Lymphopenia (total lymphocyte count below 1.0 × 10⁹/L in adults) is most commonly a transient, reactive, benign finding — it occurs routinely in: acute viral illness (almost universal in COVID-19, influenza, EBV), bacterial sepsis, trauma and surgery (cortisol release suppresses lymphocytes), prolonged glucocorticoid use, and in response to stress and catecholamines. The clinically significant lymphopenia is persistent (lasting more than 3-4 weeks), severe (below 0.5 × 10⁹/L), or accompanied by clinical features. The most important cause to exclude urgently is HIV — HIV specifically infects and destroys CD4+ T-helper lymphocytes, causing progressive lymphopenia that leads to AIDS (CD4 count below 200/mm³). The UK HIV prevalence of approximately 1 in 600 adults means that HIV should be considered in any patient with unexplained persistent lymphopenia, particularly in: men who have sex with men, people from high-prevalence countries (sub-Saharan Africa), people who use intravenous drugs, and sexual contacts of known HIV-positive individuals.

Common variable immunodeficiency (CVID) is the most prevalent primary immunodeficiency in adults, affecting approximately 1 in 25,000, and is frequently not diagnosed until the patient has experienced 5-7 years of recurrent infections. The characteristic presentation: recurrent bacterial sinopulmonary infections (sinusitis, bronchitis, pneumonia) since childhood or young adulthood, often with the same organisms (Haemophilus influenzae, Streptococcus pneumoniae), combined with hypogammaglobulinaemia (low IgG, IgA, and/or IgM) and poor vaccine responses. Lymphopenia may be present (lymphocytes fail to mature into immunoglobulin-secreting plasma cells). The immunological workup for suspected CVID: serum immunoglobulins (IgG, IgA, IgM — all low or markedly reduced), lymphocyte subset analysis (CD19+ B-cells: present but functionally impaired, or reduced; T-cells: variable), and vaccine response (failure to respond to protein or polysaccharide vaccines confirms functional immunodeficiency). GPs should suspect CVID in any patient with more than three episodes of documented bacterial respiratory infection per year despite standard treatment — referral to clinical immunology is appropriate and immunoglobulin replacement therapy is transformative for these patients.

Lymphocyte subset analysis (flow cytometry measuring CD4+ T-helper, CD8+ T-suppressor, CD19+ B-cells, and CD56+ NK cells) is the key investigation for characterising the specific type of lymphopenia — the differential diagnosis and management differs fundamentally depending on which lymphocyte subtype is depleted. T-cell deficiency (low CD4): HIV, CVID, DiGeorge, post-BMT, corticosteroid excess — risk of intracellular pathogens (PCP, CMV, toxoplasma, Mycobacteria, fungi). B-cell deficiency (low CD19): rituximab therapy, CVID, XLA (X-linked agammaglobulinaemia) — risk of encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae). NK cell deficiency: associated with specific primary immunodeficiencies — recurrent herpesvirus infections. CD4:CD8 ratio inversion (below 1.0): HIV (CD4 depleted while CD8 rises in response); lymphoma.

The PCP prophylaxis threshold decision is an important primary care immunology principle — Pneumocystis jirovecii pneumonia (PCP) is an opportunistic infection that occurs when CD4+ T-helper cell counts fall below 200 cells/mm³ (in HIV-positive patients) or when T-cell function is severely impaired by any cause. Co-trimoxazole 960 mg three times weekly (or 480 mg OD) is the standard PCP prophylaxis. In HIV-positive patients, the CD4 threshold is well-established (start prophylaxis at CD4 <200). In non-HIV immunosuppressed patients (organ transplant recipients, patients on high-dose steroids + DMARDs, patients receiving chemotherapy), the decision is more complex — PCP prophylaxis is increasingly recommended for patients on prednisolone >20 mg/day for more than 4 weeks, or on combination immunosuppression (prednisolone + methotrexate + anti-TNF). GPs who identify lymphopenia in a patient receiving combination immunosuppression should contact rheumatology or the managing specialist about PCP prophylaxis.

HIV clinic referral within 1 week of a positive HIV test is the BHIVA standard — GPs who diagnose HIV (or receive a positive result from a patient's home test) should arrange HIV clinic referral within 1 week for: CD4 count and viral load measurement, HIV drug resistance testing (genotypic resistance testing before ART initiation), vaccination status review (pneumococcal, Hep A/B if not immune, HPV), STI co-testing (syphilis, GC, Chlamydia), and ART initiation (BHIVA guidelines now recommend ART at diagnosis regardless of CD4 count). GPs should not simply send a 2WW letter — the HIV clinic referral should be direct and urgent, with the patient contacted personally to explain the positive result and the importance of early specialist assessment. The 'TasP' principle (treatment as prevention) means that ART initiation reduces plasma viral load to undetectable, making the patient non-infectious to sexual partners — every week of delay in ART initiation is a week of potential ongoing transmission.

IVIg (intravenous immunoglobulin) replacement therapy for CVID is one of the most transformative treatments in clinical immunology — patients with CVID who have had multiple hospitalisations per year for recurrent pneumonia or sinusitis experience dramatic quality-of-life improvement after starting IgG replacement therapy. The mechanism: IVIg provides pooled IgG antibodies from thousands of donors, covering essentially all common pathogens. The dose: typically 400-600 mg/kg every 3-4 weeks (adjusted to maintain trough IgG above 7 g/L). Subcutaneous immunoglobulin (SCIg) is increasingly used as an alternative — weekly or biweekly self-administration at home improves independence and quality of life. GPs should be aware of the distinction: the goal of IgG replacement is not to normalise the serum IgG level (which may remain below normal despite replacement) but to achieve a level that prevents recurrent serious infections.

The recombinant herpes zoster vaccine (Shingrix — GSK) is a significant advance for immunocompromised patients — unlike the live-attenuated zoster vaccine (Zostavax), Shingrix uses recombinant VZV glycoprotein E antigen with an AS01B adjuvant, making it safe in immunocompromised patients who cannot receive live vaccines. Efficacy in immunocompromised populations (HIV positive with CD4 >200, post-BMT, haematological malignancy) is approximately 68-90%, significantly higher than Zostavax's approximately 51% in immunocompetent elderly patients. The NHS has committed to expanded Shingrix access. GPs managing immunocompromised patients (HIV on ART, post-transplant, on long-term immunosuppressive DMARDs, haematological malignancy in remission) should ensure these patients have received two doses of Shingrix 2 months apart. This is one of the most important vaccine quality improvements for immunocompromised patients in recent years.

The U=U (Undetectable = Untransmittable) principle is the most important public health communication in HIV medicine in recent years — multiple high-quality studies (PARTNER, PARTNER2, Opposites Attract) demonstrated that HIV-positive individuals who are on ART with an undetectable viral load (below 200 copies/ml) have a zero risk of transmitting HIV to HIV-negative sexual partners through condomless sex. This is not just a statistical claim — the studies included thousands of HIV-negative partners who had unprotected sex with HIV-positive individuals with undetectable viral loads over multiple years, and not a single transmission occurred. This finding has profound implications for: the mental health of HIV-positive individuals (reducing fear of transmission), decisions about disclosure to sexual partners, and the broader public health narrative around HIV. GPs caring for HIV-positive patients should ensure they know about U=U and its implications — this information significantly reduces HIV-related psychological distress and reduces stigma.

The shared care model for HIV between HIV clinic and GP is increasingly important as HIV-positive patients on stable ART live long, healthy lives and their primary care needs (cardiovascular risk, diabetes, cancer screening, mental health) become as important as their HIV management. The GP's specific roles in shared care: annual NHS Health Check (QRISK3 — HIV-positive patients have increased cardiovascular risk from both the virus itself and from some ART medications), cancer screening (HIV-positive patients have higher risk of anal, cervical, and certain other cancers — ensure screening uptake is documented), medication review (drug interactions between ART and common medications are common and clinically significant), mental health review (depression affects 30-40% — PHQ-9 annually), and substance use screening. The GP who knows their HIV-positive patients by name and provides proactive, non-stigmatising primary care significantly improves long-term outcomes for this group.