR) and somatic hypermutation. Immunoglobulin (Ig) class switch recombination deficiencies are characterized by elevated serum Immunoglobulin M (IgM) levels and a considerable deficiency in Immunoglobulins G (IgG), A (IgA) and E (IgE). As a consequence, people with HIGM have decreased concentrations of serum IgG and IgA and normal or elevated IgM, leading to increased susceptibility to infections.

Individuals with BENTA disease have polyclonal B cell lymphocytosis (i.e. excess B cells) developing in infancy, in addition to splenomegaly and lymphadenopathy. Patients may have low serum IgM and mildly anergic T cells. These features likely contribute to the mild immunodeficiency seen with BENTA disease. Patients are generally susceptible to recurrent sinopulmonary and ear infections in childhood, and may be more susceptible to certain viruses including Epstein-Barr virus, BK virus, and molluscum contagiosum.

Affects males 50% of the time if mother is a carrier for the gene. Children are fine until 6–9 months of age. Present with recurrent infections with Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, hepatitis virus, and enterovirus CNS infections. Examination shows lymphoid hypoplasia (tonsils and adenoids, no splenomegaly or lymphadenopathy). There is significant decrease in all immunoglobulins.

BENTA disease is a rare genetic disorder of the immune system. BENTA stands for "B cell expansion with NF-κB and T cell anergy" and is caused by germline heterozygous gain-of-function mutations in the gene CARD11 (see OMIM entry #607210). This disorder is characterized by polyclonal B cell lymphocytosis with onset in infancy, splenomegaly, lymphadenopathy, mild immunodeficiency, and increased risk of lymphoma. Investigators Andrew L. Snow and Michael J. Lenardo at the National Institute of Allergy and Infectious Disease at the U.S. National Institutes of Health first characterized BENTA disease in 2012. Dr. Snow's current laboratory at the Uniformed Services University of the Health Sciences is now actively studying this disorder.

About half of US states are performing screening for SCID in newborns using real-time quantitative PCR to measure the concentration of T-cell receptor excision circles. Wisconsin and Massachusetts (as of February 1, 2009) screen newborns for SCID. Michigan began screening for SCID in October 2011. Some SCID can be detected by sequencing fetal DNA if a known history of the disease exists. Otherwise, SCID is not diagnosed until about six months of age, usually indicated by recurrent infections. The delay in detection is because newborns carry their mother's antibodies for the first few weeks of life and SCID babies look normal.

X-linked agammaglobulinemia (XLA) is a rare genetic disorder discovered in 1952 that affects the body's ability to fight infection. As the form of agammaglobulinemia that is X-linked, it is much more common in males. In people with XLA, the white blood cell formation process does not generate mature B cells, which manifests as a complete or near-complete lack of proteins called gamma globulins, including antibodies, in their bloodstream. B cells are part of the immune system and normally manufacture antibodies (also called immunoglobulins), which defend the body from infections by sustaining a humoral immunity response. Patients with untreated XLA are prone to develop serious and even fatal infections. A mutation occurs at the Bruton's tyrosine kinase (Btk) gene that leads to a severe block in B cell development (at the pre-B cell to immature B cell stage) and a reduced immunoglobulin production in the serum. Btk is particularly responsible for mediating B cell development and maturation through a signaling effect on the B cell receptor BCR. Patients typically present in early childhood with recurrent infections, in particular with extracellular, encapsulated bacteria. XLA is deemed to have a relatively low incidence of disease, with an occurrence rate of approximately 1 in 200,000 live births and a frequency of about 1 in 100,000 male newborns. It has no ethnic predisposition. XLA is treated by infusion of human antibody. Treatment with pooled gamma globulin cannot restore a functional population of B cells, but it is sufficient to reduce the severity and number of infections due to the passive immunity granted by the exogenous antibodies.

XLA is caused by a mutation on the X chromosome of a single gene identified in 1993 which produces an enzyme known as Bruton's tyrosine kinase, or Btk. XLA was first characterized by Dr. Ogden Bruton in a ground-breaking research paper published in 1952 describing a boy unable to develop immunities to common childhood diseases and infections. It is the first known immune deficiency, and is classified with other inherited (genetic) defects of the immune system, known as primary immunodeficiency disorders.

The precise symptoms of a primary immunodeficiency depend on the type of defect. Generally, the symptoms and signs that lead to the diagnosis of an immunodeficiency include recurrent or persistent infections or developmental delay as a result of infection. Particular organ problems (e.g. diseases involving the skin, heart, facial development and skeletal system) may be present in certain conditions. Others predispose to autoimmune disease, where the immune system attacks the body's own tissues, or tumours (sometimes specific forms of cancer, such as lymphoma). The nature of the infections, as well as the additional features, may provide clues as to the exact nature of the immune defect.

Lymphoproliferative disorders (LPDs) refer to several conditions in which lymphocytes are produced in excessive quantities. They typically occur in people who have a compromised immune system. They are sometimes equated with "immunoproliferative disorders", but technically lymphoproliferative disorders are a subset of immunoproliferative disorders, along with hypergammaglobulinemia and paraproteinemias.

A number of syndromes escape formal classification but are otherwise recognisable by particular clinical or immunological features.

1. Wiskott–Aldrich syndrome

2. DNA repair defects not causing isolated SCID: ataxia-telangiectasia, ataxia-like syndrome, Nijmegen breakage syndrome, Bloom syndrome

3. DiGeorge syndrome (when associated with thymic defects)

4. Various immuno-osseous dysplasias (abnormal development of the skeleton with immune problems): cartilage–hair hypoplasia, Schimke syndrome

5. Hermansky–Pudlak syndrome type 2

6. Hyper-IgE syndrome

7. Chronic mucocutaneous candidiasis

8. Hepatic venoocclusive disease with immunodeficiency (VODI)

9. XL-dyskeratosis congenita (Hoyeraal-Hreidarsson syndrome)

The bare lymphocyte syndrome, type II (BLS II) is a rare recessive genetic condition in which a group of genes called major histocompatibility complex class II (MHC class II) are not expressed.

The result is that the immune system is severely compromised and cannot effectively fight infection. Clinically, this is similar to severe combined immunodeficiency (SCID), in which lymphocyte precursor cells are improperly formed. As a notable contrast, however, bare lymphocyte syndrome does not result in decreased B- and T-cell counts, as the development of these cells is not impaired.

Diarrhea can be among the associated conditions.

A minority of patients are diagnosed with thymoma prior to manifestation of the immunodeficient state. Spindle-cell histology is present in most cases.

Because patients with GS have deficient humoral and cellular immunity, they are susceptible to a wide range of infections. Most commonly these patients suffer from respiratory tract infections. Chronic diarrhea is often related to villous atrophy rather than infection (Kelesidis, 2010).

Often autoimmune disease is associated with GS - most commonly pure red cell aplasia and myasthenia gravis. While the patients may experience hypogammaglobulinemia, a large percentage will have autoantibodies present in their serum (Kelesidis, 2010). It is theorized that the presence of thymoma may inhibit the thymus’s normal role in production of self-tolerant T lymphocytes. These T-lymphocytes may then attack the B cell precursors in the marrow, preventing maturation and ultimately resulting in hypogammaglobulinemia (Arnold, 2015).

Lymphoproliferative disorders are a set of disorders characterized by the abnormal proliferation of lymphocytes into a monoclonal lymphocytosis. The two major types of lymphocytes are B cells and T cells, which are derived from pluripotent hematopoetic stem cells in the bone marrow. Individuals who have some sort of dysfunction with their immune system are susceptible to develop a lymphoproliferative disorder because when any of the numerous control points of the immune system become dysfunctional, immunodeficiency or deregulation of lymphocytes is more likely to occur. There are several inherited gene mutations that have been identified to cause lymphoproliferative disorders; however, there are also acquired and iatrogenic causes.

The symptoms are very similar to graft-versus-host disease (GVHD). This is because the patients have some T cells with limited levels of recombination with the mutant RAG genes. These T cells are abnormal and have a very specific affinity for self antigens found in the thymus and in the periphery. Therefore, these T cells are auto-reactive and cause the GVHD phenotype.

A characteristic symptom is chronic inflammation of the skin, which appears as a red rash (early onset erythroderma). Other symptoms include eosinophilia, failure to thrive, swollen lymph nodes, swollen spleen, diarrhea, enlarged liver, low immunoglobulin levels (except immunoglobulin E, which is elevated), low T cell levels, and no B cells.

The severe combined immunodeficiency (SCID) is a severe immunodeficiency genetic disorder that is characterized by the complete inability of the adaptive immune system to mount, coordinate, and sustain an appropriate immune response, usually due to absent or atypical T and B lymphocytes. In humans, SCID is colloquially known as "bubble boy" disease, as victims may require complete clinical isolation to prevent lethal infection from environmental microbes.

Several forms of SCID occur in animal species. Not all forms of SCID have the same cause; different genes and modes of inheritance have been implicated in different species.

There are no formal diagnostic criteria (Kelleher, 2003) and many informal definitions exist. Most commonly thymoma is present with mixed humoral and cellular immune deficiency. T and B cells are both depleted so patients suffer from both encapsulated organisms as well as opportunistic infections (Miyakis, 2005). Some have defined GS as a subset of common variable immunodeficiency (CVID). Unlike CVID, there are reduced B cells in the periphery in GS (Kelesidis, 2010).

More generally it can be defined as an adult-onset primary immunodeficiency associated with thymoma, hypogammaglobulinemia, diminished B and T cells, and inverted CD4/CD8+ ratio(Kelesidis, 2010).

Omenn syndrome is an autosomal recessive severe combined immunodeficiency associated with hypomorphic missense mutations in immunologically relevant genes of T-cells (and B-cells) such as recombination activating genes (RAG1 and RAG2), IL-7 Receptor α gene (IL7Rα), DCLRE1C-Artemis, RMRP-CHH, DNA-Ligase IV, common gamma chain, WHN-FOXN1, ZAP-70 and complete DiGeorge anomaly (DiGeorge Syndrome; CHARGE).

Lymphocytopenia, or lymphopenia, is the condition of having an abnormally low level of lymphocytes in the blood. Lymphocytes are a white blood cell with important functions in the immune system. The opposite is lymphocytosis, which refers to an excessive level of lymphocytes.

Lymphocytopenia may be present as part of a pancytopenia, when the total numbers of all types of blood cells are reduced.

Reticular dysgenesis (RD) is a rare, inherited autosomal recessive disease that results in immunodeficiency. Individuals with RD have mutations in both copies of the AK2 gene. Mutations in this gene lead to absence of AK2 protein. AK2 protein allows hematopoietic stem cells to differentiate and proliferate. Hematopoietic stem cells give rise to blood cells.

Differentiation and proliferation of hematopoietic stem cells require a lot of energy and this energy is supplied by the mitochondria. The energy metabolism of mitochondria is regulated by the AK2 protein. If there is a mutation in the protein, that means that the mitochondria metabolism most likely will be altered and will not be able to provide enough energy to the hematopoietic stem cells. As a result, hematopoietic stem cells will not be able to differentiate or proliferate.

The immune system consists of specialized cells that work together to fight off bacteria, fungi and viruses. These cells include T lymphocytes (T cells), that primarily mediate the immune system, B lymphocytes (B cells) and Natural Killer cells. Patients with RD have a genetic defect that affects the T cells and at least one other type of immune cell. Since more than one type of immune cell is affected, this disease is classified as a severe combined immunodeficiency disease (SCID). A weakened immune system leaves patients susceptible to different kinds of infection. Commonly, patients who are diagnosed with RD also have bacterial sepsis and/or pneumonia.

Bare lymphocyte syndrome is a condition caused by mutations in certain genes of the major histocompatibility complex or involved with the processing and presentation of MHC molecules. It is a form of severe combined immunodeficiency.